Saturday, December 22, 2012

December 22, 2012: Draco Winding Down, More Storms On The Way

High Winds Today

As advertised by NWS, strong "sunny day" winds have been buffeting the region, with greatest gusts in the 45-50 mph range early in the afternoon. This is validation of the Wind Advisory criteria.   For the past two days, NWS has posted a somewhat confusing string of watches and warnings to account for post-Draco winds:  First, on Thursday night, a  High Wind Watch, which was downgraded yesterday to a Wind Advisory, which was upgraded early this morning to a High Wind Warning, which was downgraded this afternoon to a Wind Advisory.   Especially Friday morning there was inconsistency in all this:  NWS was predicting maximum gusts of 55 mph for Saturday, which is below High Wind criteria, yet they had a High Wind Watch for Saturday.  It seems that more work needs to be done to clarify a confusing set of watches, warnings and definitions - and be more consistent in communicating the message to the public.

The cause of the winds:  A tight surface pressure gradient, and an unstable atmosphere that has allowed vertical air currents to mix down higher pockets of wind from a few thousand feet above the surface:

The diagram shows isobars, lines of constant pressure.  Note how closely spaced they are along the Appalachians.   This means a large pressure gradient, hence strong horizontal winds.  As the large Draco low pressure region retreated slowly to the northeast, high pressure was rapidly approaching from the southwest - causing the isobars to "squeeze" together over us.

Power outages have occurred through the day, but have been isolated/spotty, numbering in the hundreds (not thousands) across area utilities.

Storminess Return This Week

My thinking on this, which I wrote about in detail in yesterday's blog, has not please reference that one.   Regarding the post-Christmas storm:  Today the models have trended toward a slightly warmer solution, meaning more rain than frozen precipitation for the metro region:

This map shows the center of the low taking more of an inland track, putting us on the warm side.  Note that when and where the heaviest precipitation occurs (solid, light-blue colored region over Baltimore), the freezing line (dark, solid blue line cutting across PA) is well north of Baltimore.  Zoom in on this figure to help see these features.  HOWEVER:  The event is still 4-5 days out.  It is normal to see the models waffle back and forth between "warm" and "cold" solutions, inland vs. coastal track I would not be surprised to see this forecast change.

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