Significant Winter Storm Expected Tomorrow

All the ingredients are coming together for a significant snowfall in the DC area tomorrow.

For most of the day, I've been trying to figure out how this forecast could end up completely wrong.  We do live in the DC area, after all, and if there was ever a place where snowfall forecasts go to die, it's here.

But I can't think of one.

The cold air will be in place, the upper-level energy is where it should be right now and the front is heading this way which will knock a good 25-30 degrees off our high temperatures today by the time the snow starts.

I suppose the only thing that could go wrong is if the storm gets its act together later than forecast by the models which would mean a less intense storm as it passes by.  At the moment, this seems unlikely.

The GFS model shows the storm as it leaves the coast tomorrow and rapidly intensifies over the Atlantic.  The trough of low pressure is expected to switch from a positive to a negative tilt which almost always means a more intense and slower-moving storm.

As the storm starts to influence our region, it will encounter the sharply-colder air and that should improve the accumulation numbers relative to the amount of precipitation produced by the storm.  The rule-of-thumb ratio is 10 inches of snow per 1 inch of liquid precipitation.  I think we do better than that tomorrow.

This will produce fluffy snow that will blow around in the strong gusty winds and that will be easier to shovel.  It will, however, be complete crap for making snowballs or snowmen.

The snow could be quite heavy in the morning - especially if banding sets up (as it should).  Forecasting the precise location of the banding is next to impossible so that's the usual reason for the range of forecast numbers.  In this case, the National Weather Service is predicting 4-6" widespread throughout the DC area.

Another metric to consider is vertical velocity - a measure of the movement of the air.  In the case of storms, higher vertical velocities imply more intense storms and heavier precipitation. 

You can see from the graphic above that vertical velocities should be fairly high during the storm, suggestive of moderate to heavy precipitation.

Some snow amount considerations:

The NAM, which is a pretty good short-range model, shows the general thinking of most mets in the DC area - albeit with slightly higher values.  The map above shows 0.5-0.6 total liquid precipitation which, converted to snowfall, produces numbers in the 6-8 inch range. 

The GFS forecasts a similar range, maybe even a bit higher than the NAM - more in the 7-10 inch range for the DC area.

I feel that the NWS mets are being a bit conservative.  It seems to me that there's potential for more than 6 inches of snow with this storm.  Not crippling by any means but certainly more than we've seen around here in a while.

The evening models will be coming soon and I'll check those out as well.  The only impacts to this forecast that I can see are the time of storm intensification which is something the models usually handle pretty well.  My confidence in this forecast is, therefore, rather high - always considering, of course, that it's a snow forecast and it's for the DC area.

Increased Snow Chances / Amounts for Tomorrow

Well.  That was sudden.

As a frontal boundary approaches our area, it has been clear for a few days that the 21st would be cold, windy and that there was a chance that snow would overspread the area due to a small wave of low pressure riding along the front.

Our big snow producers typically come from the Gulf of Mexico and ride up the coast.  It takes quite a potent little low to produce a lot of precipitation if it has terrestrial origins. 

For the last couple of days, it looked like this would be a minor event.  An "inch-or-less" type of event.  Then came yesterday evening's model run.

The models run at 18z (1 PM locally) but since the upper air data from the day isn't available yet, they basically run from "inertia" or, more accurately, a "boatload of physics calculations" using the morning data.  Thus, they are used to get a general idea of the forecast.  They are rarely used for accuracy. 

In any case... the 12Z GFS which is based on upper air data had started to suggest the idea of a stronger and just as importantly, slower moving system.  The 18Z NAM then spit this clown map out and the meteorologists of the mid-Atlantic woke up from their nap and said, "hm!":

This shows a narrow area of significant snowfall in our area and especially points south and east.  Some of those totals are quite high but it's less about accuracy in the 18Z run and more about the overall suggestion:  a stronger, slower moving storm.

Overall, the storm has the look of a "clipper" system that then moves along the coast and intensifies rapidly. 

Overnight, the trend in the models continues to be stronger - except for the Euro.  Last night's 00Z Euro run showed this:

which still shows snow but in lighter amounts.

One this is certain:  it will be cold.  Precipitation type doesn't enter the equation like usual.  I expect high temperatures in the 20's which will result in high snow rations - perhaps as high as 15:1.  The map above shows 10:1 ratios so even taking 1.5 times the above snow produces a forecast for more snow than we've seen around here in 2-3 years.

This morning's data continues to come in and it all points to an increasingly significant storm for our area.  The RGEM, for example, calls for 7-10 inches:

The morning GFS is also in that range:

There is a Winter Storm Watch for our area and with the models trending stronger, this looks like it's trending towards a major event for our region.  For now, I'd say expect at least 4 inches of snow but the chances of more (maybe in the 8-10 inch range) are not out of the question.

