To investigate the structure of ALTs, 13 events associated with severe convective storms that affected the Sterling, VA (LWX), County Warning Area between May and September were analyzed using 0.5º resolution NCEP CFSR (Climate Forecast System Reanalysis) gridded data. An ALT climatology from the Carolinas northeastward to southern Pennsylvania, referred to as the ALT Zone, was constructed based on criteria derived from the following low-level features that are common to the 13 events: (1) a wind component orthogonal to, and downslope of, the mountain barrier, (2) a mean sea level pressure trough, and (3) a thickness ridge. ALTs identified by the climatology were categorized according to their relationship to synoptic-scale cold fronts. Severe and non-severe composites of each ALT category were made using 32 km resolution NCEP NARR (North American Regional Reanalysis) gridded data in an attempt to identify environments associated with active severe convective storm events and null events, respectively.
The climatology results indicate that ALTs occur preferentially in June, July, and August, and in the afternoon and evening, suggesting that the formation of ALTs is tied to the seasonal and diurnal heating cycles. A composite of severe convective storm events in which an ALT precedes a cold frontal passage through the entire ALT Zone shows the juxtaposition of a poleward-extended plume of high convective available potential energy (CAPE) with > 30 kt of surface to 500-hPa vertical wind shear. This juxtaposition occurs over the Washington, DC, to Philadelphia corridor, where a maximum in severe convective storms occurs during these events. A case study of one such event shows two distinct severe convective storm initiation zones: (1) along a wind-shift boundary just upstream of the ALT and (2) along the trailing cold front. The storms initiating along the wind-shift boundary upstream of the ALT intensified upon entering a high-CAPE environment in the vicinity of the ALT juxtaposed with 2540 kt of surface to 500-hPa vertical wind shear.