SEVERE THUNDERSTORM DEVELOPMENT IN RELATION TO ALONG-DRYLINE VARIABILITY - A CASE-STUDY

Long-lived thunderstorms were initiated during the afternoon of 26 May 1991 ahead of a dryline in northwestern Oklahoma. Various reasons for initiation in this particular along-dryline location are investigated through analysis of observations collected during the Cooperative Okl ahoma Profiler Studies - 1991 field program, Observing systems include d in situ and radar instrumentation aboard a research aircraft, soundi ngs from mobile laboratories, a mesonetwork of surface stations, meteo rological satellites, and operational networks of surface and upper-ai r stations. Elevated moistening east of the dryline revealed by soundi ngs and aircraft observations in combination with thermal plume activi ty was apparently insufficient to promote sustained convection on this day without aid from an additional lifting mechanism. Satellite obser vations reveal scattered convection along the dryline by midafternoon and a convective cloud line intersecting the dryline at an angle in th e area of most pronounced storm initiation, extending southwestward in to the dry air. Another prominent feature on this day was a mesoscale bulge along the dryline extending northeastward into southwest Kansas. Deep convection was initiated along this bulge, but was in general sh ort-lived. Potential causes of the lifting associated with the cloud l ine that was apparently key to the preferred location for storm develo pment in northwest Oklahoma were investigated: (a) a mesoscale circula tion resulting from horizontal differences in radiative (temperature) properties of the underlying surface and (b) upward motion induced by an upper-level mesoscale disturbance. Analysis of vegetative and surfa ce temperature distributions from satellite observations suggests a po tential (more research is needed) link between surface characteristics and the development of the dryline bulge and observed cloud line thro ugh horizontal differences in vertical momentum transport. A run of th e currently operational eta model indicates some skill in predicting d ryline location and motion and predicts upward motion in the northern part of the region that was generally more convectively active, but sh ows no indication of upper-level support in the vicinity of the observ ed cloud line.