Here Comes Irene

I've decided to post a rare non-winter blog entry because of the impending hurricane here on the East Coast.  As usual, the post will be more concerned with effects in the DC area since that's where I live.

Irene is currently a Category 2 hurricane.  She has weakened a bit in the last day or so and has maximum sustained winds of 100 MPH placing it just inside Category 2 status (which begins at 96 MPH).  The intensity will likely not change significantly before landfall in extreme eastern North Carolina - probably in the vicinity of Cape Lookout tomorrow morning.

Two things to remember:

• This storm is large.  So is the extent of the rain and wind fields.  For this reason, its projected path offshore is still close enough to the coast to cause problems for us.
• However, the cut-off between significant rain and just a few showers is usually pretty sharp with hurricanes passing to our east.  If the storm follows the current forecast track, I'd guess that we'll get significant rain in the DC area - especially south and east of the city.

If this type of forecast sounds familiar it's because a hurricane's impact pattern is similar to the large snowstorms we get in the winter.  Track means a lot - although exactly where the center travels matters less...especially with a storm this large.  That's why the National Hurricane Center cautions people to concentrate less on the line that marks center track and just heed the warnings they issue.

Speaking of which...

We're under a Tropical Storm Warning here in the DC area - mostly for areas near water but including the following counties:

DISTRICT OF COLUMBIA...NORTHERN BALTIMORE...HARFORD...
MONTGOMERY...HOWARD...SOUTHERN BALTIMORE...PRINCE GEORGES...ANNE
ARUNDEL...CHARLES...CALVERT...PRINCE WILLIAM/MANASSAS/MANASSAS
PARK...FAIRFAX...ARLINGTON/FALLS CHURCH/ALEXANDRIA...STAFFORD...

SPOTSYLVANIA AND KING GEORGE.

And in St. Mary's county, the Tropical Storm Warning is now a Hurricane Warning.

So what should we expect?

The storm is likely to move fairly far off the coast so I expect the worst conditions on the Eastern Shore. 

Probably 6 inches of rain and winds gusting to 80+ there at times tomorrow.

Here, things start going downhill late tonight.  The first bands from Irene could arrive tomorrow morning

and the winds will be increasing all day.  Expect winds in the 20-30 MPH range by tomorrow afternoon,

increasing to 30-40 with higher gusts tomorrow night.  The worst should be tomorrow evening and 

overnight as the storm passes by to our east. 

Again, the cutoff for precipitation is expected to be fairly sharp.  I'd guess areas around I-95 could pick

up 1-3 inches of rain, more to the east, less to the west.  Places out in the Blue Ridge might just have a 

cloudy day.

This is a storm to take seriously but in DC, I also don't see the need to go out and buy all the bottled water

you can find.  Flooding is possible, power outages are probable and 50 MPH winds with saturated ground 

can make trees a problem.

The next official advisory from the National Hurricane Center comes out at 5 PM.

For now, here's an excellent graphic with the forecast storm track:

Boston Globe Irene Tracker

Quick Burst of Snow Tonight Into Tomorrow Morning

It's been a while since anything interesting (aside from some unusually warm weather) has happened around here but tonight we have a fairly unusual situation developing that could cause a quick burst of snow and some minor accumulations around the area.

First up is a cold front which will be moving across our area this afternoon.  Precipitation associated with the front will begin as some light rain which will increase a bit in intensity as the afternoon becomes the evening.  The cold air will take a little while to catch up but eventually it will move across the area from north to south bringing a changeover in the precipitation that's falling from rain to sleet and eventually over to all snow.

At the same time, a low pressure area will move out of the midwest and cross the region bringing the bulk of our precipitation - especially towards midnight and into the early morning hours Tuesday.

This should be a brief storm but it could be a bigger deal than it even looks now.  That "banding" effect we've talked about before could lead to locally heavy precipitation amounts and the model trends have been colder and wetter the last few runs.

Waiting for the afternoon models but I think 2 inches is a good possibility in the immediate DC area, more as you go north and west.  There is the potential for more - maybe 4 inches or more - but before we go there, I'd like to see what the afternoon models have to say.  But, just to put it out there, here's the GFS 24-hour total precipitation forecast:

You can see we're squarely in the .5-.75 liquid precipitation range.  Given that we may lose a bit to rainfall and some sleet, this argues for 4 inches or so.

One thing the precipitation will be fighting is the relatively warm ground temperatures that have developed after the past 5 days or so.  On the other hand, the precipitation could be quite heavy later tonight.

Stay tuned...

