A case study of severe storm development along a dryline within a synoptically active environment. Part I: Dryline motion and an Eta Model forecast

Through a case study approach the motion of a dryline (on 16 May 1991) with in a synoptically active environment in the southern plains, along which se vere storms ultimately developed, is examined in detail. Observations from research aircraft, surface mesonetwork stations, mobile ballooning vehicles , radar, wind profilers, and operational surface and upper air networks are examined and combined. Additionally, output from the operational mesoscale Eta Model is examined to compare predictions of dryline motion with observ ations and to aid in interpretation of observations. The dryline on this day advanced rapidly eastward and included formation of a bulge; additionally, in at least two instances it exhibited redevelopmen t (loss of definition at one location and gain at another). Aircraft observ ations revealed that an eastward redevelopment occurred in the early aftern oon and was characterized by a series of four "steps'' along the western ed ge of the boundary layer moisture. The westernmost and easternmost steps co incide with the locations of the dryline before and after redevelopment, re spectively. The retreat of the dryline in the central and southern portion of the analysis domain in the late afternoon included both continuous motio n and redevelopment toward the west-northwest. This dual-mode retreat of th e dryline was accompanied by gradual backing of the winds and moistening in low levels. The Eta Model forecast initialized at 1200 UTC produced dryline features th at were qualitatively similar to observed fields. The eastward motion of a broad area of enhanced moisture gradient agreed well with observations foll owing an initial spinup period. A north-south moisture convergence axis pre ceded the rapid eastward motion of the dryline by several hours. Lack of su bsidence in the air behind the modeled dryline leads to the conclusion that processes other than downward transfer of horizontal momentum by larger-sc ale motions (that would support eastward advection) produced the rapid dryl ine motion and observed eastward dryline bulge. Results of diagnosing physi cal processes affecting model dryline motion point toward boundary layer ve rtical mixing coupled with advection of dry air aloft as vital components i n rapid advance of the dryline eastward in this synoptically active case.