Tuesday - Wednesday

Tracking another storm system as it moves across the country, slides through the Southeast and then just off the Carolina coast on Tuesday night - Wednesday morning.

This one definitely bears watching although current model forecasts are for it to pass just to our south.

Here is the 84-hour GFS which is the forecast for Wednesday morning:

You can see the storm off the North Carolina coast.  It moves essentially NE from here in later frames.

The storm is tossing back around 0.10-0.25 of precipitation - which is light: 1-3 inches of snow or so.

Two reasons this storm bears watching.

First:  it's strong.  A 989 mb low is fairly intense.

Second: this season, almost every storm this season has trended north and west from its position in model runs this far out.

If it moves more to the north and the west, this could be a big deal.  For now, some snow Tuesday night into Wednesday morning seems likely - but light amounts also seem likely.

Don't cancel any plans - but stay tuned...

Winter Precipitation Processes - Part 1: Pressure

I thought, while we have a break, that I would post a bit about the science of meteorology - especially as it pertains to the types of precipitation we see in the winter.  Perhaps you have wondered why sometimes what falls from the sky is rain, sometimes sleet, sometimes snow.  Maybe you've seen the nature of the snow that is falling change from tiny fluffy flakes (bad for snowballs) to wet, large flakes (great for snowballs) and wondered why that's happening.

And, maybe you don't care.  If that's the case, I'd stop reading here.

But for the curious, let's talk for a moment about pressure.  Hopefully at the end of this series, you'll start to see how this ties in to some of those forecast model maps I've posted in the past month or so.

ATMOSPHERIC PRESSURE AND UNITS

What is atmospheric pressure?  Simply: it's the weight of the air in the column above any given point.

This can be measured in all sorts of ways.  In the old days, somebody figured out that if you have put a pool of mercury into a reservoir and the reservoir extended upwards into a vacuum tube, you could measure how "heavy" the air above the mercury reservoir was by observing how high the mercury was forced up the tube.

For lack of anything better, this deflection was measured in inches - which is why you hear, even today, people referring to atmospheric pressure in "inches of mercury".  Crazy and archaic, no?

Meteorologists switched to the metric (really, the "SI" - " le Système international d'unités" - or International Standard) system of units for a lot of things a long time ago.  Atmospheric pressure is no exception.

The SI unit  for pressure is a "Pascal".  Sea level pressure is typically in the neighborhood of 100 kilopascals.  Like other metric units, the prefix "kilo-" means "1000".  So, we can say that sea level pressure is typically 100 X 1000 pascals.  Or 100,000 pascals.

In the early 1900's, some dude named William Napier Shaw decided that nobody wanted to deal with a unit of measurement where the base value was going to be so high.  Cleverly, he created a substitute unit called a bar.  Since he was British, it's unlikely this has anything to do with him having a pint at the time he came up with it.  Otherwise, we'd be walking around with pressure units called "pubs".  But I digress...

A bar is equal to 100 kilopascals or, conveniently, typical sea level pressure.

Pressure decreases as you go higher into the atmosphere.  Simple enough: there's less air between you and the top of the atmosphere (where air eventually runs out and the vacuum of space approaches), therefore the air column weighs less, therefore the pressure is lower.

Because so much of our weather is defined by what happens in the atmosphere, meteorologists knew they would have to use a fractional version of the bar to still be able to have manageable values as they spoke of different pressures in the atmosphere.  So, they settled on the millibar as the unit of pressure measurement.  The prefix "milli-" means  1/1000 for SI units.  So, a millibar (abbreviated "mb") is 1/1000 of a bar and that, in turn, means that 1000 millibars is the same as 1 bar - or typical surface pressure.

Anytime you see pressure measurements on a forecast model map, they will always be in millibars.  Some TV meteorologists have even started to introduce this standard to their audiences (although you'll still always see the pressure given in "inches of mercury").



CONSTANT PRESSURE SURFACES

Pressure is very important to what mets call the "sensible weather" - or the weather that you experience outside while walking down the street.  In meteorology, we have defined what are called "constant pressure surfaces" as a way of understanding and modeling atmospheric processes that cause our weather.

Imagine, for a moment, an "idealized" or "perfect" atmosphere.  This mythic world is one where the pressure at sea level is 1010 millibars (mb) and pressure decreases in a way that is perfectly proportional to how high you are in the atmosphere.

Now, let's say you are hooked onto a big balloon and you go straight up into the atmosphere.  You're armed with a notepad and pencil, thermometer, a barometer, an altimeter (tells you how high up you are) and, hopefully, a parachute.  Here's what you're asked to do by a meteorologist:

- "When your barometer reads 1000 mb, take a measurement of altitude and temperature".

- "Take the same measurements when your barometer reads:  850 mb, 700 mb, 500 mb, 250 mb and, if you can, 100 mb" (we'll assume, in the mythic world that we're discussing, that you don't need air to breathe and that you're impervious to cold).

Shortly after takeoff, your barometer reads 1000 mb. You take a measurement of 100 meters (300 feet) and a temperature of 15 C (59 F).

As you rise into the atmosphere on your balloon, you find that your barometer reads 850 mb at about 1500 meters (5000 feet) and that it's getting cold, about 5 C (41 F).

Up you go.  The barometer reads 700 mb and you take a measurement:  3000 meters (10000 feet) and -5 C (23 F)

You continue taking measurements at the pressure markers the meteorologist mentioned.  As you do, you notice that your balloon is getting larger and larger.  Why?  The pressure in the balloon has remained constant from sea level.  But the pressure outside of the balloon is getting lower and lower.  So the balloon is expanding outward.  A short time after your barometer measures 100 mb and you take your measurements, your balloon pops.  You release your parachute, fall gently to the surface and hand the meteorologist your pad which shows something like this:

PRESSURE (mb)	APPROXIMATE ALTITUDE	TEMPERATURE
1000	100 meters (300 feet)	15 C (59 F)
850	1500 meters (5000 feet)	5 C (41 F)
700	3000 meters (10000 feet)	-5 C (23F)
500	5000 meters (18000 feet)	-20 C (-4 F)
300	9000 meters (30000 feet)	-45 C (-49 F)
200	12000 meters (40000 feet)	-55 C (-67 F)
100	16000 meters (53000 feet)	-56 C (-69 F)

Meteorologists think of the atmosphere as a series of constant pressure levels.  In a perfect atmosphere, these would be standardized, i.e. your barometer would always read 850 mb at 5000 feet.

But in the real world, that's not true.  And it's the difference in the height of those pressure levels, the amount of space between the pressure levels (referred to as "thickness") and the temperatures that make a difference.

For the purposes of winter precipitation, we'll concern ourselves with pressures from the surface up to 500 mb.

Next post: how the heights and thicknesses change and what that means for us on the ground.

Oh - and guess what?  In my little example above, you were essentially a weather balloon.  These are released by National Weather Service upper air stations (Dulles/Sterling is our closest one) twice a day (morning and evening) and record the information from the example as well as other information (dewpoint, winds, etc) on their way up.  More about this in the next post, too!

