Evolution of the Event- The leading edge of the colder, drier air was marked by a
cold front that moved south across central North Carolina on Wednesday afternoon. Surface dewpoints fell from the mid and upper
40s into the 20s in a matter of a few hours. The parent high quickly translated eastward Wednesday afternoon
evening, moving from the Upper Mississippi river valley to southern New England in a span of fifteen hours.
The OOZ Thursday upper air analysis suggested that the rapid eastward movement may slow as the 500mb flow was
slightly confluent along the eastern seaboard. In addition, a dual jet structure was noted aloft over the eastern
United States (primary jet stretching from Pennsylvania to Long Island, with a secondary jet over the southeast U.S.).
While the 850mb flow was not indicative of a damming scenario (mainly northwest versus a favored southerly flow),
the 06Z Greensboro sounding depicted that the 850mb flow had become southerly.
The 06Z surface analysis shows a distinct dry air ridge across eastern Virginia and the eastern Carolinas.
This feature is known to be a sigani for the potential for cold air damming. The precipitation overspread the
western Piedmont between 09Z-12Z. By 15z, the wedge signature was evident on the analysis over the -typical
damming region as the precipitation falling through the dry sub-cloud layer had increased the hydrostatic pressures,
causing the dry air ridge to migrate into the western Piedmont.
The 12Z upper air analysis depicted a scenario
reminiscent of previous cold air damming episodes. The aforementioned 850mb southerly flow had become established
across the Mid Atlantic coast states. The best 500mb confluence was positioned offshore, suggesting that the parent
high would continue to migrate east. The 300mb jet was exiting off the northern Mid Atlantic coast states, placing
central North Carolina in the favorable right entrance region. The divergence aloft coupled with the strong low
level convergence supplied by the low level jet crossing the Deep South likely enhanced the precipitation rates
across central North Carolina Thursday morning.
The 18Z surface analysis suggest that the evolution into a residual
cool pool was beginning. While the surface flow was still northeast in the damming region, the flow was no longer
feeding colder drier air into the Piedmont. This became more evident by the OOZ analysis. By 06Z the surface flow
across the favored damming region had become west-southwest, and skies have cleared. Thus, it appears that the actual
damming occurred for three to six hours with a residual cool pool lasting six to twelve hours. Since the wedge
signature did not develop until after precipitation had been falling for a few hours, this is probably a hybrid
damming event of short duration.
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