The Record Los Angeles Heat Event of September 2010: 1. Synoptic‐Scale‐Meso‐β‐Scale Analyses of Interactive Planetary Wave Breaking, Terrain‐ and Coastal‐Induced Circulations

On 27 September 2010 the Los Angeles Civic Center reached its all‐time record maximum temperature of 45°C before 1330 local daylight time with several other regional stations observing all‐time record breaking heat early in that afternoon. This record event is associated with a general circulation pattern predisposed to hemispheric wave breaking. Three days before the event, wave breaking organizes complex terrain‐ and coastal‐induced processes that lead to isentropic surface folding into the Los Angeles Basin. The first wave break occurs over the western two thirds of North America leading to trough elongation across the southwestern U.S. Collocated with this trough is an isentropic potential vorticity filament that is the locus of a thermally indirect circulation central to warming and associated thickness increases and ridging westward across the Great Basin. In response to this circulation, two subsynoptic wave breaks are triggered along the Pacific coast. The isentropic potential vorticity filament is coupled to the breaking waves and the interaction produces a subsynoptic low‐pressure center and a deep vortex aloft over the southeastern California desert. This coupling leads to advection of an elevated mixed layer over Point Conception the night before the record‐breaking heat that creates a coastally trapped low‐pressure area southwest of Los Angeles. The two low‐pressure centers create a low‐level pressure gradient and east‐southeasterly jet directed offshore over the Los Angeles Basin by sunrise on 27 September. This allows the advection of low‐level warm air from the inland terrain toward the coastally trapped disturbance and descending circulation resulting in record heating. Key Points Planetary wave breaking precedes extreme heat events During planetary wave breaking, trough thinning results in transverse ageostrophic circulations that focus extreme heating downscale Offshore coastally trapped disturbances result from and enhance the effect of ageostrophic circulations in producing extreme heating