Looks Like the Models Were Right...the First Time

The storm formed as expected, it snowed for a few hours after about 2 AM this morning and then dry air raced in behind and stopped the precipitation.

With the proximity of the storm, it still looks like on-and-off snow showers are possible - especially later tonight as the storm strengthens and retrogrades.

But the winds will be the principal focus.

This one was mostly a miss.  Just to our north in Pennsylvania it's snowing this morning and will likely keep doing so - especially further north into the NYC area.

Let's see how far this thing comes back to the south as it gets stuck later today.

For now:  sunshine!  (where's my smiling sunshine graphic with sunglasses?)

Downgraded to WWA - But Keep an Eye

So the NWS downgraded our Winter Storm Watch to a Winter Weather Advisory this afternoon, and I agree.

The models came west with the storm but the trend seems to have stopped for now.  This will be a glancing blow for our area with the potential for 2-4 inches of snow with the 2-inch amounts in the DC area south and slightly more as you go north.

Just to clarify:

By NWS definition at this time of year, a "Winter Storm Watch (or Warning)" is defined as either the potential for at least 5 inches of snow in a 12-hour period or 7 inches of snow in a 24-hour period.

A Winter Weather Advisory is for less than 4 inches of snow.

This remains a complicated forecast - even here on the brink of the storm arrival in the DC area.  A slight jog to the west - and I mean slight...perhaps 30 miles or fewer - and it changes forecast snow totals a great deal.

Also - even though the atmospheric temperatures support snow, it's likely that at least initially, surface temperatures will be too warm for it to stick.  So we'll be "wasting" some of the snow to melting.

Finally, sometime tomorrow the flow in the atmosphere will grind to a halt and the storm will slow down dramatically - maybe even retrograde (a process where the storm actually heads back south and west instead of north and east as the typical flow would suggest).

As this happens, the storm will send QPF back over the Mid Atlantic and the northeast.  How close?  Do we get those bands of snow tomorrow evening and overnight?  If so, then I can see the DC area getting closer to that 4-inch mark.  If not, then 2 inches sounds more believable.

Will be watching the models to see how this plays out in the runs tonight.

It's Gonna Be Close

So most all of the models shifted westward overnight with the storm track.  However, that doesn't even remotely mean they all agree on the storm track.

Most have the track still fairly far off the coast.  It makes this a complicated forecast because the cutoff line for significant and light snow (or what the NWS would call "warning-level" and "advisory-level" snow) will be very close to our area.

A slight shift in track eastward means less snow for us.  A little further westward places us "safely" in the warning-level snow.

I need to see the model runs from this morning.

For now, expect the precipitation to start tonight, probably as a period of rain or mixed rain and snow.  Eventually this will change over to all snow.

This will be a powerful storm once it's done strengthening over the Atlantic.  So whether it snows 5 inches or 2 inches, it will be windy tomorrow afternoon and tomorrow night.

Later Models Holding to the Westward Shift

The NAM still is significantly further offshore than the GFS in terms of storm track - but it, too, has come west from its earlier position.

This lends more credence to the 3-5 inch accumulation amounts forecast by the GFS earlier today.

Still watching...

It's Bigger...Possibly

Very difficult forecast situation developing for Wednesday - Thursday.

A low pressure system forming off the coast on Wednesday was forecast by most models to just brush the area with light precipitation before moving out to sea.  Run after run of the models had confirmed this situation.

Until this morning.

This morning's run of the GFS shows a sudden shift to the west with the storm track, closer to the coast and in a position to bring us more snow than originally thought.

There is likely to be a very sharp cutoff to the areas receiving significant snow and "not much" snow.  For now, we're in a zone where 4 inches are possible, more to our north and east.

It's very important that successive model runs stay with this idea and not to gravitate to one particular run.  However, it is a fairly marked change from previous runs and will have to be watched closely.

Here's the NAM total precip forecast.  When looking at this consider:  the GFS is even further west with the track than the NAM which slides all of this further onshore here in the Mid Atlantic:

Note how close the blue is...just in northeastern Maryland.  This particular run of the NAM also absolutely crushes NYC.  We'll see.

I'll keep you up-to-date as the new model runs come in.

For now, there's a chance of significant snow in our area Wednesday night through Thursday.

This Week: Rain, Some Snow

Two storms this week.  The first one goes by today and tonight bringing rain throughout the area.  Just not any cold air to work with - even though we are still in below normal temperatures.  It's not going to be a huge rain producer, but it should continue the melting process that has been going on for the past few days.

The second storm - currently forecast to arrive Wednesday-Thursday is a bit more interesting.

Right now, it looks like a storm will approach from the south and a low pressure area will form off the coast and move up into the Northeast.  At this point, it looks like the magical phase will happen too far north for us (or Philly or probably even New York City) to get serious snow out of this storm.  But it does seem to be a big storm for mid to upstate New York and places in northern New England where winter has been a dud so far.

For us, I think some snow arrives Wednesday afternoon - possibly starting as rain depending on how early it gets started - but changing to snow overnight Wednesday into Thursday.  Amounts seem light - at this point an inch or two seems to be the agreement between the forecast models.

If it turns into anything bigger, I'll let everyone know.

All meteorological eyes (at least in the mid Atlantic) are on the time period between about March 3-5 where long range models have a big storm affecting our area.  Much can change between now and then, but that seems to be our next (last?) best shot at a major snowstorm in the DC area.

Rain...

Quick post because I don't have a lot of time but:  here's how it looks for now.

The storm on Monday-Tuesday should be mostly rain for our area.

The storm on Thursday-Friday gives us a better chance of snow.

I'll post a more detailed analysis when I have the chance.

Keep an Eye on Next Week

The forecast models have found the crack pipe and I can't get it away from them.

Currently, they are woofing about three possible upcoming storm situations:

February 22-23 (Monday-Tuesday)
February 26-27 (Friday-Saturday)
March 1-2 (Monday-Tuesday)

Now, based on climatological history, it gets more and more difficult for us to get buried by snowstorms as we move into March.  The sun angle is higher, our days are longer ([our] nights are stronger than moooooonshine... +1 if you get the reference) and we're headed for Spring.

However, the change of seasons also brings a volatility to the atmosphere so if we can get enough cold air around, it certainly can snow and has.  It snowed last year (for one of the few times - remember what that was like?) at the end of the first week in March and one need only look back to 1993 to find evidence that the atmosphere can produce big snowstorms for us after February.

Back to the models...

They can't seem to lock on to a solution this time. At first they couldn't even decide whether or not we were looking at three separate events.  But it seems like at least the number of events is resolved, if not the tracks.

First, the possibility next Monday-Tuesday.  Looks like a large storm, it also looks like we could have temperature issues which could cause this to be more of a "wintry mix" kind of storm - but those are details to be worked out later.

I got the GFS off the pipe long enough to show you these:

First, at hour 126 (sometime Monday evening) it shows the storm moving into southern West Virginia and spreading precipitation ahead of it.  The blue line is the freezing line at 850mb (about 5000 feet up) which is a pretty good indicator of precipitation type (liquid or frozen).  In this case, we're just barely north of the line which means this precipitation is forecast to be snow or sleet.

