Freezing Rain Event January 10, 2011

        On early Monday morning of January 10, 2011, a rare freezing rain event took place over the northern portions of the Jacksonville Weather Forecast Office's area of responsibility.  Freezing rain occurs when rain falls into a layer of freezing temperatures near or just above the surface. The freezing rain may accumulate on surfaces such as trees, sidewalks, and roads and become very dangerous for motorists. Some freezing rain events are ice storms and can cause major problems for transportation, communications, and electrical services.   Fortunately, in this event, it did not impact the area significantly to the point where winter weather warnings were necessary.

    The event started off as a cold and shallow airmass was well entrenched over southeast Georgia early Sunday afternoon, January 9th, 2011 (Figure 1).  At this time, the temperatures were in the lower 40s over inland southeast Georgia. Dewpoints were extremely low for this area in the 3 to 8 degree range based on reports from Alma and Douglas.  This dry air more than likely played a large role in creating a situation where wet-bulb cooling would be effective at cooling the air.
Wet-bulb cooling refers to the process whereby air is cooled due to the evaporation of water (i.e., rain).  The cooling occurs as the rain falls through a layer of air and can be effective at dropping the temperatures to freezing, which occurred in this situation.  What also helped during the morning hours was a period of light and moderate rainfall that acted to enhance the low level cooling, shown by the white circle in Figure 2.  In fact, temperatures at Alma dropped from 38 degrees at 6 am Monday morning to 33 degrees by 10 am as the rain persisted.  Simple interpolation based on the observations suggests the freezing line as far south as Douglas to just north of Alma to near Jesup. 

     As far as the vertical structure of the atmosphere, an analyzed RUC model sounding taken around 9 AM Monday reveals a layer of freezing air from the surface to roughly 1000 feet high over Jeff Davis County which would be supportive of freezing rain (Figure 3).  Freezing rain was reported in a number of places across the area including Douglas, Hazlehurst, Alma, and Baxley where it was reported to form on trees and create a glaze and icicles. There were no reports of freezing rain on roads. As the temperatures slowly increased in the afternoon the thin glaze that formed melted.

Write-up by Andrew Shashy



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