Monsoon Classroom: The Great Basin High

What drives the Arizona Monsoon? The blog’s “Monsoon Classroom” breaks down the 5 main patterns that help fire up our Summer storm season.

Type II
Courtesy: NWS Tucson

The Great Basin high can be a spectacular severe weather producer for the Grand Canyon State. It consists of high pressure being shifted by a cold front grazing portions of New Mexico & Colorado. Instead of the high sitting near the Four Corners, it shifts west toward Las Vegas.

Prevailing winds aloft from the northeast don’t seem ideal to get storms forming. However, this wind shift creates shear; winds changing speed and/or direction as you go up in the atmosphere. The greater the wind shear, the greater the chance storms become organized in the White Mountains and produce severe weather in the desert floors.

StormReports063015
Storm reports from June 30th, 2015

The most recent example of this severe weather pattern occurred on June 30th, 2015. High pressure parked over the Great Basin opened the door for a mid-level disturbance to approach from Texas. Between 5:30-7:45 PM, every thunderstorm that developed turned severe & produced quarter to golf ball-sized hail. The National Weather Service in Tucson also issued its first Tornado Warning since September 10th, 2011 for the Whetstone/Huachuca City area. In all, southern Arizona recorded 17 large hail reports & 6 damaging wind reports that evening.

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