By 6 hours later (Hour 132), the low in West Virginia transfers its energy to the developing low off the coast of North Carolina.  In response to the initial low, warmer air has surged over the region and changes our precipitation over to rain (we're south of the line...but just).

Another 6 hours and the coastal low has taken over completely.  As it deepens, the counterclockwise flow brings cold air back over us and changes whatever precipitation is left back to snow before the low moves out to sea.

So - this is one to watch.  Especially because we spend so much time right on the edge of the rain/snow line.  If this were the definitive track for the storm we could start talking about details but since some models have the low pressure in West Virginia hanging on for the duration (which would mean rain for us...a bad thing considering how much snow is still out there) and others have the coastal low much stronger and moving right up the coast instead of out to sea (more snow for us), we just have to wait for more information.

Stay tuned!

Still North and West

Models have the storm tracking even further north and west which puts us in the warm sector and in the dry slot by late evening.

So - I think the forecast pattern will hold.  Snow in the morning changing to a mix of rain and snow (or possibly all rain) changing back to snow by evening before ending. 

1-3 inches should be the rule.  Around 1-2 inches for us in the immediate DC area.

Should be a nothing storm - compared to what we've been experiencing lately.

After this - nothing on the long range models until the end of the month (2/26 or so).  We'll see what happens!

Latest Models Further North

Which puts us in the warmer sector and creates a situation likely to be:

Some light snow in the morning on Monday mixing with rain in the afternoon, possibly all rain for a time and then changing back to snow late in the afternoon.

Based on the modeled position I think 2-4 inches with the 2" happening closer to the DC area, 4" further west.

I should mention that almost every snow-producing storm has shifted southward at the last moment.  In the past, this has made the difference between, say, 20 and 24 inches.  Not really noticeable.

A shift south here and we're back in the cold sector and, therefore, snowier.

Probably won't really know for sure until tomorrow night.  I'm tempted to drop it to 1-3 inches based on tonight's models but experience has taught me not to follow any forecast model run too closely.

So I'll hold at 2-4 and we'll see what future model runs bring.

A Tough Little Storm

So this one will be a difficult forecast - hopefully things will begin to clear up later tonight or tomorrow.

For now we have this situation:

A low pressure system is coming south from Canada.  These little storms are sometimes referred to as "Alberta Clippers" sometimes as "Manitoba Maulers" depending on their point of origin.  Track is everything with these types of storms.  If it tracks further to the south of DC, we see all snow.  If it tracks further to the north, we run the risk of snow mixing with sleet or even rain.

Right now, the models have this tracking just barely south of the DC area.  So, as it so often turns out, I believe the further north and west of the big cities you go, the more the precipitation that falls will be all snow.

To complicate things, these storms are usually very dry as they are continental in origin.  This means that the storm's principle source of moisture (and, as it turns out, energy) will be the Atlantic Ocean.  How quickly it intensifies and how close we are to the low center will make the difference between a little snow and some good accumulations. 

This is not a similar scenario to the last two.  This is a small storm with the potential to drop about 3-5 inches on the DC metro.  Can these storms produce more snow?  Sure.  They have been known to produce 10-12 inches sometimes.  But, for now, it looks like our snow totals will be compromised some by mixing and some by the strength of the storm when it arrives.

I'll go with 3-5 inches for now.  More model runs are coming tonight which will hopefully lock on to the track.

It looks like it will snow, but not anything like what we've seen around here recently.

More Snow...

Because we apparently still haven't been properly chastised for spending the last 4 years saying things like:

(in your best old-person voice)

"...I remember back in the 80's when we got lots of blizzards.  But not anymore!  It never snows like that here - and it might never snow like that again..."

"...I have Polaroids of back when it used to snow so that my kids can see what snow looked like..."


...first glance at the morning models for the upcoming storm look more promising for snowfall here in the DC Metro for Monday afternoon - Tuesday morning.  An initial guess would be a 3-5 inch snowfall although storms of this particular type and the path it is taking have produced surprises in the past.

Here's the latest GFS, forecasting 84 hours from now (Monday night - Tuesday morning) clearly showing a deepening low just off the coast tossing back fairly respectable QPF over our area.

Variables will be track (as usual) and how quickly the storm intensifies off the coast before moving away.

Don't Look Now

An Alberta Clipper is headed this way for Monday.  It should be smaller than these last two storms, but it's currently forecast to take a track south of us which is a favorable track for getting the biggest bang-for-the-buck from systems that are generally dry compared to these coastal storms.

I haven't had time to look at any forecast models yet.  I've got to shovel.  Or go and drink a beer.

Or something.

Where's My Toast Graphic?

 There goes the all-time snowiest winter record.  2009-2010 now becomes the snowiest winter in the history of recording in Washington, DC.

At all three major airports.

I'll let the National Weather Service explain:

...PRELIMINARY ALL-TIME SEASONAL SNOWFALL RECORDS SET AT THE
THREE MAJOR CLIMATE SITES IN THE BALTIMORE-WASHINGTON AREA...

AS OF 2 PM TODAY...WITH THE 9.8 INCH TWO-DAY SNOWFALL TOTAL MEASURED
AT RONALD REAGAN WASHINGTON NATIONAL AIRPORT...THE SEASONAL SNOWFALL
TOTAL IN WASHINGTON DC STANDS AT 54.9 INCHES. THIS WOULD BREAK THE
PREVIOUS ALL-TIME SEASONAL SNOWFALL RECORD FOR WASHINGTON DC OF 54.4
INCHES SET IN THE WINTER OF 1898-99. OFFICIAL SNOWFALL RECORDS FOR
WASHINGTON DC DATE BACK 126 YEARS TO 1884.

AS OF 1 PM TODAY...WITH THE 11.9 INCH TWO-DAY SNOWFALL TOTAL
MEASURED AT BALTIMORE/WASHINGTON INTERNATIONAL THURGOOD MARSHALL
AIRPORT...THE SEASONAL SNOWFALL TOTAL IN BALTIMORE STANDS AT 72.3
INCHES. THIS WOULD BREAK THE PREVIOUS ALL-TIME SEASONAL SNOWFALL
RECORD FOR BALTIMORE OF 62.5 INCHES SET IN THE WINTER OF 1995-96.
OFFICIAL SNOWFALL RECORDS FOR BALTIMORE DATE BACK 118 YEARS TO 1893.

FINALLY...AS OF YESTERDAY...THIS YEARS SEASONAL SNOWFALL TOTAL AT
DULLES INTERNATIONAL AIRPORT STOOD AT 63.5 INCHES. THIS WOULD BREAK
THE PREVIOUS SEASONAL SNOWFALL RECORD OF 61.9 INCHES SET IN 1995-96.
AS OF 1 PM THIS AFTERNOON...THE TWO-DAY SNOWFALL TOTAL AT DULLES IS
8.5 INCHES...WHICH WOULD MAKE THIS YEARS SEASONAL SNOWFALL TOTAL
72.0 INCHES. OFFICIAL SNOWFALL RECORDS FOR DULLES DATE BACK 48 YEARS
TO 1962.