Snow Totals Currently From 2-6 Inches Around the Area - Snow Will Continue

Here in Arlington, I measure 2 inches of wet snow that is packing itself as it falls.  If the atmosphere were colder, we would probably have 3-4 inches at this point.

Snow will continue into the night ending from southwest to northeast around 10-11 PM.

A few of those small-scale bands have formed and are producing snowfall rates of 1-2 inches per hour.  I expect snow totals will vary greatly from place to place depending on those bands.

In the radar below, you can see the upper-level energy moving to the east where it will phase with the surface low offshore.  As it starts this process there's a "twisting" motion to the precipitation which will keep things going for a while - even though you can see the back edge of the snow around Charlottesville.

Enjoy!

More Data Coming In - Just Keeps Getting Better

The new data looks supportive of more snow - but a lot will depend on those small-scale conditions we've talked about before.  Get under one of the bands and you could see some hefty accumulations.

Right now 6-8 inches seems likely, 10 is within reach here in the immediate metro.

Interestingly, the bullseye is setting up where the models had it this morning - right over DC and up to the northeast.  We may end up with more snow in the immediate DC metro than in points to the west in Loudoun County.

Rain starting here in Arlington - a little windier.  Reports of lightning and thunder in Woodbridge - mostly rain for now...

Major Band of Activity Headed Into DC Area

A line of strong radar returns is moving through the Fredericksburg area and heading north towards us.  Reports with this line have included heavy rain changing to sleet and then to snow fairly rapidly.

This is an unusual situation where you'll probably be able to watch as the thermal profiles in the atmosphere change and support frozen precipitation.

To add to the fun, there are numerous lightning strikes associated with the line.

Shouldn't be long now before the activity starts around here.

Current Radar

You can see the precipitation in western Virginia and developing now to our south.  The low pressure center is clearly defined in the swirl that, at this moment, is located just along the border of Virginia and North Carolina.  I'm expecting snow to start falling - perhaps initially as a bit of rain which should rapidly change to snow - within the hour.

Models Continue to Show More Snow

The short-range models and radar trends continue to advocate more snow for our area and an unusually high probability of convection - thundersnow - this afternoon.

5 inches or more seems likely at this point with higher amounts out to the west.  Portions of Fairfax and Loudoun counties could see almost 10 inches.

The Winter Storm Warning has now been extended for all but the southern portion of Maryland.  The second round of precipitation is moving into the Charlottesville, VA area now.  The center of the upper-level energy seems to be in western North Carolina.

I worry that the worst of this will be right around rush hour and if you're reading this and you're in the DC/Baltimore Metro area (especially if you're to the north and west of the cities), you might consider leaving early.  The Federal Government just announced that they will close 2 hours early.

From the National Weather Service:

...RAPIDLY DETERIORATING CONDITIONS FOR THE MID TO LATE AFTERNOON
AND EVENING RUSH HOUR COMMUTE...

AFTER A BREAK IN PRECIPITATION THIS MORNING...ACCUMULATING SNOW
WILL OVERSPREAD THE AREA THIS AFTERNOON AND EVENING. A MIX OF RAIN
AND SNOW WILL QUICKLY CHANGE TO ALL SNOW DURING THE AFTERNOON AS
IT BECOMES HEAVY. THE PERIOD OF HEAVY SNOW WILL COINCIDE WITH THE
LATE AFTERNOON AND EVENING RUSH HOUR COMMUTE ACROSS MOST OF THE
REGION...INCLUDING THE WASHINGTON DC AND BALTIMORE METROPOLITAN
AREAS. CONDITIONS IN SOUTHERN MARYLAND WILL DETERIORATE AROUND
SUNSET AS PRECIPITATION CHANGES TO SNOW.

THE HEAVY SNOW WILL REDUCE VISIBILITIES BELOW A QUARTER MILE WITH
SNOWFALL ACCUMULATION RATES AT TIMES OF 2 INCHES PER HOUR. THIS
WILL PRODUCE DANGEROUS TRAVEL CONDITIONS DURING RUSH HOUR. SNOW
WILL MOVE OUT OF THE AREA BY MIDNIGHT.

12Z Models Coming In

If anything, the NAM is snowier for us.  Need to see the rest of them but the NAM really lays down some moisture over our region tonight.  This is down to what we call "nowcasting" which is observing the trends that are happening now as a basis for prediction rather than putting so much stock in the models.

Still - look at the NAM total precipitation for the storm.  It's got us in a mini-bullseye - almost 1.25 inches liquid equivalent.  All snow and a 10:1 ratio would give us over 12 inches but with the ratios and losing some already this morning to rain and slush, it's less than that.

But how much less?  Let's see the trends once the snow starts falling.

