Weather Summary for December 18th and 19th 2009

Weather Summary for Dec. 18-19, 2009

Angie Enyedi, NWS Jacksonville

A stacked low pressure center tracked east-northeast across the Gulf Coast states during the early morning on Friday, December 18th. A surface warm front lifted from central Florida and positioned across north Florida in advance of the low (Figure 1, 15Z surface analysis and observations). The low center expanded as it crossed southeast Georgia in the afternoon, then it re-strengthened offshore of the Carolina coast during the evening (Figure 2, 21Z surface analysis and observations). A strong cold front trailing from the surface low pressed southward across the Jacksonville county warning area late Friday night and early Saturday morning. Several severe weather ingredients traversed the Jacksonville county warning area (CWA) in advance of the low, and prior to the event it appeared as though these ingredients would phase across much of north Florida and extreme Southeast Georgia Friday morning and early afternoon.  There was also some indication that a pre-frontal squall line could develop in advance of the surface cold front during the afternoon. 

Thankfully, no severe storms developed across the Jacksonville CWA with this event; in fact, there was not even a thunderstorm observed over land in our area!  Post event analysis reveals that strong mid and upper level subsidence began to overrun the surface low during the late morning then filtered over the Jacksonville CWA during the afternoon.  The presence of the dry air significantly limited the extent of convection, and the strong subsidence, or sinking motion of the air aloft, prevented storms to even develop despite the presence of low level instability and moderate dynamics.  The window of opportunity for thunderstorm development across our land based zones was missed due to lack of phasing of these convective ingredients.  Take a look at the following images that give a history of the surface low track and the infiltration of dry air over the area through the course of the day (Figure 3, Water Vapor and surface analysis).  Note when moisture was most abundant (early in the day), the most convection developed; then through the course of the day, as drier air moved over the area from the west (indicated by the orange hues), the areal coverage of convection significantly decreased (Figure 4, KJAX Radar Imagery).

Although storm development was still possible with dry air aloft, storms did not develop in the afternoon or the evening due to strong subsidence, or downward motion, over the area (ideally strong ascent or lift would be present which helps support rising motion and thunderstorm updraft formation).  The 00z Jacksonville radiosonde observation indicated significant warming in the mid levels (also an indication of subsidence and stabilization) as well as a lot of dry air (Figure 5, 00z 12/19/09 JAX RAOB).

The most significant sensible weather, other than periods of moderate rainfall, occurred Friday night and early Saturday morning trailing the frontal passage.  Strong pressure rises, on the order of 5 mb/3 hr, promoted a tight post-front pressure gradient across the area (Figure 6, MSAS 06Z pressure rises and surface observations). Strong low level cold air advection forced cold air above the surface downward (cold air is more dense), and as this air descended, it brought down 30-40 mph winds toward the surface (Figure 7, JAX Velocity Azimuth Display (VAD) Profile). Just after midnight, much of the Jacksonville forecast area was reporting sustained non-convective winds of 15-20 mph with wind gusts of 30 to 35 mph.  The only significant weather alert for this entire event was issued around 1 am and it was not even for thunderstorms!  It was to highlight the possibility of minor wind damage due to the gradient wind gusts of 30 to 40 mph. 


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