Sitting and Spinning

If you look at the radar image in the last post (which is automatically updated because it links directly to the source), you can see that the area of precipitation has moved very little.  As the storm off the coast strengthens (meteorologists also say "deepens" because the pressure is decreasing) it is actually retrograding slightly. 

This means that the low is moving against the prevailing flow and is either stationary or even moving slightly south and west.

This has kept bands of precipitation from just south of DC northward for the past few hours and looks like it will continue into the mid-afternoon.

9 inches or so here in south Arlington although measuring has become a guessing game due to the wind.

A For-Real Blizzard

Sometimes, the National Weather Service issues a Blizzard Warning because we in the Washington area are not used to heavy snow and strong wind.  Rarely in the past has the weather actually reached blizzard conditions.  Officially, those are:

1. Sustained wind speeds of 35 mph or more, or frequent gusts to 35 mph or greater;
2. Considerable falling and/or blowing snow that reduces visibility frequently to 1/4 mile or less.

Today, it's the real thing.

Winds are gusting to 43 MPH this morning, snow is falling at the rate of 1 inch per hour, whiteout conditions are occurring, National Airport is reporting visibilities of 1/10 mile, the air temperature has fallen all morning into the lower 20's and the windchill values are in the single digits:  6 at National Airport.

Have we moved to Buffalo?

The storm formed as the models promised and at around 2:45 AM, the sleet and light freezing drizzle that fell for a few hours last night changed back to snow.  At around 6 this morning, the storm began to bomb off the coast and the snow started falling heavily around the area.

I have 8 inches here and reports are of generally 8-12 inches around the area.  Measurement of snow becomes next to impossible in conditions like these - even for people not at National Airport.

The radar shows the back edge of the precipitation advancing slowly to the northeast.  Still a few hours of moderate to heavy snow before things start to let up a bit.  The winds will continue all day however.

Stay inside if you can!

What a winter.

So We're in the Dry Slot...Again

A lull in the precipitation here in the DC Metro area as the storm to the west shifts its energy to the storm off the coast.  In between, a gap in the precipitation field has developed through most of Virginia although in the last few frames I see some snow is redeveloping in central VA.

I have just shy of 4 inches on the ground and we will see how this new storm develops and what track it takes.

A few things of note that were not forecast by the models:

- the dry slot has worked its way much further north than forecast.  It actually pushed north of DC when it was originally forecast to be significantly to the southwest

- mixed precipitation and a changeover to sleet from snow has occurred much further north than forecast.  DC was not expected to changeover and Baltimore was considered safely in the snow range.  In the last hours, areas north of Baltimore have reported sleet and areas south of Baltimore have reported sleet or no precipitation for periods this evening as the dry slot worked in.

- the transfer of energy from the storm in the midwest to the storm off the coast has not happened as quickly or efficiently as forecast.

All of these suggest that the final snow totals will be somewhat less-than-forecast.  A lot depends now on the next few hours and how the storm off the coast forms.

But I'm going to bed.  The storm will do what it will do.  I'll take another look later this morning.

Good night!

Sleet Takes Over

A mixture of sleet and snow is falling in Arlington right now, I suspect more sleet than snow to the south and less sleet to the north.  This should all change back over to snow after midnight when the storm off the coast really gets going.

The National Weather Service has officially decreased the forecast snow totals for the DC metro area to 8-12 inches.

The heaviest snow should fall tomorrow morning and with it, another serious concern: winds.  It's possible that winds could gust to 45 mph tomorrow - which could cause big problems with trees and power lines.

Oh and...

you know how this morning the Winter of 2009-2010 ranked 3rd on the list of all-time snowiest winters ever officially recorded in Washington, DC?  In second place was the Winter of 1995-1996 which featured a total of 46".  As of this evening, that record is toast.  Move over 1995-1996, we're in second place all-time with this season.  And we still have a realistic chance of reaching #1 all-time...perhaps not in this storm (considering who will be officially measuring...see earlier posts for my rants about that)...but probably before this winter is over.

Moderate to Heavy Snow Continues

I have 3.25 inches here in Arlington since about 4:30 PM when the first flakes came down.  Snow didn't really get started until after dark.  Radar shows more bands of heavy snow - particularly south of DC and headed northward.

Snow Heavier in Arlington

And radar shows some heavy stuff headed this way...

Light Snow Has Started, Storm is Forming

Reports of some intermittent light snow across the area, mixed with a bit of sleet to the south, as expected.

The map below is a plot of how many millibars the barometric pressure has changed in the last 3 hours.  You can use this plot to see that in the southern states, the pressures are rising - indicating that storminess is moving out of that area.

You can also see that in the Great Lakes region, pressures are falling.  That's the initial area of low pressure that is transferring its energy to the real storm which will be forming off the coast of the Carolinas.  Evidence of the storm forming can be seen in this chart.  Notice that the pressure is falling, quite rapidly, just off the coast.

I still think the NWS is on the high side of the forecast amounts.  Their amended afternoon forecast is 10-14 inches.  I'll stick with 8-12 here around Washington, more to the north and east, less to the south.  We'll see how things go once the storm gets going later tonight.

The radar is filling in and snow should become fairly steady in the next few hours.

There is the chance that the snow will mix with sleet even as far north as Washington, DC - but I expect it to be more of an issue to the south of town.

Evening models coming in now.  I'll let you know what I see.

I'll Leave It Alone

For now, I'll leave my expectations at:

4-8 south of the city
8-12 from DC northward to Baltimore
12+ north of Baltimore

But based on what I've been seeing for the last hour or so, even that might be too high.  Things could change, in either direction, once the storm actually forms.  But for now, this looks like a big hit for our friends in New England - particularly north of New York City and into the Boston area.

We'll see how things evolve.  I've left the official wording of the National Weather Service warning above.  They are still calling for 12-16 inches in the DC area.  I'll let you know if that changes.

This is A Really Difficult Forecast - Maybe Less Snow?

Here we are, a few hours from the onset of precipitation and things aren't very clear yet.  This sort of storm setup has so many things that could make a forecast go wrong.

This scenario is very complex and I don't envy the official forecasters at the National Weather Service who have to make a final call that will affect so many people.

I have some concerns based on the latest models and radar.

The newest model runs show the heaviest axis of precipitation shifting somewhat to the north and east.  They also show the low pressure area itself forming closer to the coast - which could give us problems with mixed precipitation.  Finally, some of the upper-level patterns suggest that we in the DC area could experience issues with the dry slot again (like we did early Saturday morning which cut down on accumulations somewhat).

On the other hand, the storm that is expected to develop will be intense and slow-moving.  Models continue to put out a lot of QPF for us which increases the confidence for a major snowstorm.

I'm going to watch things develop for a bit.  In no way am I suggesting that this storm will not happen.  There's just much more uncertainty about exactly how all of the elements in the atmosphere will come together - a process that's happening even as I type this.

