A preliminary analysis of severe mesoscale convective systems (MCS) crossing the Appalachians

It is well documented that Mesoscale Convective Systems (MCS) are capable of producing severe weather, such as damaging winds, large hail, and tornadoes. Operational forecasting experience suggests the complex terrain of the central and southern Appalachians Mountains (peak elevations 1000-2000 m MSL) can influence the evolution of MCSs across the region. Examples of complicating factors associated with the mountains include the potential modification of mesoscale cold pools/gust fronts, generally reduced surface-based instability in the higher elevations, downslope effects east of the ridge line, and damming of a cool/stable air mass east of the Appalachians.

This study serves as a preliminary observational-based examination of severe MCSs that interacted with the Appalachians over a 6 year period (2000-2005). The study incorporated more than 50 severe MCS cases that occurred adjacent to and/or moved across the central and southern Appalachians (approximately an 800 km extent from southern Pennsylvania to far northeast Georgia). Based on WSR-88D radar data and storm report information, severe MCSs were fundamentally classified as either crossing (MCS produced severe reports west and east of the mountains) or not crossing (MCS produced severe reports only west of the mountains). The majority of the MCS cases examined were from the late spring and summer months, although a few cool season cases also occurred. Preliminary findings suggest that severe MCSs approaching the Appalachians from the west are twice as likely to weaken below severe levels when they cross the mountains as they are to remain severe. Furthermore, severe MCSs that cross the Appalachians are climatologically most likely from May to August. This tends to occur in association with a low amplitude shortwave trough and at least moderately strong mid/upper level flow, coincident with a warm/unstable air mass over the Appalachians and across the Piedmont and Coastal Plain areas. Diurnally, while severe MCSs can approach the Appalachians at any time of day or night, those that cross tend to reach the western slopes of the mountains in late afternoon or early evening, while those that do not cross tend to arrive overnight.

In addition to climatological information, comparisons of composite upper air, surface, and thermodynamic data will be presented to discriminate between crossing vs. not-crossing severe MCS cases.