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.