But I think there's a chance we get less snow than is forecast.  Right now, the forecast is the same as it was last night:

8-12 inches from about Fredericksburg north to DC
12-16 inches from DC to Baltimore and points west
16+ from Baltimore northward to Philly

but after looking at things this morning, I think those amounts may need to be lowered some in the DC area and southward.  Maybe to an average of 10 inches in the immediate DC metro, less south, more north.

In fact, if I had to do it right now, I'd just shift those values northward and put in a new range for south of DC.  So:

4-8 from about Fredericksburg north to DC
8-12 from DC to Baltimore and points west
12-16 from Baltimore northward to Philly

More information when I have it!

Quick Note

Just a quick note for anyone who didn't know.  You can click on any image I post and see it full-size.

And One More Graph to Illustrate This Winter

Thanks to Jessica for finding this.  It shows various snow statistics for the DC area, including that the average snowfall is 15 inches...which we eclipsed in December.  In one storm.  To this chart you can now add the most recent storm which, officially at National Airport, recorded 17.8 inches.  Ridiculously low (inaccurate and not-representative), but official is official.

A Record Winter

With this coming storm, we have a chance to break the all-time snow record for DC in one winter.  Even with National Airport as the location of record, we are at 45 inches for the year.  That's third behind

1898-1899:  54.4"
1995-1996:  46"

That 1995-1996 record is toast.  Starting tomorrow, probably around 5 PM - but with a lot of February and all of March to go, that 1898-1899 record is reachable.  It might even be gone by Wednesday.

And while this has been an amazing winter (we have 10 more inches, offcially, this year at Reagan National Airport than in the previous 4 winters...combined) it's very interesting to note that, based on climatological cycles, it may not have been too difficult to assume that this winter would be snowy (image below from the Washington Post).

We'll talk about why we seem to be in the bullseye this year and a bit about teleconnections (like El Niño) when we get some breathing time between storms...

Another Big Storm

So this one started out looking average and winds up looking pretty impressive.  Overall, this storm would be considered an unusually-strong winter storm for the DC area, except that we're kind of getting used to that now, aren't we?

Based on the model outputs and QPF's that are shown in the forecast, it seems like we're in line for at least a foot of snow from DC northward.  As a first call:

8-12 inches from about Fredericksburg north to DC
12-16 inches from DC to Baltimore and points west
16+ from Baltimore northward to Philly

The NWS is going slightly lower, some TV mets slightly higher.

There is a somewhat lower-than-normal confidence with this forecast because although the components are in place, the storm has yet to form.

But look for snow to spread across the area beginning in the early afternoon and increasing in intensity.  I think the majority of the snow will fall overnight tomorrow but we might have as much as 2-4 inches on the ground by nightfall tomorrow.

Mixed precipitation should not be an issue from DC northward but may be a factor south of DC - especially the further south you go.

I'm still going over model data so more posts to come.  Stay safe, everyone!

It's Actually the Precipitation Fields

The low track looks fairly consistent from model to model, but the strength of the low that develops after phasing is intense.  The models have responded by extending the precipitation field southward.  New model runs coming in now...but the midday runs aren't usually the most accurate.

Still, most models have us in the 8-12 inch range and this is looking more likely.

Oh - and just as I was typing this, the NWS has put us under a Winter Storm Warning.

In the text it warns of 10-20 inches...the lower end of that range happens the further south you go.  So in Arlington, that argues for 10-14 inches.

Still trying to sort it all out - but right now, significant snowfall - at least 8-10 inches and possibly a foot or more seems probable for a time period beginning tomorrow afternoon and ending in the afternoon on Wednesday.

QPF Plot for this storm.  I expect ratios slightly above the 10:1 rule...

More Snow Coming

I'm checking out the morning/afternoon models.  Things look a little stronger and a little further south at first glance.  Many models now suggesting at least 8 inches of snow, some hint at a foot.

I'll post more once I've had the chance to look them over...

The Game is Still On

And I don't mean the Super Bowl.

The nighttime run of the GFS still holds on to the timing of the storm.  This is going to be a big deal for southern New England - from Philly to New York City.  They could get a storm much like the one that just hit us this past weekend.

But it won't be negligible for us - not if the timing and positioning continues the way it looks like it will.  Here's the GFS in hour 72 showing the total QPF over us for this storm:

This puts us between 0.75 and 1.00 inches of QPF.  To the GFS, that translates to the following snow forecast:

Overall, the models seem to show us in the 6-10 inch range with higher totals northward to Baltimore - where they could see another foot.

Unlike the previous storm where the energy could be tracked as it moved across the country, this storm won't even form until the phasing takes place.  So even though it's only a couple of days away, there's still a fair amount of uncertainty with this storm.  A slight shift to the south puts us in the heavier snow.  A shift to the north and we have less snow and more mixing issues (with sleet or rain).  And all of it depends on the timing of the jet stream phase.

*whew*

So - for now, it looks as though snow will start falling sometime in the second half of Tuesday - maybe late afternoon.  The heaviest snow will fall overnight and into the morning on Wednesday and then continue with decreasing intensity through Wednesday afternoon.

6-10 inches seems like a good forecast at this point.  And we are under a Winter Storm Watch which, for the NWS, means the risk of 5 or more inches of snow.

I'll be watching the forecast models...

Good night.

It's Just a Phase...but Maybe We Should Worry

First, a few words about jet streams and then we'll talk about Tuesday - which could be yet another significant snowfall for the DC/Baltimore region...

Perhaps you have heard of the jet stream.  It is a relatively narrow, fast moving belt of air high in the atmosphere.    What you might not know, is that there are several jet streams.  The two that matter for us are called the Polar Jet Stream and the Subtropical Jet Stream.

The jet streams have a lot to do with the type of weather we experience.  Areas of the country north of the Polar Jet experience cold air outbreaks.  Sometimes meteorologists (especially on TV) will refer to the Polar Jet "diving" southward but what really happens is that the cold air is forced southward by other atmospheric interactions which, in turn, causes this ribbon of fast moving air to move along with it.

In any case, low pressure areas form as ripples in this wave of air.  These are the storms we experience.  The Polar Jet, also referred to as the "Northern Branch" sometime brings snow to our area - but it's usually relatively light.  Have you heard of Alberta Clippers?  These are fast-moving storms that come down on the Polar Jet from Canada.  Since they are continental in origin, they don't have a lot of moisture to work with and we typically get a few inches at the most from these storms.

Low pressure systems also develop in the Subtropical Jet.  These are very wet storms as they usually pass over the Gulf of Mexico and draw moisture from there and the Atlantic Ocean.  In the Winter, subtropical storms bring a lot of precipitation - but without any cold air, this can be rain.

Sometimes, these branches of the jet stream will join.  This is referred to as phasing and when it happens, we can get some pretty big storms.  To give you a visual sense of what this looks like on a map, the diagram below shows the northern and southern (Polar and Subtropical) jet streams phasing over Kansas back on December 6, 2009:

Essentially, phasing combines the energies of both jet streams and produces a strong area of low pressure.

