Observations of Mountain Wave–Induced Precipitation Shadows over Northeast Pennsylvania

Abstract

WSR-88D depictions of two mountain wave?induced precipitation shadows observed near the Wyoming Valley of northeast Pennsylvania are presented. These mountain waves developed in similar synoptic environments that featured a strong south to southeast low-level jet, a stable layer situated near mountaintop level, and cross-barrier flow that decreased with height. One event was associated with a well-defined, singular precipitation shadow, while the second event displayed multiple precipitation shadows. Subtle differences in the vertical distribution of temperature and wind shear between the two cases appeared to be instrumental in defining what type of structure the mountain wave and their associated precipitation shadows displayed. This is supported by calculations of Froude number, Brunt?Väisälä frequency, Scorer parameter, and horizontal wavelength for the two events. These mountain waves appear to have a significant effect on the local precipitation distribution in and near the heavily populated Wyoming Valley, with amounts reduced within and up to 15 km downstream of the valley. These effects are evident in the radar-estimated storm total precipitation products for both cases, and implied by rain gauge data for one of the events. Precipitation drift appears to play a role in the actual surface location of these precipitation minima with respect to the radar-indicated position, in cases of strong low-level flow.