There's unusually little variance with this storm as far as forecasting it goes.  I don't see any indications that it could end up producing nothing at all - but with the way winter forecasts go around here, I'm going to keep watching.

I'll update again later.

Forecast on Track, Winter Storm Warnings Are Up

The models have come into better agreement for this quick but potent storm that should be starting in a few hours.  I'd expect snow to begin around daybreak and really pick up in intensity between about 7 AM and 10 AM.  3-5" is forecast by the National Weather Service and that seems reasonable to me. 

Banding is going to set up as I mentioned in a previous post and that could create locally higher snowfall amounts.  Thundersnow is not out of the question!

There is still a chance that most of the precipitation gets pushed south of our area which would result in less snow overall but that doesn't seem likely at this point.

Round 2

For a few days now, both the GFS and the NAM (which is a good short-term model) have been suggesting a reinforcing shot of cold air would come through our region overnight and into the day tomorrow along with enhanced upper level dynamics.  This combination was indicative of a second burst of snow that would begin early tomorrow morning and continue into the afternoon.

It now looks more likely that this will happen.

The only thing that gives pause is the Euro model which has insisted for a while now that any precipitation will be pushed south of our region.

The primary difficulty in this forecast is less what type of precipitation will fall (as it was in the last storm) but how much.  A lot of the snow totals could largely depend on small localized banding events.  If one of those bands forms overhead, it could mean the difference between 1-3 inches and 3-6 inches of snow.

One thing we have going for us is the push of cold air aloft which should mean enhanced liquid-to-snow ratios. 

Here's the GFS snow total map for this storm:

you can see the bullseye in the Baltimore area but we in the DC metro are still doing pretty well - in the 3-6 inch range, generally.  When I look at the projected temperatures at the 850 mb level:

I see a lot of -4 and -5 degree readings overhead which suggest to me a liquid-to-snow ratio a little higher than the standard 10:1.  Maybe as much as 15:1.  This means that whatever falls will accumulate and quickly.

The DC metro is under a Winter Storm Watch for tomorrow morning for 3-5+ inches of snow.  I think this is reasonable based on the GFS and NAM model forecast.  I'm going to wait to see the Euro before I get too excited but, in summary, prepare for more snow starting early tomorrow morning and accumulating quickly.

Snow, Then Ice, Then Rain and Some Combination of Everything in Between

I thought about just posting that headline and calling it a day because that's pretty much what the forecast boils down to.

There's a good chance that we'll see every possible winter precipitation type tomorrow at some point.  As to exactly when the changeover occurs and how much precipitation will be falling at what times - that's where the complexity of the forecast really lies.

A couple of things:

- The models have been trending towards less overall moisture for this storm but have also been shifting the axis of heaviest precipitation right over our region.

- The NAM has a wet bias so I wouldn't get too excited about snow amount forecasts and instead concentrate on the area modeled for maximum precipitation.

- Models in general are pretty bad at forecasting overrunning events like this one (cold air at surface hemmed in by the mountains, warm moist air aloft).


Here's the NAM clown map from this morning:

again, focus less on the amounts for now and more on that stripe of heavier precipitation that goes right over us.  This modeled local maximum in precipitation has been shifting southward with consecutive runs of the models and appears to be in pretty good position to give us some precipitation.

Now, what kind will fall?

My best guess is that the precipitation will start mid-morning in the DC area as snow and it will snow for the rest of the morning into the afternoon.  The changeover will begin from east to west, first as sleet (which could be heavy for a bit), then to freezing rain (liquid precipitation that freezes on contact with the surface) then to plain rain as temperatures even at the surface rise above freezing.

I don't think temperatures get much higher than 32 tomorrow or so - maybe less if enough snow and sleet fall in the morning.

By Monday morning, it's all rain and then we'll have to watch and see what happens.  Some models suggest a continuation of the moisture flow into our region and with the cold air firmly in place by Tuesday, we could have another shot at some snow then.

This graphic (called a meteogram, tells some of the story):

Since this is built off of the NAM run, we should probably take the precipitation totals with a grain or two of salt (if this verified, for instance, it suggests 6-8 inches of snow by Tuesday night).  But check out the precipitation types and the difficulty the model has in resolving them.  Blue bars with an "S" caption is snow, red bars with a "P" caption is sleet, orange bars with a "Z" caption is freezing rain and the green bars with an "R" caption is plain rain.

You can see the secondary burst of snow Tuesday morning over there on the right of the graph.

At this point, I think a Winter Storm Warning will be needed for points north and west of DC and a Winter Weather Advisory for DC and points east. 