On Tuesday, a low pressure system is forecast to move in from the Midwest.  Sometime on Tuesday, the northern and southern branches of the jet stream are forecast to phase.  If this happens where the models currently suggest it will, a strong low pressure will form off the coast of North Carolina or southeast Virginia and will move up the coast and give us some snow.

Right now, that is the most likely scenario but there are others.

The jet streams could phase later, which could cause a weaker low pressure to form which would, in turn, result in much less precipitation for us.

Forecasting the timing of jet stream phasing is difficult - and meteorologists rely on models and satellite images to try and make the call.

Timing of phasing will be everything with this system.  For now it looks like this could be yet another significant snow event for the DC/Baltimore region.  We may have more issues with possible rain/sleet/snow mixing during the event, but we could still see accumulating snow.

How much?

The GFS shows 1.00 - 1.25 QPF forecast.  (QPF is a shortcut way to say "liquid equivalent precipitation")  Even with mixing issues, that at least 6 and maybe as much as 10 inches of snow.  But again - it all depends on the phase timing.  This is the type of forecast that could change radically 24 hours away from the storm if the atmospheric conditions don't come together just right.

Stay tuned!

On to the Next!

This storm did not disappoint, even if it didn't break the all-time record of the Knickerbocker storm of 1922 (28 inches).  National will end up reporting about 17-18 which is about 2-4 inches lower than the surrounding areas. For them, that's pretty "accurate".

Dulles, on the other hand, checks in around 30 inches as do other places to our west and north.  Here in Arlington, I recorded just over 20 inches as did Loran and Joan in Alexandria.  Reports are still coming in.

We did, however, break a 110-year-old record for snow in a single day - even with National Airport's useless measuring system (I picture an NWS guy holding his hands in the snow, one at the bottom, one at the top, and then walking 200 yards back to the tower with his hands held apart to have a colleague measure the space between them).

Now for Tuesday.  Models have a pretty good storm and this could be the one that New England has been waiting for all Winter.  While we here in the Mid-Atlantic region have been unusually snowy, places from New York City northward haven't seen much - both relative to us and relative to their seasonal norms.

We're relatively close to this storm, temporally, so the models are already coming into line.  Storm track, as always, is a factor.  Below you can see a snapshot from this evening's GFS model at 84 hours (overnight Tuesday - morning Wednesday) showing a pretty good low pressure system on the coast, the DC area north of the blue freezing line (indicating that our precipitation would be of the frozen variety - probably snow) and showing about a half inch of liquid equivalent precipitation being tossed back over the area.

Since this is a snapshot, you can expect that there was a little precipitation on the model for us in the time frame before and after this snapshot.

At the moment, that argues for at least a 5-inch storm.  But this time, the models don't agree all that well and some have a stronger storm, some have it closer to the coast.

It bears watching - especially since there are a couple of scenarios that could give us issues with precipitation type.  We don't need any rain on top of this crazy amount of snow.

More to come on this next storm in an incredibly busy season.

Hope everyone enjoyed this storm and is safe and warm indoors!

OBS: WDX and NAX

Observations from two of our reporting stations:

First my parents (WDX) report snow rates of an inch an hour throughout this morning and total snowfall approaching 15 inches.  All that remains of the company name on the official measuring stick is "Reamer Company".

And from reporting station NAX, in north Arlington, Bryna and Russ report 17 inches and these scenes.  Cleverly, the furniture rental place included their contact information twice on their official measuring device:

Mother of God. Part II

Shoveling that snow takes some effort.  Wow. 

Almost 19 inches here in Arlington at 1:00 PM.  Snow still falling at a moderate to occasionally heavy rate.

Awesome!

The Dry Slot

Well, as the transfer of energy between the low out west and the low off the coast happened overnight, some dry air began to intrude into northern Virginia and the DC metro area.  Very early this morning, that caused snow to temporarily decrease in intensity.  Meteorologists refer to this as a "dry slot".

The dry air never really made it much further north and as a result, the snow totals in the DC area this morning range from 14-19 inches whereas in Baltimore they have already seen 2 feet and in Columbia, MD they are reporting 28 inches.

Snow continues to fall and will continue for most of the daylight hours into the evening tonight as the main surface low pulls away from the coast and the upper level lows rotate through the region.  Today's snow would be a normal "big" snow for DC - probably 4-8 inches of accumulation on top of whatever you have already seen.

For us, that's 17 inches and counting!  Measurement is becoming difficult as the winds have picked up and snow is blowing everywhere.

Thundersnow!

While watching a particularly heavy snow burst at 1:28 AM (that had to be at least a 3-inch-per-hour burst), I saw lightning in the southern sky twice and heard thunder.  Incredible.  I've only seen thundersnow one other time - during the Veteran's Day storm of 1987.

We're over a foot now here at Potential Vorticity central.  The ruler has disappeared.  Going to get the yardstick involved next.

And, of anecdotal interest: while watching the Weather Channel earlier this evening, both the current conditions and the forecast for Saturday simply had one word:

Blizzard.

Radar at 12:14 AM

Tonight I believe I have seen some of the heaviest snowfall I've ever observed.  Rates are 1-2.5 inches an hour and more very heavy bands are headed this way.  In the radar below, you can see two things.  The first is that as the low in the Appalachians transfers its energy to the coastal low off of North Carolina, it has brought a surge of warmer air with it.  Radars tend to be poor at judging rain/snow lines, which is why it looks in the radar below that rain has edged into DC and points south when I can see outside that it has not.

But there is some mixing going on in central Virginia and further south of the DC metro.

As the low off the coast rapidly intensifies (mets refer to this effect as "bombing") more cold air will be drawn in and will change the precipitation that is mixing back over to snow, possibly as far south as Norfolk.

The other thing you can see is the dramatically heavy precipitation that's going on.  Those dark blues are the bands I spoke of and they are where the snow is falling at tremendous rates.  See that yellow line above Richmond?  That's another locally heavy band of what is likely sleet.  As it moves due north and hits the colder air, it will be yet another burst of heavy snow for our region.

11.5 inches here in Arlington.  More to come...

Radar at 9:15 PM

Heavy snow continues to fall in the DC area and on the radar you can see, it's not going anywhere anytime soon.  The low pressure center in the midwest is transferring its energy to the low developing off the coast of North Carolina.  That will ride NE off the East Coast tomorrow.

A couple of power flickers here at Potential Vorticity central - so if I disappear from blogging, it's because of a power issue.

Oh and 7 inches at 9:30, which is an inch per half hour or two inches per hour of snowfall rate now.  You can see all the dark blue setting up over our area in the map below.

OBS: NAX

From Arlington, at Bryna and Russ' home, just shy of 6 inches at 8:04 PM (and a handy number for a furniture rental place in Maryland)

OBS: WDX

Photo from the Woodbridge Reporting Station (WDX) - also known as my parents' house.  A dogwood tree and heavy snow:

Snowfall Rate: 1 inch per hour

4.5 inches at 8:04 PM - one inch in the last hour.

