An assessment of 26 April 2011 Pre-Frontal Squall Line in Kentucky

In this research the characteristics of a pre-frontal squall line are examined. This event consists of a cold pool driven pre-frontal squall line that propagated into western Kentucky and produced a maximum three second wind speed within the five minute data collection period of 100.7 mph (45 m/s) in Murray located over Calloway County in Kentucky on the night of 25 April 2011. Although this wind gust was associated with a squall-line, it was notable when compared to the other four Kentucky Mesonet stations that adjacent to the station from the north, east and west whose wind gusts averaged 20 mph (8.9 m/s) during the same five minute data collection period. However, the squall line also produced further damaging winds from 60-65 mph that were recorded by further stations as it progressed through western and central Kentucky. This research focuses on diagnosing the synoptic and mesoscale contributions that led to the development of the pre-frontal squall line and subsequent anomalous wind gust. The data used in this study consisted of Kentucky Mesonet 5-minute observations from the surrounding region, North American Regional Reanalysis (NARR) and Rapid Update Cycle (RUC) datasets. The latter two datasets were visualized and analyzed using Interactive Data Viewer (IDV). Radar imagery from the HDSS Archive System from the National Climatic Data Center for the Paducah radar was analyzed using Gibson Rich Level 2 software.

This high wind producing system, under favorable synoptic and mesoscale conditions, became cold pool driven, separated from the main system and eventually propagated to eastern Kentucky. Radar analysis indicated notched rotation within the squall line winds in excess of 35 mph inbound and 46 mph outbound with a collocated normalized rotation maxima of around 1.0. The anomalously high wind gust is believed to be the product of synoptic preconditioning to an area in the southeastern United States for thunderstorm development and mesoscale contributions within western Kentucky that enhanced the thunderstorm development and the resulting squall line. While CAPE values for instability were marginal since the early morning timing of the event, a strong low level jet and DCAPE values in excess of 1000 J/kg suggested a strong damaging wind potential from downward transport of momentum in rain cooled downdrafts. This in comparison with the inverted ‘V' shape from the strong surface dew point and temperature difference of the sounding suggested the occurrence of damaging straight-line winds, microbursts, downbursts, or tornadoes. The low lifting condensation levels with 0-3 km helicity in excess of 200 m2 s-2 and ample shear also supported a marginal tornado threat.

Damage assessments made by the National Weather Service revealed wind damage and tornado reports across a broad swath of eastern Kentucky as the line passed through. Calloway County had only one high wind report confirmed by the Kentucky Mesonet data. This research will explore all the possible atmospheric interactions and work to determine the most probable cause for the spike in the wind data.