Forecast Pretty Much on Track - Rain Coming, Then Snow

So after a night when the data showed some uncertainty to final snow totals for today, it seems that the general idea of the forecast is holding pretty well.

This morning, many of us have a coating of slushy snow on the ground thanks to an early-morning burst that has changed in some spots to sleet and in most spots to rain or drizzle.  Expect this to continue for a while - probably with a total changeover to rain before the atmosphere begins to cool.

By the afternoon the reverse of the morning change begins.  Rain and sleet will change to snow from west to east and should start falling with intensity - especially as the afternoon goes on.  By rush hour, travel conditions could be pretty bad both with heavy snow falling and accumulations.

With the thermal profiles in the atmosphere and a changeover-based forecast, location is going to be everything in this storm.  As an example: the National Weather Service has Washington, DC under a Winter Storm Warning for 3-6 inches of snow but has Arlington County in a Winter Weather Advisory for 2-4 inches of snow.

But I expect some of these totals could change as the day goes on - especially once the snow starts seriously falling later this afternoon.

As far as timing:  it looks like the changeover will start during the mid-afternoon hours - I'd guess around 2 PM.  The main thrust of the storm should hit our area from around that hour until about 8 or 9 tonight.

2 PM Satellite Shot (GOES-13)

Shows our storm gathering strength along the Gulf coast and strong to severe thunderstorms moving through Florida:

WSW Extended to all Metro, Atmospheric Profiles Suggest Thundersnow Possible

Reviewing some of the data from this morning/early afternoon.  Because of the thermal profiles of the atmosphere and the amount of energy available with the storm and especially because the main impetus for our snow will be the upper-level energy which is what we call "cold-core" - there's a chance that we could see some convection tomorrow night and the exciting phenomenon of thundersnow.

The guidance (especially the NAM) is also trending earlier with the rain to snow changeover: more like mid-afternoon than "after sunset".

And the National Weather Service has extended the Winter Storm Warning to cover the bulk of the DC Metro area.

GFS Total Precipitation

A quick map:  GFS total precip for the storm.  We're well in the 1-inch range which is something along the lines of 7-9 inches of snow equivalent.  We'll lose some (.25?) to slop, some more (.25?) to melting when the changeover happens and that leaves around .50 - .75 of liquid equivalent or around 4-6 inches with the bad snow ratios I expect.  Usually there's a 10 to 1 rule (1 inch of liquid precipitation is 10 inches of snow).  If the snow is exceptionally dry (cold atmosphere) the ratios can go up to 15 or 20 to 1.  With the borderline atmosphere tomorrow, I'd expect less than 10 to 1.

Things Get MUCH More Interesting

Washington DC and points north and west are now under a Winter Storm Watch for tomorrow night through Thursday morning.  I imagine the rest of us will be under some kind of advisory eventually as well.

Latest models in with 4+ inches for our area - the average forecast is 6 and the extremes are 2-3" on the low side to over 10 on the high side.  I think 4-6 is a good first call.

The time of the changeover is crucial.  Looks like "early evening" for us here and if so, there will still be plenty of energy and moisture to work with.  Because of the preceding slop (rain/snow/sleet) we will probably lose a couple of inches to melting (snow doesn't stick to water) but the snow could fall rather heavily tomorrow night into the early hours of Thursday morning.

Still a lot of data to ponder.  Thermal profiles are still borderline and any little shift in track or timing could change the whole thing - one way or the other.

For now - a bit of snow in the morning, a bit of slop in the afternoon and some significant snow tomorrow night seems to be the trend.

Still watching the data come in - more to come.

Things Get More Interesting

The general forecast continues essentially on track with the thinking from yesterday: a period of snow to start tomorrow morning, mostly a rain/snow mix (or all rain) during the day Wednesday and a changeover back to snow tomorrow night as cold air gets pulled into the storm.

But...

Remember how the NAM yesterday showed a fairly good chunk of moisture passing through after the temps get cold enough for snow tomorrow night?  That idea has been picked up by many of the models and it looks like there could be some accumulation of snow tomorrow once the changeover takes place - if indeed the upper level energy has enough moisture to work with.

I'm checking out the 12Z models as they come in today. 

Just a heads up though: if this changeover takes place early enough and if enough cold air can hang around and if there's enough moisture associated with the upper level energy (if, if, if) - we could be looking at some heavy snow tomorrow night.  Some of the models are saying some ridiculous stuff right now.  I'm trying to sort it out.

More soon.