Snowfall rates could double, or even triple, as the night goes on!

All Continues on Schedule and Track

The evening update from Arlington, VA at 7 PM, I have 3.5 inches on the ground.

Snow is falling heavily but hasn't yet reached maximum intensity.  I expect 18-20 inches to fall overnight, so snowfall rates of up to 3 inches per hour will be possible later this evening.

Storm track is as expected and we're down to looking at small-scale features like the "banding" I described in a previous post.  We're entering a bunch of small scale situations that are very favorable for heavy snow.  Some of these processes are the same as those you get during thunderstorms in the Spring and Summer.  For this reason, thundersnow is possible later tonight - something I find very exciting.  I've only seen it once in my lifetime.

Post your observations in the comments!  It's good to know how much everybody has seen.  And especially let me know if you have seen any lightning or heard any thunder.

And share your photos!

Enjoy the snow!

A Massive Storm

Snowfall extends from Minnesota to southern Maryland in the Delmarva Peninsula and a line of strong to severe thunderstorms extends from South Carolina off the coast of Florida. 

One Last Thing to Consider

Remember the snow-to-liquid rule? 

Generally, one inch of liquid precipitation = 10 inches of snow.

This ratio is true as an average but it is never a rule.  The ratio changes based on the temperature in the lowest layers of the atmosphere, including here at the surface.

We notice the difference immediately, but not in a mathematical way - in a physical way.  Remember the storm in December?  The first thing my friend said when he walked outside and picked some up was: "it's not snowball snow".  Indeed it wasn't.  It was loose and powdery and didn't pack well.  This means that it was more ice crystal than liquid.

The 4-5 inch storm on Tuesday night was different.  That snow packed almost as soon as you picked it up.  It was the kind that stays stuck to tree branches and leaves them looking beautiful in the morning sun.  That snow had a lot of water content.

Our snow from this megastorm will have more water content than the first, but less than the one on Tuesday night.  I expect ratios of 10:1 or maybe 12:1 from this storm whereas Tuesday night the ratios were more like 8:1 and back in December the ratios were 15:1 or more.

From the same amount of water, wet snow produces lower accumulations and dry snow produces greater accumulations.

So - if the snow is "wet", it will pack down and the measured accumulations will be somewhat lower.  If it's mostly "dry" snow, like we expect, then we have a real chance to be in the mid to upper 20's in snow totals from this storm.

At this hour:  moderate snow in Arlington, 33 degrees.  Roads and sidewalks are now getting icy.  Take care out there...

One more GFS model run...

Just to prove the point.  We could hardly be better positioned as a metro area if you like snow.  Remarkable liquid water equivalents for this time of year.  Just a powerful storm that will drop record snows across our area.

Precise Scientific Measurement

In college, I took an entire course on meteorological measurements.  It covered units, precision and the devices used to take measurements of various atmospheric phenomena.

In the class, as a way of explaining how important precision is during every step of the measurement process, the professor told us the story/joke about the scientist who was building a dog house.  He took his measurements with a micrometer to be completely precise, but when he went to mark the wood for cutting, he couldn't find a pencil, so he marked the spot with a thick piece of chalk.

Then he couldn't find his laser-sighted circular saw, so he used an old rusty handsaw to finish the job and was amazed to find that none of his boards were the same size.

Of course, the lesson here is:  it makes no difference if you start with perfect precision if along the way, you use imprecise devices.

Ever notice that the official snowfall amounts at National Airport are always lower, and sometimes significantly lower than surrounding areas?  National maintains the official total snow amounts for Washington, DC - which is funny, because National Airport is in Arlington...Virginia.

Anyway, I think their measuring device must be near a 757's exhaust or maybe near the hotplate where they cook their afternoon Cup o' Noodles, because you watch:  it will be at least 6 inches fewer than anything reported in Arlington or DC.

Fortunately, here at Potential Vorticity, I have crack statisticians keeping track of snow totals around the area.  In Woodbridge, my parents have cleverly constructed a measuring device that I already guarantee is more accurate than whatever-the-hell they use at National Airport.

It's a highly-precise (if a little off vertical), scientific-orange, snow measurement stick with markers at 12 inches, 20 inches and then every 2 inches up to 32:

I especially like that it's not certified by NOAA.  This almost guarantees its accuracy.

Like any good scientific institution, the Woodbridge reporting station also has a backup device.  Generously donated by the Texas Reamer Company (For All Your Downhole Drilling Tool Needs - and you know you have some), its accuracy is undisputed, its durability unquestioned and it's already about two inches in the snow, so we'll have to do some subtraction.

What will you be using to measure?  Send me an e-mail and let me know (preferably with a photo).

Moderate snow here in Arlington at this hour.  Much of it melting on the roadways and sidewalks as we're still at 34 degrees.  This will change...

First Flakes!

From our on-the-ground reporters in Woodbridge, we have first flakes at 9:35 AM and here in Arlington, at approximately 10:10 AM.  Snow has quickly picked up from flurries to steady light snow.  Buckle up, it's going to be a long ride.

20-22 inches now seems like the low end of the range for the DC area, there will be many areas of 2 feet or more.  I'm still reviewing the models and radar to check storm track.

3-5 inches by late afternoon, driving should still be ok but will deteriorate rapidly as evening approaches.

GFS Says: I'll See Your 24 Inches and Raise You to 30

The latest GFS run is in.  2.50-3.00 inches of liquid equivalent.  That's 25-30 inches of snow...assuming no mixing issues.  I'll wait for other models to agree but, wow.

Expected Timeline

Right now, based on model data, here's what I expect in terms of timeline and accumulations:

Late Morning to Noontime Friday:  Snow begins from southwest to northeast expanding in coverage and intensity through the afternoon.  3-6 inches on the ground by evening.

Friday Evening - Saturday Morning:  The heaviest snow develops and we could be looking at a foot to a foot and a half by morning on Saturday.

Saturday:  Snow will continue, lessening in intensity through the afternoon and ending overnight.

Total accumulations?  I think, so long as we avoid any issues with sleet mixing in - and I think, at this point, that we will - you can expect a general range of 18-26 inches.

Predicting totals in any one spot is tough because very small-scale events take place within storms like these - almost like storms-within-a-storm.  Mets refer to this as "banding" because their presentation on radar is of darker bands within the existing precipitation area.  If enough bands set up over one place, I can see localized areas 28-30 inches as possible.

This storm is a big deal.  We haven't seen one like this in a number of years.  It won't quite be the Superstorm of 1993 in intensity at its center, but the impact could be nearly as big for areas around DC.

"Variables", or, "What Could End Up Causing This Forecast to Suck"


At this point, confidence is as high in a forecast as I can remember it being.  There are no outlier models, no strange European model screaming for the storm to come to our west and douse us in rain rather than snow, no bizarre forecast model created by some dude in his garage that forecasts the storm to go out to sea and miss us entirely.  All the models, to some degree, agree.  That's unusual.

In an uncertain science, this is as close to certain as it gets.  But stuff can still go awry. 

