Now that winter has arrived, at least unofficially, we wanted to pass along a recent paper by WFO Charleston, SC Lead Forecaster Steven Rowley regarding the cold air damming (CAD) associated with a significant ice storm that affected the Southeast U.S. , particularly southeast SC and southeast GA. The following is the abstract….
During the morning of 10 January 2011 an ice storm affected most of the National Weather Service (NWS) Charleston, South Carolina (SC) forecast area. This event featured a cold air damming (CAD) regime marked by a strong surface high pressure and a dry, cold air mass with boundary layer wet bulb temperatures at or below 0°C. Freezing rain developed as warm, moist air was forced above this environment. Advection of cold, dry air sustained boundary layer evaporational cooling and outweighed warming processes produced by the moderate to heavy freezing rain, which supported a period of rapid ice accretion on exposed surfaces at all but the far southern locales of the NWS Charleston, SC forecast area. Numerical model forecasts of this CAD event and associated rapid ice accretion proved accurate across most inland counties of the NWS Charleston, SC forecast area. Across coastal and southern counties of the NWS Charleston, SC forecast area, model guidance prior to the onset of precipitation depicted erosion of the periphery of the CAD regime by a combination of processes including inland penetration of the coastal front, changing any freezing precipitation initially supported by evaporational cooling to warm rain before major travel impacts occurred. Indeed, this scenario conforms to climatology and was assigned a high probability of occurrence by objective guidance and operational forecasters. However, during the morning of 10 January 2011 cold air expanded slightly farther south and east than expected during the period of heaviest precipitation. This unusual expansion of the CAD regime into coastal areas was facilitated by a chain of events that began with the ageostrophic maintenance of cold, dry air and dominance of the inland CAD regime. Mean sea level pressure rises within this inland cold dome and concurrent mean sea level pressure falls associated with the coastal front enhanced the isallobaric component of the ageostrophic wind, which eventually dominated the surface flow. The resulting offshore surface wind advected cold, dry, stable boundary layer air into population centers near the coast and, in turn, disrupted the normal onshore progression of the coastal front and associated warmer marine layer. A post-mortem, concurrent examination of isallobaric winds and the evolution of the surface wet bulb temperature offered important clues to CAD expansion and associated precipitation type changes along the coast during the morning of 10 January 2011. Additionally, this post-mortem adds to prior research by showing that routine evaluation by forecasters of specific meteorological fields and diagnostics may lead to more effective assessment of the mesoscale aspect of CAD evolution. Forecasters can apply this diagnosis to more accurate near-term precipitation type forecasts during CAD events in the coastal southeast U.S.
You can access the full paper at: http://www.erh.noaa.gov/er/hq/ssd/erps/ta/ta2014-01.pdf.