** Winter storm watch Wednesday night and Thursday **

A major winter storm is looking increasingly likely Wednesday night and Thursday, but widespread crippling snow amounts are still far from a lock.  This article will explore the possible ways that this storm could range from almost historic for some to just a run-of-the-mill snow to rain storm  (with just a few inches of snow before a changeover to slop).

There should be no shortage of moisture.  Most models are now predicting over an inch of liquid equivalent precipitation over the area (the one holdout is the GFS).  That would equate to double digit snow totals throughout the region IF all of it fell as snow. However, some models say ‘wait a minute, a major snowstorm still may not be in the cards’.   Some of the latest models suggest double digit snowfall totals  could well happen well west of the city, but getting these amounts in the city will be more difficult.

Technical discussion

Why it could either boom or bust

The Short Range Ensemble Forecast (SREF) plume diagram, run at 4 a.m. this morning (9z), hints that despite our fairly bullish feeling about this storm’s snow potential, there is still some potential for the storm to either boom or bust.  The top diagram shows the 21 simulations of total precipitation associated with the storm.  The rate that the individual lines climb tells you how intense the precipitation is falling from that model simulation.  Where the lines are flat, no model precipitation is occurring.

The dark black line is the average of all members, a pretty bullish mean forecast of over an inch of liquid precipitation.  However, the spread of the simulations still is huge.  One member predicts over 2 inches of liquid equivalent, more than 20 inches if it all were to fall as snow.  That member is very similar to last night’s European in its evolution of the upper level disturbance that ultimately produces our storm.   Two members of the 21, essentially predict that the storm will slide out to our south, leaving us dry.  But those two members are distinct outliers as all the operational models from the various centers are considerably heavier.  The operational model forecasts from the European, Canadian and UKMET models all predict more than an inch of liquid equivalent across the area.  The GFS was not as bullish but still was around 0.70”.

Why are there still such differences in the forecasts? The model runs last night were still dealing with an upper level system that had not yet entered the U.S.  That feature and the upper level jet on the western side of it will ultimately determine the evolution of our Nor’easter.  Satellite data fills in the void of data over the oceans but features generally are better sampled once they get inland.  That feature should be better resolved on upcoming model runs.

How a bust could happen

The figure below shows two model upper air forecasts from the SREF ensemble suite.  The member on the left ended up producing the copious precipitation amounts predicted by the European model while the run on the right ended up with a weak low well off the coast with essentially no snow.  Note on the right panel there’s no circle or closed-off area over Tennessee.  In this forecast, there’s also a disturbance (sometimes referred to as a “kicker”) coming into the Great Lakes to keep the northern stream moving which probably helps shear out our potential storm maker.

By contrast, the left hand figure (above) closes off an upper level low. As it tracks eastward  just to our south, it places us in the storm’s comma head where heavy snow often falls.  That’s essentially is what last night’s European model and to a lesser extent, this morning’s NAM model was simulating.  Such  a wrapped-up storm can be double-edged sword.  If the upper low wraps up too quickly and the southern end of the trough progresses faster than the northern end,  the storm will track farther to the west and may start  pulling warm air aloft into the region leading to a changeover to sleet or even rain.  Today’s European model has that scenario giving D.C. several inches of snow but then changing the snow to rain.   However, it still plasters the far western portions of the area.   That’s certainly one, pretty believable bust scenario for areas near the city.

This morning’s Canadian model shows just how close we are to changing over to rain Thursday morning despite a pretty good track for snow.   Today’s European model, with a more westerly track, brings the warm air farther to the west than the Canadian.   Areas south and east of the city change over to rain by 7 a.m. on the model.  That’s our biggest worry concerning the storm.  The NAM model staves off the warming because of its heavy precipitation rates and more easterly track than forecast by today’s European model but even its temperature profiles are perilously close to one offering sleet during the morning hours on Thursday.  The bottom line is that some accumulating snow is likely but how much is still very much up in the air.

How a boom could happen

The boom scenario is predicated on two things.  One, the Gulf origins of the storm suggests it will have plenty of moisture associated.  A decent track like the latest Canadian, UKMET and or last night’s European model almost always results in more than one inch of liquid equivalent.  However,  the really big storms usually have an upper low that almost closes off a center similar to the SREF boom-like ensemble member discussed above. Such a feature usually results in strong frontogenesis.  That’s a big word meaning that aloft the temperature lines are packing closer together with time.  This process often results in a band of snow with very intense precipitation rates and sometimes is associated with thundersnow.  Last night’s European model formed just such a band holding it stationary for an extended period of time.

Above you see frontogenesis in action: the dotted red and blue lines on the figure above are an indication of the mean temperature in the layer between around 5000-10,000 feet.  During the period when the lines really press together a band of very heavy precipitation develops just to the west of those lines.  That’s pretty indicative of a frontogenesis-related band. In such bands, snowfall rates of 1 to 2 inch per hour rates are often the norm.  The strength of the upper level low and strong surface low development on the UKMET, European and Canadian models argue that such a band might develop.  If you end up under such a band, your neighborhood is likely to be plastered with snowfall amounts similar to the amounts during some mega-storms.  Of course, not everyone will end up under one of the bands and there still is some chance that the more bullish models may still be overdoing amounts if the GFS is correct.  The best chances to be band struck is to be located west of the city, probably out towards Leesburg and Frederick or even a little west if the latest European model is right.

So what do I think?    

The models are still struggling with the evolution of the upper level low (disturbance) and therefore the track of the storm.  As the high pressure system to the north – supplying our cold air –  is sliding off the coast,  we will be challenged to get very high snow amounts in the city unless the storm track is close to perfect.  That still may or may not happen.

Irrespective of the exact storm track, a period of accumulating snow Wednesday night is likely.  The storm’s evolution on Thursday will determine whether the storm is a boom or bust for snow lovers.  It will come down to the wire until we can get a good handle on that.