How much longer will the heat wave last in the southwestern US?

By Alex Sosnowski, AccuWeather senior meteorologist

The weather pattern responsible for the hottest weather of the year so far over the interior Southwest will gradually break down into next week.

For many locations, this past Thursday may end up being the hottest day in the first heat wave of the year. The temperature reached 109 degrees Fahrenheit at Death Valley, California.

Temperatures reached or exceeded 100 in Palm Springs, Imperial, Thermal, Blythe and Needles, California, on Thursday. Highs were 99 in Phoenix and 96 in both Las Vegas and Fresno, California.

Phoenix was one spot that topped Thursday's high level mark on Friday when the city saw its first 100-degree day of the year. 

Following the intense heat, high temperatures are forecast to trend downward slightly through this weekend.

During much of this past week, a northward bulge in the jet stream coincided with an area of high pressure near the surface. The pattern brought building heat, combined with dry and sunny conditions.

"Beginning as early as late this weekend, the jet stream will dip southward and some moisture will be drawn in from the Pacific Ocean," according to AccuWeather Senior Meteorologist Brett Anderson.

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"This will have the effect of producing a cooling trend with more cloud cover at first, followed by an increasing chance of at least spotty showers and thunderstorms by early next week," Anderson said.

A storm from the Pacific Ocean may help to focus the showers and thunderstorms and has the potential to bring locally heavy rain and gusty winds in a west to east fashion from Sunday night and Monday to Tuesday over the region.

Those who have hiking trips planned should use caution as a downpour from miles away can trigger flash flooding in canyons and the normally dry stream beds known as arroyos.

The shower and thunderstorm activity is not associated with the monsoon, which is officially noted by several days in a row of high humidity that leads to a daily uptick in showers and thunderstorms. The monsoon does not typically begin until June.

In the wake of the storm, the jet stream will retreat northward and high pressure will build over the region once again during the middle and latter part of next week, along with a warming trend.

Download the free AccuWeather app to see how high temperatures will climb into this weekend and if your area may be hit with thunderstorms next week. Keep checking back for updates on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier and Verizon Fios.