It's Been One of Those Winters

So I've held off posting until now in the hopes that, given time, the picture of what is to happen with this mid-week storm would clarify.  I hoped that the models would arrive at a consensus and that the biggest problem with the forecast: the absence of a strong cold air source during the start to middle of the storm -  would also present itself more clearly and allow me to make solid conclusions.

But - nothing like that has happened.  Nothing.

To have a good "Miller A" Nor'easter - the type of snowstorm that is likely to dump 6+ inches of snow (or more) on our area, you need a few ingredients:

- A persistent cold air source.  This is provided by a strong high pressure over New England of the sort we've had for the past few days.  The problem?  It's moving east.  The further east it moves the more the clockwise flow around the high stops funneling cold air from the north and begins to funnel warmer air from the east and southeast.  So this is something of a question right now.

- A surface low pressure system near or over the Gulf Coast.  This we have - no problem here.

- An upper-level low moving out of the Plains states and phasing with the surface low somewhere in the southeastern United States.  Most all of the models agree this will happen although there are variations with respect to timing.  All of the important models have the phase happening well before the storm enters our region.  This is a good thing.  You'll remember that the majority of the problems we've had with storms and forecasts this year have been due to the difficulty in predicting the exact time and location of the phase and that the phasing has happened almost overhead which has caused dryslot problems.  Not this time.

- Favorable storm track.  Too close to the area and you have mixing problems: sleet or even rain mixes in with the snow and keeps accumulations down.  Too far off the coast and you get missed by the juiciest part of the storm and that keeps snow totals down.  The models are still in disagreement about the final track except that all the important ones agree that the track will be somewhere east of here.  In this case especially - with the cold air and temperatures being borderline at best - a small change in storm track will probably mean a huge change in what we experience. 

So - where are we with this thing?

To begin with, the storm forecast positions are slowing down.  What was once a Tuesday night into Wednesday storm now looks like a Wednesday morning - Thursday morning storm.  This has been a trend for the last couple of days so I wouldn't be surprised to see things slow down even further as we get closer to the event.

Now lets look at some of the big players in the model world and where they stack up.

We'll begin with the forecast frame for 30 hours from now (36 hours from 12Z - the run time of the model) or about 7 PM EST Tuesday night.  Here are both the NAM and the GFS for comparison at the same time frame.  The NAM will be on the top, the GFS on the bottom.   Remember that you can click on any image in this blog to see the full-size version.

I want you to keep an eye on two things.  First, notice that the position of the low (NAM just south of the Florida Panhandle, GFS right over it) is pretty similar in each model as is the precipitation field.  Heavy rains are breaking out from Georgia into South Carolina and severe thunderstorms are on their way into Florida.  Clearly there is a lot of energy and moisture available to this storm. 

But notice, also, that blue line that runs from the Missouri/Arkansas area northeastward towards the mid Atlantic.  That's the 0-degree isotherm and it can be used to approximate where the precipitation would remain snow (north and west of that line) and where it might change to rain (south and east of that line).

Notice that the NAM is quite warm - forecasting our entire area to be above freezing even at this frame while the GFS is slightly colder and actually paints the 0-degree line right over our area. 

Onward to 42 hours - around 1 AM Wednesday morning.

The storm is strengthening and is moving to the northeast.  Heavy precipitation is falling in Florida and along the Georgia and Carolina coasts.  We still have no precipitation in either model though it's beginning to break out just to our south in the GFS version of events.

Again the low position is pretty similar in both models but the precipitation field is more inland on the NAM bringing heavier rains to interior Georgia and South Carolina.  For us, note the 0-degree line.  The NAM now has it right over us.  The GFS continues with a colder forecast showing the line now sagging south of us putting us in a regime where snow could be expected once the precipitation starts falling.

6 more hours and its 7 AM Wednesday morning:

In the NAM, precipitation is still not quite over us but in the GFS it has begun to do something in our area.  Again the 0-degree line is right overhead in both models making the precipitation type a very difficult call.  The storm positions are a little different now with the NAM showing the center around the GA-SC border on the coast and the GFS slightly faster: along the NC-SC border at the coast.

6 more hours and it's 1 PM Wednesday afternoon.

We have ourselves a storm.  Both models agree on that and both have a fairly powerful one located somewhere near the VA-NC coast.  However, the other thing both models have for us is precipitation type issues.  The NAM is really warm with the 0-degree line now bisecting Pennsylvania while the GFS has it closer to us but still to the north.  This would argue for at least a mix of snow and rain if not all rain.

To 7 PM Wednesday evening we go:

The storm is quite intense now and just to our east.  It's powerful enough that it's drawing its own cold air down with the counterclockwise flow around the low and the 0-degree line creeps southward again.  Significantly more precipitation exists over us in the GFS forecast when compared to the NAM at the point where this takes place.

