Analysis of the September 2010 Los Angeles Extreme Heating Event

Abstract:

The Southern California coastal region has a temperate climate, however, there are days with extreme heating where temperatures may reach above 37°C, stressing the region’s power grid, leading to health issues, and creating environments susceptible to fires. These extreme localized heating events occur over a short period, from a few hours to one to two days and may or may not occur in conjunction with high winds. The Santa Ana winds are a well-studied example of this type of phenomena. On September 27, 2010, Los Angeles, CA (LA), reached a record maximum temperature of 45°C during an extreme heating event that was not a Santa Ana event. We analyzed the event using observations, reanalysis data, and mesoscale simulations with the Weather Research and Forecasting Model (WRF) to understand the mechanisms of extreme heating and provide guidance on forecasting similar events.

On 26 September 2010, a large synoptic ridge overturned and broke over the midwestern United States (US), driving momentum and internal energy to the southwest. A large pool of hot air at mid-levels over the four-corners region also shifted west, moving into southern California by 26 September. This hot air resided over the LA basin, just above the surface, by 00 GMT on 27 September. At this time, the pressure gradient at low levels was weak. Based on WRF model and wind profiler/RASS observations, we propose that separate mountain-plains solenoids (MPS) occurred on both 26 and 27 of September. The MPS on 26 September moved the hot air into place just above the surface over the LA basin. Overnight, the hot air is trapped near the surface due to the action of gravity waves in conjunction with orographic density currents and remnant migrating solenoids that form over the mountains surrounding LA. When the MPS forms during the late morning on the 27th, the descending return branch flow plus surface sensible heating creates a mechanism to move the heat to the surface, leading to record temperatures.