Thursday, January 24, 2013

Jan 24, 2013: Arctic Clipper #2 Bringing Friday Snow

We're still on track for another "snow event" (note my avoidance of the word "snow storm"),  tomorrow.   Even with a forecast 1"-2", timing here is bad, bad...right during peak Friday rush hour.

The culprit is a weak wave in the upper atmosphere, moving rapidly from west to east across the Ohio Valley.  The forecast map for tomorrow evening shows the weak area of low pressure moving off the Delmarva, with a pocket of light snow (green hatched area) exiting our region from the northwest:

Surface forecast chart for Friday evening
And yes, the 1032 mb high pressure over the northern Plains is part of the "Siberian Express", a sequence of sub-freezing air masses that have been invading the Mid Atlantic all week, from deep interior Canada.  Another Arctic front will sweep through Friday night, ushering in a new cold blast for the weekend.

Here are the snow totals envisioned by the NWS for tomorrow's event:

Predicted snow accumulation for Friday's snow event

Compared to last night's event, the snow is expected to be more widespread, without the strong north-south gradient.   The system has a bit more moisture to work with (but still not anything near what a coastal low or Nor'easter taps into).  And as this piece of energy passes through, it will merge with another wave to our south...phasing together and intensifying into a more powerful, single storm over the ocean.  But similar to last night, we anticipate a snow:liquid ratio in the 15:1 to 20:1 range, meaning very light, powdery snow that will accumulate on untreated roads instantly. 

However...the devil could still lurk in the details.   There are some aspects of Clipper-type systems that we don't forecast very well.  Among these is the occasional appearance of an embedded snowband, containing moderate-to-heavy snow, within the larger region of light snow.  These bands are small-scale, meaning the computer models do not resolve the processes that create them very well.  I can recall several instances of Clippers Past in which a seemingly benign snow event mushroomed into a headache for a subset of our metro region.  In other words, Clippers are capable of generating small swaths of surprise, Warning Criteria snowfall.  Witness last night's Clipper, which dumped almost 6" to the southeast of D.C., and was therefore under-forecast for that region.   So I am always a bit wary of these beasts.  Sometimes the only way to anticipate these bands is to monitor the storm in real-time, i.e. radar surveillance, and rush out the necessary warnings.   You can bet I'll be keeping a close eye on the situation tomorrow afternoon.

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