6 hours later - 1 AM Thursday morning:

Here we see a bomb of a storm with the GFS center position slightly to the west of the NAM position.  However, the NAM keeps the precipitation going for us with as the upper level energy rotates through.  The 0-degree isotherm hovers right overhead on the NAM however.  The GFS is colder - the 0-degree line is to our south - but much drier with just a little leftover precipitation in our area.


So, as you can see, even with a reasonable consensus on track and strength, two of the biggest models are still quite different in terms of thermal and precipitation profiles.  Following either of these models strictly is probably a bad idea but the general idea in either case is for a day when snow may begin in the morning on Wednesday and change to or mix with various other types of precipitation including sleet and rain.  Towards the evening, precipitation would change back to snow and if that upper energy swings through like the NAM suggests, we could see snow falling through most of Wednesday night into early Thursday morning.

Thermal profiles are tough for models to forecast and a lot is probably going to depend on your locale.  Further north and west is the better chance for this to remain all snow however it's also a place where less precipitation may fall. 

Accumulations are difficult to predict as some of the potential could be lost to liquid precipitation instead of accumulating snow.

I should note that other models have a similar story but a couple of them hold onto the snow longer.  One thing is certain:  any snow that does fall will be wet and heavy.

This has been a difficult year for forecasting.  In every case it seems that one element has been missing: phasing, precipitation field or, in this case, source of cold air.  That makes for difficult forecasts and this is one of the toughest.

The afternoon models are running (as is the 12Z Euro).  If they contain any surprises, I'll post them.

By The Way

Long-range models are showing the first credible shot at a snowstorm around here since the Boxing Day Storm that just missed us for mid-next week (Wednesday-Thursday as it looks now).

Still a LONG way to go (and you remember how the Boxing Day storm waffled back and forth before giving us nothing) - so don't get too excited.

One at a time, I guess.  Let's wait for this one to pass before we get excited about next week.

Quick Update

Haven't had all that much time to look at the models but it seems like it's GFS vs NAM.  The GFS model suggests a more southerly track to the low across VA this evening which would give us some snow - mostly from midnight into the early morning hours tomorrow.  An inch to slightly more is probably all we can look for from this.

The NAM is even worse.  A more northerly track almost over us decreases the amount of moisture available, introduces the dry slot and makes atmospheric temperatures questionable with regards to the rain/snow line.

Based on the morning model runs, I'd say around an inch or two - but I'll take a look at the afternoon models as well.

Either way - shouldn't be a big deal and I don't expect it will start until closer to midnight tonight - maybe even later.

Storm Watch: Thursday Night - Friday

First of all:  sorry about not posting for this latest storm.  I was out of town with no Internet connection.  This was an interesting storm and at some point on Monday night I think I saw every possible type of winter precipitation possible:  snow, sleet, graupel and freezing rain.

Before we get into the possible storm Thursday night into Friday, I thought it might be good to talk about the different types of precipitation and how they form.

Snow - this is the easy one.  Temperatures in the clouds are cold enough to produce ice crystals that we recognize as snow.  As it falls from the clouds, the atmosphere is sufficiently cold all the way from cloud base to the ground (or almost the ground) for the ice crystals to remain intact.

Variants:

- "wet" snow which we sometimes see when the air between the cloud and the ground is hovering around freezing.  Some melting takes place which makes the snow "sticky".  You know this because snow with a higher water content makes better snowballs than dry snow which we get when it's very cold.  The snow flakes collide with each other and stick together making for the streaky, sometimes large flakes we see during "wet" snow.

- "graupel" which is a phenomenon that almost always happens when the precipitation type is transitioning (e.g. from snow to sleet) and is caused by snowflakes hitting droplets on the way down that are colder than 32 degrees but remains in a liquid state.  This is called supercooling.  The snowflake develops a coating of ice which continues accumulating (accreting) around the ice crystal until it resembles a small ball rather than a snowflake.  This is not the same as sleet.

Sleet - as snow falls from a cloud into air that is too warm to support ice, the snowflake melts into a water droplet.  At some point in its travel toward the ground the air cools again for enough distance to refreeze the droplet of water into an ice ball which we recognize as sleet.

Freezing Rain - as snow falls from a cloud into air that is too warm to support ice, the snowflake melts into a water droplet.  The temperature near the surface is at or below freezing, however (meteorologists refer to this as the cold air being "shallow").  Therefore the liquid water freezes on contact with anything on the surface that is below freezing especially cars and trees.  Over time, the ice accumulates (we say "accretes") and produces that glazed look like the one we saw this morning.


