March 4, 2013 3:30 PM Major Snowstorm Remains Possible, Devil In Details

11:00 PM UPDATE

Still on track with forecast as laid out below;  no change in reasoning;  still thinking 4"-6" heavy wet snow across the metro belt.   Evening runs of the forecast models keep solid accumulation bulls eye over mountains of NOVA.   One of the models (GFS) has tapered off the snow totals across central MD.   NWS forecasters have tweaked back their numbers a bit, thinking that lack of cold air will prolong changeover of rain to all snow. 

7:00 PM UPDATE

Here are the NWS forecast snow totals for Wednesday's storm.  The Jackpot is in the mountains, well to our west and south.   NWS foresees about 4"-6" for DC and Baltimore.   This is entirely consistent with my initial call for a 4"-6" swath encompassing Baltimore, made at 3:30 pm today (read below).

3:30 PM UPDATE

The overall thinking outlined in my 9:30 AM blog from today remains intact.

The core of the event is still 48 hours away, so we are just beginning to codify likely zones of accumulation...subject to significant adjustments in the next 24-36 hours.

The reason this one is difficult, for the metro region:  Somewhat mild air is expected to filter in from the east (off the Atlantic) in low levels, setting up a rain-snow line somewhere along the I-95 corridor, that may very well shift around from hour to hour. But the warming influence may be countered aloft by strong "dynamical cooling" of the atmosphere, caused by vigorous uplift of air.   Over the mountains to our west and southwest, the elevation effect means cooler prevailing surface temperatures from the start...combined with dynamical cooling...producing all snow from start to finish - hence the highest snow accumulation potential there.

Today's models are showing the intrusion of low-level air with temps 37-39 F, on strong easterly winds off the Atlantic, as the storm intensifies over Norfolk:

The left panel shows the NAM model, the right the GFS model, at 7 AM Wed.  The westward push of milder air off the Atlantic gets over the I-95 corridor, and remains through early afternoon.  The dark red solid line is the 32 F isotherm;  near-surface temperatures east of this line are warmer than freezing.  If there is enough low-level warmth to melt snow, this will significantly cut down the snow accumulation.

Here is the NWS Hydrometeorological Prediction Center's probability of snowfall > 8" for the Mid Atlantic region, issued 3 pm this afternoon:

The metro area's probability hovers at 10%...and as discussed, the bulls eye for accumulation is over the mountains of NOVA, WV and western MD.

Here is what seems most likely at this time:

1)  About 12-15 hours of precipitation, with snow starting 7 AM Wed, tapering 7 PM Wed -- but the changeover could be delayed along I-95 if the warm air intrudes (the scenario discussed above);
2)  Heaviest precipitation falling from late morning until mid-late afternoon;
3)  Snow starting heavy and wet, remaining wet for several hours...ending up a bit drier and fluffier toward the end, as colder and drier air filters in at all levels;
4)  Likely total precipitation 0.5" to 1.0" across the metro region and a 10:1 average snow:liquid ratio would give about 5"-10" BUT this assumes early transition from rain to snow, and does not consider likely changes in the snow:liquid ratio as the storm evolves;
5)  Highest snow totals in mountains of NOVA and western MD, possibly > 12";
6)  Winds gusting to 25-30 mph during Wed afternoon and evening (higher in mountains);
7)  Likelihood of a heavy snow band setting up north and west of storm center Wednesday afternoon -
where this happens, expect heavy snows (2"+/hr) and higher local accumulations

At this time a SW to NE oriented swath of 4"-6" seems reasonable cutting through the Baltimore metro region.  Of course, this figure is subject to change!!!!

Next update:  11 PM tonight