Triple digit temps in Southwest -- and it's still April

By Tim Ballisty, weather.com meteorologist

It's only April, but some extreme heat has arrived over the Southwest U.S. this weekend.

Why is this happening? A major upper-level ridge of high pressure has taken up shop across the region.
This upper ridge provides subsidence or sinking air. Sinking air actually compresses and pressure builds. This build-up in pressure leads to an air mass temperature increase.

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We are forecasting high temperatures in the Desert Southwest to reach the upper 90s and triple-digits; coming close to or exceeding the record highs for the date in some cities.

Before this weekend, Phoenix had not reached the century mark in 2012. The average first 100-degree reading comes around May 2. Last year, the city first hit 100 degrees on April 1.
The major warmth won't only be confined to the Southwest but also permeate into the Sacramento and San Joaquin Valleys of California (Sacramento, Fresno) and also into the heart of the intermountain West (Salt Lake City, Grand Junction, Boise). In fact, record highs may be broken in the Pacific Northwest too including in Portland and Medford, Ore.
Highs across the region are forecast to be 10 to 20 degrees above average.
This intense heat will continue through at least the early part of the work week with some slight mitigation of the heat by midweek.
Look for Wednesday highs in the 90s again in the Desert Southwest, mid to upper 70s in the Salt Lake Valley and 80s to near 90 on the Strip in Vegas.

Will this pattern stick around? Unlike the previous pattern in place, this shift will be relatively short-lived. Computer weather models currently show the western ridge weakening as a new storm system approaches the California coast late Wednesday into Thursday.