Ok - class time is over!  On to the possible Thursday storm.

As has become the norm this year, the models disagree on the track - but moreso on the intensity - of this storm.  We'll look at the NAM and the GFS for this discussion.

Oh, and a quick note: you can click on any of the images in the blog and get see the full-sized version.  This can help if you're having trouble seeing details in the smaller maps.

We'll start about 54 hours from now:  Thursday morning.

Here's the NAM forecast for that time:

You can see our storm in the mid-Mississippi Valley.  The NAM already has a strong low - especially when compared to the GFS at the same time which forecasts a weaker and more "elongated" area of precipitation:

Later that night (12 hours later) the NAM shows the storm really getting its act together.  Precipitation extends from the Northeast into the southern states - although not quite in our area yet.  You can see a storm center forming as the precipitation is forecast to be heavier in Kentucky and Tennessee at this point.

The GFS is not quite as excited about the storm although it does "close off" the low and shows its position in southwest Virginia at this forecast time.  You can tell the intensity isn't as high mostly because the precipitation field is less impressive - especially near the center.  However, unlike the NAM, the GFS has light precipitation breaking out over our area by this time. 

Moving ahead 6 hours, the NAM has a pretty nice system starting its turn up the coast.  We are in a region of moderate precipitation and this would argue for a few inches of snow overnight Thursday into Friday morning.

The GFS has a similar solution for this time frame - the storm is wrapping itself up over the Atlantic - but the overall precipitation field is somewhat lighter especially from our area northeastward into Philadelphia.

So - the timing of this storm between the two models is fairly close.  I think we can expect any snow that does fall to happen sometime after sunset on Thursday night.  The models would argue more towards midnight.

We'll need another few models runs before talking seriously about possible accumulations and snow totals.  For now, I'd say the outlook is favorable for snow on Thursday night into Friday morning and there's a 50/50 chance that the snow would be "significant" - i.e. 2 inches or more.

This isn't a big storm for us by any stretch - no Snowpocalypse this time.  But it could be a nice, quick snowfall of a few inches if things come together over the next day or two.

Stay tuned...

18Z NAM Confirms Eastward Shift, Less Snow

The eastward shift continues bringing our area into the one-inch range for snow fall.  Pretty disappointing but this was never going to be a large snowstorm.

So enjoy what does fall.  Here's the NAM forecast total precipitation for those interested:

Looks like we're in the 0.10 inch liquid range which translates to a 1-1.5 inch snowfall.

Point to the Dryslot

Check out this radar.  Can you see the dryslot?

Inland low is pretty strong but we're getting caught in the dryslot between it and the storm off the North Carolina coast.

Let's see if we can get more than an inch out of this thing.

The snow drought this season continues.  Guess we are paying for last year!

Stupid Dryslot

It's going to be close around here.  The shortest-term models we have are slightly east of the NAM positions yesterday which isn't that good for us.  I think the gradient of snow shifts east a bit and we're talking lower totals of 1-3 inches here in the DC Metro with less to the west and more up around Baltimore.  I felt pretty confident about 2 inches yesterday and I still think so.

Still reviewing data this morning.  There's even less consensus as you get to small-scale modeling and a lot of it will have to be figured out once the snow starts.

You can see the eastward shift in the NAM in this precipitation total forecast map (compare to the one in the post just below this).

18Z NAM Precipitation

Here's the NAM precipitation forecast. We'll be running right around 10-12:1 ratios so multiply by the liquid precipitation in the map above by 10 or so and you get forecast snowfall in inches.

As you can see, this fits nicely with the forecast so I'll leave things how they are for now.

More model guidance coming - especially later tonight. Those will be important runs.

18Z NAM

Here's the NAM from the 18Z run (earlier maps were from the 12Z run).  Usually the 18Z models are taken with a grain of salt and usually just used to identify trends in the forecast or inconsistencies.

The NAM is pretty consistent with its slower, stronger philosophy although this low position is about the same but the precipitation shield is further west than that of the 12Z run at the same forecast time.

Here we see the phase occurring and some light to moderate snowfall over our area at about 1 AM on Wednesday.

No change to forecast at this point.  No significant model trends to suggest that the forecast is off track.

At This Point, a Light Event

Elements are coming together for our snow event tomorrow. 

This will be a storm track and low pressure interaction pattern known as Miller B, the hallmark of which is two low pressure centers interacting - one from the west over land and one from the southeast.

Our first two big storms (December 19-20 and February 5-6) last season were both Miller B storms - so we know they can produce large amounts of snow.  However, it is the timing and location of the interaction between the two lows and their original intensity (especially of the inland storm) that are crucial components.

