Monday, December 31, 2012

December 31, 2012: The Wild Winds of December

December, 2012 closed out with a series of three wind storms, from December 22 through the 30th.  Each time, the region endured several hours of cold winds gusting between 45-50 mph...during which the NWS issued Wind Advisories, and area utilities dealt with 100's and even 1000's of regional power outages.

Here are the maximum wind gusts (mph) reported at our region's three airports for each of these events:

Winter Storm           Date         BWI        IAD       DCA

Draco                       Dec 22     44            51          43
Euclid                      Dec 27     46            46          45
Freyr                        Dec 30     46            47          49

It is certainly unusual to experience three Advisory-level wind storms in the space of a single week.  But the series testifies to the ferocity of this season's early winter storms, and the overall pattern of a very energetic jet stream. 

In each case, the winds were generated on the back side of a retreating mid-latitude cyclone (low pressure region).   Draco was a powerful storm that generated a blizzard over the Great Lakes, then moved over interior New England.  Euclid was a Nor'easter (coastal low), as was Freyr.

As each storm moved out of our region (to the northeast), it intensified - meaning the central pressure dropped.  At the same time, a cell of strong high pressure approached from the west.  The difference in surface pressure between each low and its high - called the pressure gradient - determines wind strength.   The gradient became very strong for several hours over the Mid Atlantic during each of these weather systems.   An example of this is shown on the surface weather map for Winter Storm Draco:

Surface weather map December 22, 2012 showing Draco's pressure gradient. Adapted from Unisys Corp
Additionally, clearing skies behind each storm allowed the surface air layer to warm up a bit, while the upper atmosphere cooled (as cold air was swept in from Canada).  This destabilized the air layer, causing it to bubble and stir.   Pockets of fast wind were mixed down to the surface, adding momentum to the already rapidly streaming air, in the form of high wind gusts.  Meteorologists call this effect "mixing down of high momentum from aloft".  Consequently, tree limbs snapped onto power lines, leading to periods of brief power outages across the Baltimore-Washington region.

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