Summary of the Weekend Snow Events of February 18-20, 2006

A cold front (Polar Front) swept across eastern Kentucky early on Friday February 17th followed closely by a second frontal system (Arctic Front) later in the day.  After setting record highs around 70 across most of the region on Thursday, a much colder airmass filtered in across the Bluegrass State through the day Friday and on Friday night behind these fronts.  This shallow cold airmass remained locked in place through the weekend, setting the stage for two snow events. 

Saturday February 18, 2006

Flakes began to fall over eastern Kentucky during the early morning hours of Saturday February 18th and continued through the early afternoon hours.  Just a cursory glace at the surface map early Saturday morning wouldn’t have given much clue that an accumulating snowfall was about to happen as a large bubble of high pressure was building over the Commonwealth.  However, along with the high pressure was a very cold and dense airmass, which was oozing into the region.  Just above the surface a strong temperature gradient had developed over the Midsouth, visible on both the 925 hPa (~2500 ft msl) and 850 hPa (~5000 ft msl) charts, and a weak low pressure area was visible on the 925 hPa chart over Louisiana.  This weak circulation pulled the warm Gulf of Mexico airmass over the cold low-level airmass, a process known as isentropic upglide.  Aiding the whole process was a strong 300 hPa (~30,000 ft msl) jet located over the Great Lakes region placing eastern Kentucky in the Right Entrance Region of the jet, a location which maximizes upper level divergence and lift.  All of these processes working in tandem produced a large shield of precipitation along and to the north of the 850 hPa front.  The snowfall lasted until the weak 925 hPa low shifted to the east and drier air filtered in aloft later Saturday afternoon.

The largest accumulations were had along the Tennessee and Virginia borders over extreme southeastern Kentucky, where the deepest moisture was located.  A general 1 to 3” accumulation was found in this area, with higher amounts, up to 5”, found over the higher elevations of Bell, Harlan and Letcher Counties .  Accumulations over the remainder of the area were generally around an inch. 

 Sunday Night and early Monday February 19 & 20, 2006

The second snow event broke out over the Bluegrass State Sunday evening and continued through the overnight hours.  The process behind this event was very similar to the Saturday event, however a few minor differences resulted in slightly higher amounts in some locations.

While a cold and dry high pressure remained in control of the east Kentucky surface chart, a weak low was analyzed over the northern Gulf of Mexico .  Deep southwesterly flow was evident above the surface on the 925 hPa and 850 hPa analyses, as was a continued strong thermal gradient over the lower Tennessee Valley.  Divergence was maximized in the upper levels as was visible on the 300 hPa analysis and combined with the southwesterly flow in the mid-levels to once again transported warm Gulf of Mexico air over the cold low-level airmass.  While initial indications were for a light snowfall generally 3” or less, the surface chart revealed a trough over the southern Appalachians extending north in extreme southeast Kentucky and southwest Virginia.  This feature likely enhanced surface convergence and lift and helped produce slightly higher amounts (up to 6”) in parts of Leslie, Bell and Harlan Counties and much of southwestern Virginia.

 

 



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