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Western US weather

Heat surges in the Desert Southwest to end the week as monsoon starts to kick up

By Brian Thompson , AccuWeather meteorologist
7/10/2019, 12:27:24 PM

Some of the hottest weather of the season so far is on tap for the Southwest through the weekend, and the monsoon will start to show signs of life too, especially in the higher terrain.

There was a bit of a cooldown earlier this week across much of the Southwest. In fact, it was enough to set a few record low temperatures. Douglas, Arizona, set a record low of 58 degrees on Monday morning and 60 on Tuesday morning.

Even Flagstaff, Arizona, dropped to a chilly 35 degrees Monday morning, which was just a degree off the record.

Phoenix got down into the mid-70s Monday and Tuesday, and Las Vegas even failed to reach 100 on Monday, which is the only time that has happened so far this month. Nothing earth-shattering in either place, but still noticeable.

Things are changing, though, as a huge ridge in the jet stream is going to build over the Four Corners, which is going to bring some of the hottest weather of the year so far to some places.

Upper-Air Map for Sunday 7/10

An upper-level forecast map from the GFS valid Sunday. This illustrates the general flow around 18,000-20,000 feet, and helps show the very large ridge building over the Southwest and South Central U.S. The colorful area near the TX/LA border is what will likely be Barry.

Las Vegas will likely get up to around 109 degrees over the weekend, which would best the top number so far this year, which is 107. Phoenix hit a seasonal high of 112 last month and should at least match that with this wave of heat.

Salt Lake City will also have the hottest weather of the season so far, with temperatures approaching 100 degrees at times through the weekend.

The large ridge in the jet stream will be pretty persistent across the Southwest through much of next week, as it looks now. However, some monsoonal moisture is gradually going to return.

What this moisture will do is up the chances for daily showers and thunderstorms, especially in the higher terrain of Arizona and New Mexico. Afternoon highs will likely come down a bit but will be replaced by an increase in humidity at the surface, so there really won’t be much of an improvement in comfort.

In my blog post from last Monday, I used the upcoming stretch as a way to show how we can use ensemble forecasting to gauge how likely it was for the monsoon to pick up around the middle of the month.

It looks like that test case has worked out pretty nicely, as the moisture in the atmosphere will be increasing right on cue heading into the weekend. I used precipitable water quite a bit in last week’s blog, because it’s one of the best ways to quantitate how much moisture is in the atmosphere at a given spot.

Here’s the precipitable water forecast for today (Wednesday)…

Precipitable Water for Today 7/10

and here’s what they end up being by the end of the weekend. The values have almost doubled by this point from New Mexico and Arizona over in some of the deserts of Southern California, indicating a surge in moisture.

Precipitable Water for Sunday 7/10

With the moisture increasing, there will be more instability to produce thunderstorms, and the natural terrain of the Four Corners acts to add additional lift that aids in thunderstorm development.

In the coming days, we'll see an increase in thunderstorm coverage over the high terrain of New Mexico as well as northern and eastern Arizona due to that increasing instability. There can be some dry thunderstorms, or thunderstorms that have lightning but no rain, which can spark wildfires.

By the weekend, the instability will start to ramp up in some of the deserts, too, so we may start to see some stray thunderstorms make their way out of the mountains and into the deserts.

When this happens, a flooding downpour will be possible, and any stronger thunderstorm can lead to a dust storm as well. We'll be able to fine tune the details a little better as the time gets closer, so check back with your local AccuWeather forecast daily to see which days have the risk for thunderstorms.

Farther to the north and west, even with moisture content in the atmosphere increasing toward places like Las Vegas and Palm Springs, there generally won't be enough instability to produce thunderstorms.

The threat of monsoonal thunderstorms, especially in the higher terrain, will persist through much of next week.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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Western US weather