Chesapeake Bay Effect Snow Event of December 25, 1999

            On December 25, 1999, another "Chesapeake Bay" effect snow event occured within the Wakefield CWA. As in the November 30, 1999 case, very cold 850 MB temperatures were indicated across the area. 850 MB temperatures were in the -10C to -15C range across the area where the "bay effect" plume was generated. Bay water temperatures were around 4C. This yielded an 850 MB - Bay surface temperature difference on the order of 14-19C, in excess of the 13C temperature difference required to generate "Bay effect" or "Lake effect" snow.

            Wind direction is also critical for the generation of a "Bay effect" snow event. Surface winds must be between 340 degrees and
010 degrees. 360 degrees is optimal. The direction of wind is oriented along the length of the Bay to provide the maximum over water trajectory conducive to the development of "Bay effect" snow bands.

           Enhanced IR satellite imagery from 1615 UTC December 25, 1999 shows a plume of low clouds (indicated by the blue enhancement) across the southern Chesapeake Bay extending across portions of Norfolk and Virginia Beach. The Wakefield WSR-88D was in clear ir mode at the time. The 0.5 degree base reflectivity image from 1600 UTC December 25, 1999 shows the plume of low clouds and "Bay effect" snow quite clearly. Some reflectivity values were in the 16-21 dBz range. Light to locally moderate snow was occuring over portions of Norfolk, Chesapeake and Virginia Beach at the time. Temperatures were in the Middle and Upper 20s at the time.

The following is a sequence of surface observations from Norfolk International Airport - Times are in EST
 
05:51 26.6° F    12.2° F    30.33 in   10.0 mi   340/12 Mostly Cloudy
06:51  26.6° F    12.2° F    30.39 in   10.0 mi   350/13 Mostly Cloudy
07:51 26.6° F    14.0° F    30.39 in   10.0 mi   350/14 Mostly Cloudy
08:51 26.6° F    15.8° F    30.42 in   10.0 mi   360/10  Snow/Light Snow 
09:51 26.6° F    17.6° F    30.42 in     7.0 mi   350/13  Snow/Light Snow 
10:51 28.4° F    12.2° F    30.42 in   10.0 mi   340/12  Scattered Clouds
11:51 30.2° F    10.4° F    30.39 in   10.0 mi   330/14  Partly Cloudy
12:51 30.2° F    12.2° F    30.39 in   10.0 mi   330/12  Partly Cloudy