Last year, in both cases, the low from the west passed close to our area before transferring its energy to the developing offshore low well to our southeast.  As the storm moved up the coast this infusion of energy and time over the Gulf Stream caused the "bombing out effect" that buried our area in snowfall.

This time will be a bit different.

So let's look at some near-term model maps to see where this transfer might take place.  This close to the storm, I prefer to look at the NAM which is usually a very good short term model .  However, I should note that for this storm - at least so far - the NAM is an outlier.  It is slower with timing overall and stronger with the inland low.  Making it even more suspect is the fact that, generally, there IS consensus between the Euro, GFS and UKMET models and all of them are a little faster and a little weaker with the storm for this area.

However, the NAM essentially agrees with the other models for the time period leading up to early morning Wednesday.  So, I'll show you the NAM because it represents the strong side of the guidance available - but my actual forecast rationale will be like a buffet:  an entree of Euro/GFS/UKMET and a NAM dessert.

That darker-green area in Illinois is our inland storm moving off to the northeast.  Our coastal storm is developing off the Carolinas.  This would be about 15 UTC or around 10 AM tomorrow.  As you can see, not much going on around here according to the NAM.  I would agree.  Maybe some flurries or light snow showers but I think most of the action will be later in the afternoon.  NAM is a little slower than the other models but not significantly.

Six hours later and the NAM is forecasting the edge of the steadier light snow in our area.  This is late afternoon tomorrow - around 4 PM.  The low to the west is a bit better organized as is the one off the coast.  These will continue to function semi-independently for another few hours before the energy transfer and phase begin to take place.  I expect the light snow to begin falling steadily sometime tomorrow mid to late afternoon.  This rationale is supported by the other models.

Here's where the models disagree - or really where most of the other models start looking at the NAM with a suspicious glare.

In the NAM map above (9 hours later - around 1 AM Wednesday morning) you can see the two low pressure areas characteristic of a classic Miller B presentation - one in western Pennsylvania and one just off the Delmarva coast.  This is where the NAM forecasts the transfer and phase to really get going.  The NAM is slower to make this transfer and, because it's also forecasting a stronger inland low, our region experiences a more extended period of moderate snow before the transfer is complete and the storm off the coast takes over.

Compare the NAM to the GFS forecast for the same hour:

The GFS/Euro/UKMET gang advocate a weaker inland low (it's not even rendered with an "L" in western PA unlike the NAM), a quicker energy transfer and a faster end to the snow in our area.

One other thing to worry about: when the 850 MB low passes to the north of our area in a Miller B scenario, there is a high likelihood of being dryslotted around here.  We essentially end up in a "hole" between the storms as the phase takes place. Check out the precipitation totals by the time the storm is off the coast of Maine Wednesday night:

 See the hole in the precipitation field over our region and to the west and south?  Dryslotting.

It's worth noting that even the NAM - on the strong side of the guidance - forecasts the coastal storm takeover to be too far north (off the Delmarva coast) to affect us much.

Here's what I expect:


Based on the previously-discussed issues and scenarios and a few glances at upper level data which suggests that there will be a fair amount of precipitation ahead of that coastal low even before the Miller B transfer begins, I would say a chance of flurries or light snow beginning late morning Tuesday but especially by the afternoon.  By the late afternoon/early evening I think the light snow becomes steadier and and more widespread and continues into the night.  There's a good chance that the snow becomes moderate for a time Tuesday night (especially the further east you are) before it ends from west to east overnight on Tuesday into early Wednesday morning.

This is another storm where the lightest amounts will fall to the west (except once you get over the mountains) and the heaviest amounts to the east (except possibly along the coast and in southern Maryland where mixing of snow with sleet and rain could occur).


Generally in the DC Metro I'd expect 2-4 inches in a southwest (low end) to northeast (high end) gradient up to about Baltimore.  North and east of Baltimore things could get interesting quickly with 5+ inch amounts.  Similarly, towards the coast amounts could reach 5+ inches.


By Wednesday morning this is all out of here and in the wake we get more of the same so far this winter: cold and windy into the weekend.


What could change things?
 
- The NAM could be the start of a trend rather than an outlier.  Slower and stronger means more snow for us.

- The phase could happen earlier and further to the south.  The more time we spend under the influence of the coastal low, the better.  If the phase has already happened at a time when precipitation is still falling from the coastal low, we'll get more snow.

- The UKMET model could be right.  It represents the weakest edge of the guidance.  The UKMET solution is for a storm further out to sea.  We get even less of an impact from the coastal low than is already forecast by most of the other models and, as a result, less overall snow.

