Tuesday, February 21, 2012

Baltimore’s Only Brush With Winter: February 11, 2012 Arctic Front and Snowfall

This has been an unusual, but not unheard of, mild and snow-free winter.   Heavy snow in Baltimore can best be described as “episodic” in nature, a feast-or-famine in terms of total seasonal snowfall.   Recall the record-setting 2009-2010 winter in Baltimore, with a season total snowfall of 77 inches.   This season, cold air masses have sequestered well to the north of the Mid Atlantic and the jet stream has been largely zonal, running flat in a west-to-east flow.   Without big, cold meanders in the jet building down the east coast (these are called jet stream troughs), storminess and winter air outbreaks have been minimal.    Whether we can blame this behavior on La Nina, and/or the North Atlantic Oscillation, remains to be carefully assessed once the season ends.
Baltimore endured a brief winter blast on February 11-12, when a chunk of that cold air mass over Canada pushed south into the Plains and Great Lakes.   The anticyclone that propelled this cold air south and east was intense, with a surface pressure of 1046 mb (fewer than 5% of all anticyclones over CONUS ever reach this intensity).   The leading edge of this cold surge invaded the Mid Atlantic in the form of an arctic front – essentially a cold front on steroids.   The synoptic weather map at 7 AM on February 11 (below) shows the front (blue line running through West Virginia) poised to blast through Baltimore:

Ahead of the front, there was sufficient moisture and an unstable (overturning) air mass.   As this air was lifted along the front, a narrow band of convective snow showers erupted east of the Blue Ridge Mountains.   Around 2 pm, this sudden squall slammed through the area.   Visibility plummeted as a heavy fall of snowflakes and graupel (tiny white icy spheres) fell in a volley, accompanied by gusty winds, rapidly falling temperatures and even lightning.   Many areas picked up a quick ½”-1” of accumulation in a matter of 20 minutes.   The regional weather chart (below) shows this squall as detected by weather radar, along with snow streamers blowing off the Great Lakes and a patch of upslope snow along the Allegheny Plateau.

In this image, isobars (lines of constant pressure) are drawn at 2 mb pressure intervals.  The very compact squeezing of isobars implies an intense pressure gradient, which drives the wind.   Winds from the northwest rapidly increased trough the late afternoon as the heart of the cold air mass began to filter in.  The arrival of cold air from the northwest was very swift.  The diagram below shows this cold air advection at about 5,000 feet above the surface.  Cold air advection is a calculation of how rapidly the temperature changes at a location, due to the influx of cold air by the wind.  Dark blue shades in this diagram show where the chilling effect was most pronounced.   Note that the air temperature at 5,000 feet above Baltimore was 18° F!  

But really cold air was only part of the story that night.   Wind speed steadily increased through the evening, as surface pressure surged in Baltimore.    The winds were also very gusty, peaking at 49 mph.  The diagram below, called a time series, plots hourly observations of wind gust and air temperature at Dulles airport.  Note the arrival of the arctic front at 3 pm on February 11, followed by the precipitous drop in air temperature and surging winds.

Intermittent snow squalls continued through the evening, exemplified by the patch of moderate snow captured by weather radar just before midnight on the 11th:

The air temperature never climbed above 32° F the next day!
What’s interesting is that none of this extreme weather – arctic chill, thundersnow, high winds – were actually produced by an area of low pressure, but rather by an exceptionally strong anticyclone.    We normally think of low pressure regions or cyclones as bringing disturbed and stormy conditions, while anticyclones or regions of high pressure herald fair weather.  But in the case, the tables were turned!

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