This should be the type of storm which will keep you inside on a Sunday and, since the Redskins are 3-9 and playing a 9-3 team, should keep attendance at FedEx field on Sunday to about a dozen of Daniel Snyder's family members.

More updates if the afternoon model runs show anything different.

Model Runs Trending a Bit Colder - Still a Mess of a Forecast for DC Area

As Paul pointed out in his comment on my last post, the models have been trending colder overall with this storm - especially with regards to the depth of the cold air and the time it will take the warm air to erode the dome.

Other forecast elements still in place:  high pressure funnels cold air southward starting Friday night.  Saturday will be noticeably colder.  The cold air is in place as warmer air starts to overrun early Sunday morning.

At this point, it looks like a shot of snow followed by sleet, possibly significant freezing rain before changing to all rain late Sunday night into Monday.

There are still a lot of questions about timing and how much front-end precipitation will fall but it seems somewhat more likely today that frozen precipitation will fall on Sunday and we could have some icing problems before things changeover to rain.

Here's this morning's Euro snowfall forecast:

which still suggests a few inches of snow for the DC area.

More models are coming in - I'll post later today.

First Significant DC Metro Winter Precipitation Possible on Saturday-Sunday

Humm.  Good to be awake again here on the PV blog! 

That was a strange offseason.  No real summer to speak of and then a cold autumn which seems to want to transition to winter as soon as possible.

Fine then.  Let's begin.

We've had our share of cold weather but precious little wintry precipitation.  I'm loathe to credit the seventeen flakes of snow that fell here just before Thanksgiving as "wintry precipitation" but either way, it certainly wasn't significant.

Saturday night into Sunday could be a different story.

Let's set up the scenario and I'll chat about the issues and complexity of the forecast.

First off, we have a couple of storms.  The first one goes through here Thursday night through Friday as a front passes through the area and brings colder air.  Could it end as a bit of snow?  It could.  But it won't amount to much.  The majority of what falls around here will be rain so, for the purposes of this blog, we'll ignore it, no matter what the "incredibly thoughtful and clever" folks at The Weather Channel have decided to call it. 

And by "incredibly thoughtful and clever" I mean, "galactically moronic lunatics desperate for ratings".

What the hell is a "Cleon", anyway?

Onward.  The second storm (I can think of a compound "d" word the Weather Channel can use if it needs a suggestion) is the more interesting one around here.

The synoptic situation is this: 

A strong high pressure will be moving east over New England but will have enough influence on our region to pump cold air southward.  This cold air will funnel its way into our area - channeled against the mountains like a quarter pipe.  This is a classic setup for what we call "cold air damming".  Essentially, since cold air sinks, the air just sits in a pool bounded by the mountains to the west and the warmer ocean to the east. 

Now here comes the storm.  The main low pressure center is forecast to take a path well to the west of here through the upper Midwest and into the Great Lakes region.  This is normally a horrible setup for us if we want any kind of frozen precipitation.  The counterclockwise flow around the low brings air from the south which is quite moist but also quite warm.

However.  With the cold air settled in a pool, the warm air won't dislodge it right away.  In fact, initially it will ride over top of the cold dome, condensing along the way.  If the cold air is deep enough, we'll see snow or sleet for a time Saturday night into Sunday.  As the warm air erodes the cold air below it, any snow will change to sleet and any sleet will change to rain. 

Here at the surface, the temperatures might stay cold enough for some (most?) of Sunday that any liquid precipitation will freeze on contact with the surface.  That's the dreaded freezing rain which would leave a glaze on things.  No fun.

At some point on Sunday evening, the warm air will begin to win and by Monday, plain rain will be falling with much warmer temperatures as the last of the cold air is scoured out.

QUESTIONS

- How deep will the cold air be?
- How long will the cold air last?
- How much precipitation will fall while the cold air is still around?

These will make all the difference between a forecast that is just for a bit of sleet followed by rain and a forecast that includes every possible frozen precipitation type followed by rain later Sunday night and Monday.

Most of these details have yet to be resolved.


MAPS

Here's this morning's Euro model output for total snowfall from this storm:

as you can see, the DC metro falls between the 3-6 inch contour.  This represents what I believe to be fairly ideal conditions.  Deep cold air, lots of precipitation on the front end of the storm.

I'd say, at this point, there's a 20% chance of verifying in the DC metro.  A greater chance as you move north and west of the city.

I'm concerned about how much cold air there will be and how much precipitation will be out on the front-end of this storm.  But I expect we'll see some frozen precipitation on Saturday night into Sunday.  How much and for how long remains to be seen.

I'll update when new information comes in.

Welcome back!