* Winter storm warning Tuesday for entire region *

3:30 p.m. update: You may wonder: It’s 58 degrees outside, how is it going to snow and stick tomorrow? Answer: An arctic front in advancing south and east (coming through this evening), and temperatures will steadily drop to below freezing by dawn. In the upper Midwest, readings are currently in the single digits and below zero.

If temperatures end up a little warmer than forecast or are a bit slow to cool, we might “lose” a little accumulation to melting early on in the storm, but we’ve factored that into our forecast and feel confident that enough cold air and enough moisture will arrive to overcome today’s warmth.



Weather map from 2:30 p.m. shows cold front northwest of D.C., but headed southeast (National Weather Service)

The latest NAM (18z) and SREF (15z) models support our forecast.

From 12:46 p.m.: Accumulating snow is likely on Tuesday as a disturbance from the north (or “clipper”) dives to our south and then intensifies as it tracks across North Carolina to the coast.

Periods of moderate to heavy snow are likely as temperatures gradually fall through the 20s. Winds will increase in the afternoon so some blowing and drifting snow is also possible by late in the day. A winter storm warning has smartly been issued by the National Weather Service.

Our initial accumulation forecast is for 4-7 inches across the region with a 15 percent chance of more than that, and a 15 percent chance of less.



Capital Weather Gang snowfall forecast first call, issued 12:45 p.m. Monday.

Last week, we discussed that this pattern was capable of producing moderate snowstorms and on a few occasions (1995 and 1961) had yielded snowfalls in excess of 4 inches. Tomorrow’s storm appears to be morphing into a similar magnitude storm.

This is one of those rare cases where there are no precipitation type questions as the rain-snow line is expected to be in southern Virginia when the snow develops across the area. Therefore, we are unusually confident that a moderate to high impact snowstorm is on tap for tomorrow.

What is the timeline for the storm?

Snow is expected to start across the western portions of the area between 6 and 9 a.m. (Frederick, northern Fauquier and Loudoun counties), the immediate metro region between 7 and 10 a.m. (within one county radius of D.C.), and the eastern suburbs between 8 and 11 a.m (counties along the western shore of the Chesapeake Bay). The snow should continue through the day ending in the 7 to 11 p.m. window, west to east. The heaviest snow is expected from the mid-morning through the afternoon hours. While the onset of the snow may affect the morning rush, especially in western areas, the bigger problems are likely for the afternoon and evening rush.

Impacts

Roads are expected to rapidly become snow-covered during the day. The cold temperatures pretty much assure that there will be accumulations on the roads and they will be become slick. Complicating the situation will be reduced visibilities due to periods of heavy snow and possible blowing snow late in the day. School cancellations are likely. Flight cancellations or delays are also likely during the day tomorrow.

We will post SchoolCast and FedCast later this afternoon

– – – Some of the discussion that follows is technical – but most readers should be able follow the gist of it. Feel free to ask questions in the comment area below – – –

Why are you so confident in accumulating snow?

All of the main computer models now are predicting enough moisture ( 0.3-0.5 inches of liquid equivalent precipitation across the region) to produce over 4″ of snow given the cold temperatures and fluffy nature of the expected flakes. This morning’s GFS and NAM models both boosted their snow forecasts and are predicting a very favorable storm evolution for maximizing snow with this type of system.

This morning’s SREF models simulations (shown below) for the region were unanimous in forecasting accumulating snow. Each line on the plot below is a set of simulations for Reagan National Airport. The steeper the slope of the lines, the heavier the snow is falling during that time interval. All the members are predicting that the precipitation will fall as snow (with melted liquid amounts varying between about 0.20 to 0.80”).

The average liquid output of the various simulations of 0.46″ was about the same as the amounts being forecast by this morning’s NAM and GFS models. Given the normal snow to liquid ratio of around 10 to 1, 0.46” of water would equal 4 or 5 inches of snow. However, the ratios with this storm are more likely to be around 15 to 1. Therefore, all the models suggest that Reagan National would break there two inch snowstorm drought and most would suggest snow in the 4 to 7 inch range.

Another reason for thinking this will be a pretty strong storm is the evolution and track of the upper level disturbance. The track is very similar to the one that occurred during the January 2 storm that generally produced 2-5 inches across the area. This storm’s upper level system is actually a little stronger than that one. This one also has the added advantage of having cold enough temperatures so that all the precipitation should fall as snow.

An old forecast rule back when models were not as good as they are now is: really watch storms closely when the spin (vorticity) center tracks across Tennessee and then starts taking on a tilt where the base of the trough (dip in the jet stream) gets a little east of the top end of the trough. That negative tilt promotes strong upper level divergence aloft usually leads to storms developing strongly along the Carolina coast. Whenever you have mass evacuating at the top of the atmosphere, it is almost like sucking on a straw. The air below the upper level divergence is forced to rise strongly which leads to the development of heavy snow on the north side of the upper disturbance. The NAM forecast of upper level system is shown below.

This morning’s GFS was almost identical. Its evolution helps explain why the low strengthens as it is tracking to the coast and also why today’s GFS and NAM are now predicting snow totals in the 4 to 7 inch range.

What about snow to liquid ratios?

The snow from this storm should be fluffy because of the cold temperatures but also because of the temperatures at which the snow is being formed. Snowflakes with long filament-like branches (dendrites) tend to have more spaces to trap air than snowflakes having a plate-like structure so accumulations from aggregates made out of dendrites tend to have a higher fluff factor. Dendrites form at temperatures around -12 to -16 C and such temperatures are forecast in the layer snow would form Tuesday.

The sounding or temperature profile (see below) for Reagan National at 1 p.m. tomorrow shows the red (temperature) and green (dew point) lines almost on top of each other in the snow forming layer. The air mass is saturated so snow crystals should be forming. The dendritic growth zone is sandwiched between the two blue lines – see below.

Note how a large portion of the saturated part of the sounding is located within those two blue lines. The sounding suggests snow to liquid ratios are more likely to be in the 15-1 or even 20 to 1. Therefore, our chances of this storm’s snow totals reaching or exceeding 4 inches are high.

So what is the bottom line and how could forecast be wrong?

This is one of those storms where we’re more confident than normal about getting accumulating snow. Our best bet is that the storm ends up delivering moderate to heavy snow producing accumulations in the 4 to 7 inch range across the area. However, given the likelihood of high snow to liquid ratios and as a similar storm from the past produced as much as 9 inches, there is some “boom” potential – for snow to exceed 4 to 7 inches by a couple inches. Without moisture from the Gulf of Mexico, much more than that is unlikely.

Less snow than forecast (a bust) could occur if the storm does not develop as quickly as forecast, keeping the heavier precipitation a little farther south and east than we are expecting. That would still probably lead to 2-4 inches across the area. Based on this morning’s runs, we think that probability of such a bust is relatively low but we do live in Washington. We are not gamblers but would bet that the (record long) two inch snowstorm drought in D.C. (dating back to January 26, 2011) will come to an end tomorrow.