High-Impact Squall Line Events of 2023 in the Greater Washington, DC CSA

The Washington, DC-Baltimore, Maryland (MD)-Arlington, Virginia (VA) combined statistical area (“CSA”) is among the top five most populated areas in the contiguous United States. This region is also characterized by a local maximum in severe convective wind frequency that peaks during the summer months and is often generated by linear mesoscale convective systems (MCSs), also referred to as “squall lines” or quasi-linear convective systems (QLCSs). During the 2023 convective storm season, the DC-MD-VA CSA was impacted by three significant and damaging squall lines, focused from late July to early August. One of the most noteworthy and potentially hazardous events occurred on 7 August 2023. During the afternoon of 7 August, an area of intense thunderstorms developed into a squall line over the Appalachian Mountains region of Virginia, Maryland, and Pennsylvania along a low-pressure trough ahead of a strong cold front. The squall line tracked eastward, intensified in a volatile air mass over the Maryland and Virginia Piedmont region, and produced numerous strong downbursts (thunderstorm downdraft-induced straight-line winds). A significant downburst of the squall line event impacted the city of Westminster in Carroll County, Maryland, resulting in widespread tree damage and more than 30 fallen utility poles in which live electrical wires fell on about 34 vehicles.

This paper will provide a comparison of the closest radiosonde observation (RAOB) and NOAA-20 NOAA Unique Combined Atmospheric Processing System (NUCAPS) sounding profiles over three hours before the damaging downburst event in Westminster. The sounding intercomparison illustrates close agreement in mid-afternoon (1400 EDT) vertical temperature and moisture patterns and significant convective instability with elevated storm outflow wind potential, thus providing good lead time (three hours) to anticipate hazardous straight-line (downburst) winds and resulting structural damage. Microburst windspeed potential index (MWPI) values of 3 and 4, as indicated by the NUCAPS and RAOB soundings, respectively, correlated to wind gust potential of 47 to 56 mph. At 2105 UTC, a downburst wind gust of 54 mph was recorded at Carroll County Regional Airport. Successive Integrated Multi-satellitE Retrievals for GPM (IMERG) rainfall rate product images at 2030 UTC and 2100 UTC, respectively, showed the eastward progression of the squall line and the location of strong downburst occurrence (white-circled region) at Westminster, Maryland, most likely generated by an embedded supercell storm in the squall line. Convective storm-generated downbursts in squall lines are an operational forecasting challenge due to the spectrum of time, space, and intensity scales in which they occur. This paper presents the governing physical processes of downburst generation employing the strategic application of polar-orbiting meteorological satellite datasets, ground-based radar datasets, and Weather Research and Forecasting (WRF) model analysis to build a three-dimensional model of the thermodynamic structure of the ambient environment and a conceptual model of the downburst-producing QLCS.