Wednesday, December 21, 2011

Updating Our Christmas "Threat"

With the wagon train of weak storms forthcoming through the Mid Atlantic over the next few days, small differences could yield big changes in what ultimately falls from the sky and can make a relatively larger difference in the resulting forecast for the region. For a couple of days, the European computer model (which is the best of the computer modeling out there) suggested a storm system would track through the Mid Atlantic on Christmas Day, bringing unsettled conditions with rain and higher elevation snows. The GFS, which is the second best of the lot, was also on board with some sort of event for Christmas Eve until yesterday, when the storm was suppressed farther southeast and a non-event for our region.

 The Euro last night joined the GFS in going with "no storm" for Christmas Day. Why did it shift?   It's a matter of how the model is showing the upper level evolution of this storm system.    Yesterday's midday Euro run showed a relatively stronger mid level disturbance in the South, which helped fire up a little bit stronger storm as the mid level energy kicked east through the Mississippi Valley.   On the graphics below, you can see the yellow shading digging down into Oklahoma and Arkansas early Sunday morning (the 06z time frame is 1 AM ET) and how twelve hours later (18z, 1 PM ET) you have a pretty decent slug of precipitation and a decent low pressure center organizing across the Appalachians.

Contrast those two graphics with last night's run of the European computer model -- which showed two key differences aloft and a much weaker reflection at the surface.   The first difference is the much weaker mid level wave in the South.  Those yellows that were into Arkansas aren't even south of Illinois on this particular run of the Euro.  Secondly, the northern branch of the jet is much flatter (the green shaded areas are farther north and there is less "dig" to that jet).  The northern jet's flatter look helps prevent the southern wave from getting a sharper wave to develop.  The result of these two interactions is a much weaker system that passes harmlessly to our south, through the Carolinas, and brings little more than clouds to the region.

Given that there is another wave to come before the Christmas "threat", the Thursday night/Friday wave will pave the way for how the Christmas system behaves.  I don't think that the Euro depiction to be right...we've seen a bit of a trend with computer modeling over the years to nudge these storms a bit stronger and closer as we close in so I would not be surprised to see this system pop back up on the radar in another day or so.   In terms of what will fall, odds would favor rain in the city and south/east...but there would be some shot of snow in the higher elevations north/west of the city at the onset of the system.