On the Formation, Structure, and Evolution of an Eastern Pacific Upper-level Disturbance that Contributed to Severe Weather East of the Rockies during MPEX

The field phase of the Mesoscale Predictability Experiment (MPEX) was held from 15 May to 15 June 2013. During MPEX, the subtropical jet (STJ) was active and proved to be a source for upper-level disturbances that crossed the southern Rockies and subsequently triggered convection east of these mountains. Anticyclonic wave breaking (AWB) over the eastern Pacific on 5-6 June 2013 resulted in the formation of an elongated potential vorticity (PV) streamer that stretched from California southwestward to east of Hawaii. AWB progressed inland over western North America on 7-8 June while to the south the aforementioned PV streamer thinned, elongated, and evolved into a closed mesoscale vortex (PV anomaly) off the coast of California. This mesoscale PV anomaly was noteworthy for being very dry in middle levels, highly compact, and unusually symmetric.

The purpose of this presentation will be to document the formation, structure, and evolution of the aforementioned upper-level mesoscale PV anomaly. Barotropic instability likely played a role in the formation of this mesoscale PV anomaly over the eastern North Pacific. This mesoscale PV anomaly maintained a coherent structure as it moved inland on 10 June, crossed the Great Basin and Rockies on 11 June, and emerged over the High Plains on 12 June. The structure and evolution of this mesoscale PV anomaly as it moved inland and crossed the Great Basin and Rockies will be investigated with the help of special dropsonde observations obtained during MPEX. Emphasis will be placed on documenting the interaction of the mesoscale PV anomaly with the complex terrain of the western United States. A linked second presentation by Guastini and Bosart (separate abstract) will explore the role that the aforementioned mesoscale PV anomaly played in an MPEX-sampled severe weather outbreak over the High Plains and Upper Midwest on 12-13 June 2013.