Snow Thursday night? Maybe. But the bigger deal is likely two shots of bitter cold

December 31, 2013

We’re still watching the chance of snow Thursday night and probably some rain Sunday night into Monday. But just like that we’ve buried the lead, because the much bigger deal is likely to be really cold temperatures Friday into early Saturday, and potentially crazy cold temperatures midweek next week. These outbreaks will be the coldest we’ve seen this season, and possibly the coldest in a number of years.

That’s the short story. Below is a detailed look…

Thursday-Friday storm

cwg_Junker

The evolution of our potential snowstorm Thursday night is complex. The models have trended toward a southern track, but a messy one with more than one area of low pressure and significant potential to leave us in a dry slot. Most models are now only giving us light precipitation. The latest NAM pretty much places D.C. in the proverbial precipitation hole, leaving us dry. Today’s GFS is a little more bullish, developing enough snow Thursday evening to possibly put down light accumulations. The latest European model is between the two, suggesting a chance of snow showers but probably little or no accumulation.

Right now, conversational snow or a light accumulating event is possible late Thursday night into early Friday. Even that hinges on there being enough moisture around as the main system moves out to sea and upper-level energy moves through on the back side, which is always a risky proposition. It’s too early to make a final call on whether we will see accumulating snow in the area, and if so how much. But for now the probability of DCA getting an inch is about 20-30%, and not really much higher to the north and west, which even if colder may see less precipitation.

Why is Thursday night snow more likely to be light than heavy?

This morning’s plume diagram of the forecasts from the various SREF ensemble members is pretty bearish on snow. On the diagram below each of the 21 lines represents a model run in which either the initial conditions or physics have been tweaked slightly to assess the predictability of the pattern and ascertain some feel about what type of storm we’re dealing with. The diagram also depicts the type of precipitation that the member is forecasting. The ensemble members heavily favor light precipitation, with 17 of the 21 members predicting amounts at or below 0.20” of liquid equivalent, and not all of them are predicting snow. The average precipitation of the members is only .18”, slightly lower than this morning’s GFS run. Two members argue that the chance of a decent snow, while remote, is not completely dead.


(Penn State University)

How cold Friday into Saturday?

The coldest air of the season so far will arrive Thursday night, and by Friday morning temperatures are expected to be frigid. The latest Euro is predicting lows in the teens to near 20 around DC by Friday morning (see below) and for temperatures to struggle to reach the mid-20s on Friday during the day. Friday morning the cold will be augmented by breezy conditions leading to nasty wind chill readings. Friday night into Saturday morning lows may dip into the teens across most areas. The cold lingers into Saturday but temperatures by afternoon should be flirting with 30.

Friday morning low temperatures as predicted by the European model. (WxBell)
Friday morning low temperatures as predicted by the European model. (WxBell)

A possible rain storm, and then even colder?

Another storm is on track to cross the area Sunday into Monday. Models differ somewhat on the timing of the storm but are in fairly good agreement that low pressure will pass to our north, providing enough warming to support rain rather than snow. Though confidence is still on the low side given we’re still several days out.

The European ensemble mean forecast (below) of the temperatures and streamlines at 850 millibars (around 5,000 ft.) helps explain why the next storm will probably be rain, but also why the next shot of cold air could be even colder than the first. Streamlines tell you which way the winds are expected to blow and provide a sense of where the strongest winds will be. The forecast below indicates that our winds at around 5,000 feet will be from the south and that temperatures at that level will be above freezing Sunday evening at 7 p.m.


More important is what is happening over Canada. The streamlines indicate that winds from the north extend from far northern Canada southward into the Plains. The pink colors indicate temperatures around minus 30C or lower at about 5,000 feet. Such cold temperatures aloft usually mean below-zero readings at ground. Some of that air will be coming our way. Yes, it will modify some but still will be frigid.

By 7 a.m. Tuesday (not shown), the European ensemble mean is predicting 850 mb temperatures across our region may drop to around minus 25 C or lower. My friend Jay Hanna put together this table of the observed high and low temperatures at DCA when such cold 850 temperatures were observed across the region. Almost all yielded single digit lows:

jay_hanna_chart

Today’s GFS is not quite that cold with its 850 temperatures, but it too shows another really cold shot coming. Bottom line: Next Tuesday and Wednesday could be frigid, with highs perhaps struggling to reach 20 and lows in the suburbs possibly sinking to the single digits one of the nights.

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Rick Grow · December 31, 2013