1. Storm Track - the big one.  Forecast models have been pointing to a track that puts the bullseye over Washington DC and Baltimore, MD.  You've seen the output from these models below in my previous posts.  But, especially as you go further north, the gradient of total precipitation gets VERY tight.  In other words, a few miles difference makes a huge difference in storm totals. Right now, Philly is forecast to have 12-18 inches but most models put New York City in the 3-6 inch range and some miss it entirely.  So, too much of a shift south and we end up in lighter snow.  This doesn't appear likely.  Too much of a shift to the north and we're back with that sleet mixing in problem again.  This doesn't appear likely either.
2. "Dry Slot" - If you look a few posts down, you can see some satellite images I posted of when the storm was still southwest of Texas.  See the comma shape?  Notice that between the head of the comma and the tail there's a southwest to northeast aligned area of no clouds.  That's the dry slot.  If you get under that, there's no precipitation.  Now the atmospheric setup here should preclude that from being an issue.  But it's the only other thing I can think of to go wrong with this forecast.  We'll put this in the "not likely" column as well.

 More model runs still coming later this afternoon.  I'll try to stay on top of them.

 

Slack-u-weather Radar

I don't have a whole lot of respect for Accuweather, but I'm happy to take this lovely radar and repost it here.

Man, that's a big storm.

GGEM Model Run Complete

It's a Canadian model (eh!) and it just finished its run.  No change in the forecast track.  Confidence remains high.

The More the Merrier

So - even though we frequently use the NAM forecast model this close to the storm because it is a good short-term model, a meteorologist would never (or should never) make a forecast based on one model.

It's nice to have corroboration.  To that end, here's this morning's GFS model (Global Forecast System) run for total liquid-equivalent precipitation from this storm.  You can see its similarity to the NAM which I posted in my last note.

When the forecast models agree, confidence in any particular forecast increases.  Forecast confidence for this storm is getting higher all the time.

Remember:  take the amounts you see below and multiply by 10.  That gives you inches of snow forecast:

A Thousand Words

We're mostly using the NAM now because it excels in the short range and we're about 24 hours away from the storm.  Here's the NAM forecast for total liquid water equivalent precipitation on Saturday.  On the basis of this map, 2 feet looks possible in the DC metro.  I'm still looking at the model runs from this morning.  More to come!

NAMtastic

A slight shift to the south in the latest NAM forecast model track for the storm.

Actually:  The Storm.

Anything that stretches from east Texas to Hawaii deserves capital letters.

It's just one model run of at least 10 more to come before the actual storm arrives, but I should mention that a slight shift to the south is good for us...

Oh.  Let's get this out of the way now.  I like snow.  In fact, I love snow...and, specifically, snowstorms.  So in this blog: good means more snow, bad means less snow.  Thanks for your attention, now back to your regularly scheduled blog update...

...it takes that "mixing with sleet" issue and moves it south and repositions us in the figurative "bullseye" of the storm (we were just south of it earlier this afternoon).  Meteorologists refer to a southward shift in model forecast track as a "colder" solution because more cold air is generally trapped north of the storm and a southerly track means more of the forecast area is in the cold "zone".

Anyway, the effect of this is seen below in the NAM forecast total liquid equivalent precipitation.  Remember the 10 to 1 rule.  Mets (and mathematicians, for that matter) write this ratio as "10:1".  10 inches of snow per 1 inch of liquid equivalent.

Now, without further ado, the NAM

*crash*

as they say in northwestern Turkmenistan:

boo-ya!

Remarkable.

The storm is dragging moisture in from the Pacific Ocean all the way back to Hawaii:

Mother of God.

Our little storm, hanging out southwest of Texas at the moment:

Lunchtime Update: What are the Variables?

Some snow is a near certainty for the DC area.  So what will local mets be freaking out about for the next 50-60 hours before the first flakes start to fall in the DC metro?

1. Storm Speed:  when will the first flakes actually fly.  Of course, this has monster implications in terms of local commuting (just think if the snow starts at 3 PM on Friday...can you imagine the traffic issues?) and in terms of having the road crews ready to get rid of the snow once it falls.  Models tend to forecast storms moving too quickly at first so local mets are likely expecting things to start a little later than modeled.  For us, that would be sometime Friday afternoon.
2. Storm Track:  this is really the big deal...the difference between some snow, lots of snow and biblical snow.  And here's where the models are still disagreeing.  Some indications are that sleet could mix with the snow here in the DC area for a period of time on Saturday.  This would cut our accumulations from "crippling" to "significant".  There are currently models that suggest 8-12 inches because mixing takes place and others that suggest no mixing and totals more in the 18-24 inch range.  This will be the hardest thing to forecast and may really only become clear as the storm begins to increase in power on Saturday.  Obviously, there's a big psychological difference between 8-12 and 18-24 but there's a pretty big operational difference between 8 and 24.  8 inches is an average "big" snowfall around here.  2 feet is crippling for airports, roads, etc.  I'll be using the models as best I can to figure out what the final track will be.

Right now, significant snow is still expected, mixing could become an issue, crippling snow still possible.

More to come.

Oh, and...

...as Bryna pointed out in her comment to my last map post:

We could be doing this all over again on Tuesday of next week.  I can't even look at that storm yet - but early indications are for another good snowfall...

Geez, what a Winter.

A Little Wrinkle

In the Northern Hemisphere, low pressure systems induce counter-clockwise circulation of air around their center.  The stronger the storm, the stronger the counter-clockwise flow.

For strong winter storms in the East, this flow becomes an important factor because not far from us is a huge body of water that is at temperatures much warmer than the air.   If a storm tracks too close to our area, the counter-clockwise flow around the center brings warm air from the Atlantic Ocean into the region.

This can cause the snow to change to sleet or even rain for a time before it changes back to snow as the storm center passes by.  The net effect is to reduce overall accumulations of snow.

All of this makes the final track of this storm particularly important to us.  If it tracks too close to our area, I think there's a chance that sleet mixes in with the snow on Saturday afternoon.  If it tracks further to the south, then I think we end up in the all-snow area.

The difference?

Well, I think mixing precipitation could lower expected snowfall amounts from 2 feet to more like 15-20 inches and that points from Baltimore northward into Pennsylvania (where the warm air doesn't reach) could remain all snow and have the jackpot from this storm (2 feet +).

The models are in disagreement as to final track but it's becoming more and more likely that this is a question of "how much" rather than "if".

Stay tuned!

And Now, a Deep Breath

A little pause before the weekend festivities.   *inhale*  *exhale*

Ok!  Let's get rolling.  Sometime in between my actual job, I'll take a look at the forecast models and get a sense for the weekend.  Indications at this point are:

- Snow begins Friday afternoon sometime and picks up in intensity through Friday night.

- Saturday will feature moderate to heavy snow all day long, probably ending Saturday night

How much?  Still too early to be tossing amounts around.  However - for something like the 4th consecutive model run, I've been seeing maps like this forecast for snow amounts from the NAM for this storm (this one is in "inches of snow"):

O.o