Appalachian lee troughs and their association with deep convection

Appalachian lee troughs (ALTs) can play an important role in the development of convective storms, some of which may become severe, to the lee of the Appalachians from the Carolinas northeastward to southern Pennsylvania. ALTs typically are mesoscale features that can induce convection by acting as sources of low-level vorticity, moisture, and convergence. Accurately forecasting the location, mode, and severity of convection associated with ALTs is important due to the proximity of the convective initiation region to the densely populated Eastern Seaboard. However, forecasting convection associated with ALTs can be challenging, especially in weak upper-level flow regimes that are characteristic of the region to the lee of the Appalachians during the summer months.

To investigate the structure of ALTs, 14 cases of ALT events associated with convection between May and September were analyzed using 0.5° resolution gridded data from the CFSR (Climate Forecast System Reanalysis). The data were visualized using horizontal maps and vertical cross sections created in GEMPAK, and were examined for common mesoscale features. A climatology of ALT events is being constructed based on criteria derived from features that are common to the 14 cases.

This presentation will include the results from the ALT climatology. Based on results from the ALT cases, we will also show that ALTs are characterized by a surface wind component normal to the mountain barrier and by the following low-level mesoscale features: (1) a thickness ridge, (2) a thermal vorticity minimum, and (3) a geostrophic relative vorticity maximum. Horizontal maps of the thickness ridge and negative thermal vorticity, as well as vertical cross sections of geostrophic relative vorticity and potential temperature, suggest that ALTs are shallow, warm-core phenomena.