Summer 2012 North American heat wave

In the Summer of 2012, a heat wave took place, leading to more than 82 heat-related deaths across the United States and Canada. An additional twenty-two lives were lost in the resultant June 2012 North American derecho. This long-lived, straight-line wind and its thunderstorms cut electrical power to 3.7 million customers. Over 500,000 were still without power on July 6, as the heat wave continued. Temperatures generally decreased somewhat the week of July 9 in the east, but the high pressure shifted to the west, causing the core of the hot weather to the build in the Mountain States and the southwest U. S. shifting eastwards again by mid-July. By early August, the core of the heat remained over the Southern Plains.

The heat wave formed when high pressure aloft over the Baja of Mexico strengthened and moved over the southern plains around June 20–23 and then spread east and northward, remaining fixed over the center of North America through July 2012.

The direct cause of the heat is that, under high pressure, the air subsides (sinks) toward the surface. This sinking air acts as a dome capping the atmosphere. This cap helps to trap heat instead of allowing it to lift. Without the lift there is little or no convection and therefore little or no convective clouds (cumulus clouds) with minimal chances for rain. The end result is a continual build-up of heat at the surface resulting in drought conditions over wide areas.

This heat wave, like all extreme weather events, has its direct cause in a complex set of atmospheric conditions that produce short-term weather. However, weather occurs within the broader context of the climate, and many scientists agree that global warming has made it more likely that heat waves of this magnitude will occur. A NASA-sponsored study indicates an increase in the drying tendency from spring to summer over the U.S. Central Plains, especially during the transition from June to July. The intensified deficit in precipitation is accompanied by increased downward shortwave radiation flux, tropospheric subsidence, enhanced evaporative fraction, and elevated planetary boundary layer height, all of which can lead to surface drying. These conditions are similar to those accompanying the 2012 drought and intense heat that occurred in mid-summer.

The intense heat wave in the West was initiated around June 20–23 when the high pressure system centered over the Baja of California shifted upward into the plains and caused temperatures to approach or even surpass 110 degrees for the next several days, breaking many records for the area.

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