So what can we expect over the next two days?
UPDATE: The National Weather Service has just upgraded the storm watch to a Winter Storm Warning. It takes effect at midnight tonight and continues through 3 a.m. Thursday. Latest NWS forecast calls for 4 to 8 inches of snow in the immediate Fredericksburg region, with the highest totals coming west of town.
FORECAST: Check here for the latest from the NWS.
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At this point it’s obvious the ‘Burg will be dealing with impacts from the upcoming storm system. So what will those impacts be? First of all this morning’s NWS Sterling snow accumulation forecast graphic is shown here:
As I indicated in the last couple of posts the gradient between a lot of snow (10-14 inches in Culpeper and Fauquier counties) and just some snow (2-4 inches in eastern King George county) is very tight. This is mainly due to the fact that this storm has little cold air to work with. Starting tonight the position of the rain/snow line will be dictated by elevation differences with most of the snow falling over higher elevations to our west. In their morning forecast discussion the folks at NWS Sterling indicated their concern in this quote:
“CONCERNED ABOUT THE RA/SN LINE…AM MUCH LESS CERTAIN OF THE END RESULT IN THE I-95 CORRIDOR.”
So how will it get cold enough to snow around here? A process called “dynamic cooling” occurs when atmospheric pressure decreases, which is what will happen as this storm reaches the coast and begins to rapidly intensify. The storm will create its own cold air as it strengthens, pushing the rain/snow line east of I-95 most of tomorrow (Wednesday) to create most of Fredericksburg’s snow.
But this strengthening of the storm will have other effects as well, one of which is the creation of “snow bands” which are wintertime equivalents of squall lines that can rapidly dump a lot of snow over very localized areas. This forecast graphic for 10 a.m. tomorrow morning shows upward velocities at the 700 mb (~10,000 feet altitude) level:
The white circle I added encloses an area of strong uplift that will create the conditions for these snow bands to develop. (Another feature that will likely develop in these snow bands will be lightning and thunder, a very unusual companion to snowfall.) It’s very difficult to predict where these bands may set up, but if one forms over or near the ‘Burg the snow totals could ramp up in a hurry.
So to sum up the impacts from this storm we will see rain beginning after lunch today and mixing with snow tonight. If the rain/snow line behaves like NWS Sterling believes it will the city of Fredericksburg will see some heavy wet snow accumulation by dawn Wednesday, perhaps 2-4 inches worth. Then, once the coastal storm begins to strengthen the snow intensity will increase during the daylight hours Wednesday as will the winds, which combined with the heavy wet snow will provide the potential for power outages should tree limbs and powerlines fail under the stress. After another 2-4 inches of snow accumulation the precipitation will wind down by midnight Wednesday but with snow showers possible through mid-morning Thursday.
I should also point out what should be obvious in that if you must travel west of Fredericksburg the next couple of days keep in mind those locales will have a lot more snow to deal with, meaning their roads and streets will take longer to clear. Although all parts of the region will experience strong northerly and northwesterly winds Wednesday this will be an extremely location-dependent event for snow depths. Oh, and by the way the high temperatures this coming weekend are forecast to be in the 50′s…