Why is forecasting winter weather so complicated and so often inaccurate?


I could write 6 posts about this and not be done but there are a number of reasons for it:



- The guidance is, for the most part, forecasting massive-scale events and we try to translate them to the small-scale.  Take the storm after Christmas, for example.  The distance between Ocean City and Washington, DC is about 100 miles (as the crow flies).  You'll remember that we received a dusting of snow from that storm here in the DC area.  Ocean City?  14 inches.  We were around half that distance away from 6+ inches.  50 lousy miles in a storm hundreds of miles in diameter.  


- The psychology of snow around here.  Let's take last season's largest snowfall for example.  Midweek forecasts that week were for between 24-32 inches of snow.  Some had higher amounts up to three feet.  By the time the snow started falling, forecasts were trimmed back by about 4 inches to allow for the reduced snowfall during the Miller B transfer.  Do you think anybody cared that their snow forecast was now 20-27 inches instead of 24-32?  


But now let's say I'm 4 inches off in my forecast for tomorrow.  That's the difference between snow and no snow at all. 


- Storm track is almost the whole ballgame.  In our case for tomorrow it's actually TWO storms we have to track.  What if the one from the west actually gets closer to us?  What if the track of the storm from the south is too far east?  Everything changes.


- Storm intensity is the rest of the ballgame.  Our storms for tomorrow are just getting organized.  How strong will they be?  You can see the models already disagree about that with the NAM having a stronger inland storm than the rest of the models.


- Small-scale features.  Things like "mesoscale banding" can cause big differences in snow totals.  Check out this radar image from the December 26th storm in New Jersey/New York.  You can see how a forecast of 12-14 inches ends up being 20+.  Those yellow bands of snow are almost impossible to forecast in duration and extent.  But if you get under one, it's like the snowpocalypse all over again.

Hey - I'm not making excuses - sometimes winter weather forecasts around here can be wretchedly bad.  Sometimes meteorologists ignore their better judgement and their intuition in favor of computers.  But it's a crazy science and even in this age of incredible technology advancement, there's still a lot of guessing to be done.

I'll be watching the afternoon model runs.  If anything changes, I'll make a quick post to let you know.

Complex Pattern for Monday Night Through Tuesday

I didn't post for the last couple of snowfalls because they were expected to be minor and turned out to be.  Just some snow showers on Thursday night into Friday morning and a few more on Saturday morning provided the dusting that the DC metro area received.

A much more complex situation is in the offing for a time period beginning Monday night through Tuesday night - possibly lingering a bit longer than that.  Much is still undetermined but there's a likelihood of accumulating snow here in the DC metro.  How much and where are both things that need a few more model runs to iron out.

As has become customary this winter, the models are all over the place and there's not much consensus beyond a good chance of snow on Tuesday.  This begins with a low pressure area forming in southeastern Texas and moving along the Gulf coast on Sunday night.  I expect this low to cross off the coast of the southeastern U.S. by Monday - probably around central Georgia.

From the north comes the polar jet and a disturbance rides along with it from the plains into the Ohio Valley.  This is expected to be pretty strong and it, alone, would likely produce some snowfall around here as it tracks across our region Monday night.

Here's where the models start to diverge.

Scenario 1, at this time the more likely, has the northern energy missing phase with the southern energy until the storm is well northeast of us.  This scenario means a weak coastal low, a more dominant inland low and light accumulation amounts depending on the strength of the inland disturbance and the proximity of the coastal low - weak as it might be - as it moves northeastward into the Atlantic.

Additionally, the models hint a secondary low forming in southwest Virginia and moving northeastward which would continue our snow chances through Tuesday afternoon and evening.

Even with the extended period of snow, it seems like a light event.

Scenario 2 would have the northern energy phasing with the southern energy which would strengthen the coastal storm and also having the effect of "holding" the storm closer to the coast as it moves northward.  This scenario is a classic nor'easter scenario - but, as in previous situations, the timing of the phase is everything here.

Right now the model consensus is for a later phase but when and where is very uncertain.

An early call would be a chance of snow Monday night with better chances Tuesday and Tuesday afternoon possibly extending through Tuesday night.  I can say that there's a good chance this will be "accumulating" snow which would mean, for me, "2 inches or more" but lots of details still have to be ironed out before we can have serious discussion about totals.

There's also the chance that storm track gives us mixing issues as temperatures will be borderline (low to mid 30's) during the balance of the day Tuesday.

I'll be watching the model runs pretty carefully for the next few days.  I'll start posting some maps in my next post.

Hopefully this one decides what it wants to do sometime before the day of the event!