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Western US weather

Heat shifts north in the West next week; Monsoon storms expand this weekend

By Brian Thompson , AccuWeather meteorologist
7/16/2019, 12:28:42 PM

The heat that has been baking the Southwest in recent days will eventually spread northward by next week. The coverage of monsoon thunderstorms will increase starting this weekend, and the increase in humidity will bring down high temperatures a bit.

In the short-term, though, the heat that has been impacting the Desert Southwest will start to ease a bit later this week. An upper-level trough swinging across Central California will erode the ridge that has brought the extreme heat to the Southwest in recent days.

Phoenix saw a new highest temperature of 114 degrees back on Thursday and then again on Monday, while Las Vegas reached 111 degrees on Saturday, the first 110+ degree temperature of the year so far. Tuesday will be another hot day.

The trough helped kick up the winds in San Francisco on Monday, and on Tuesday the winds will be pretty gusty toward Las Vegas. With the wind picking up in the dry air mass in place, Red Flag Warnings are up for Tuesday for high fire danger from northwestern Arizona through southern Nevada and into portions of Utah and Colorado. The fire danger will be high across much of this area on Wednesday as well.

While it's not much of a trough moving in, it will turn the flow aloft out of the southwest for a few days, which will limit the spread of monsoon thunderstorms.

The monsoon has been slowly commencing over the past several week. Some very spotty thunderstorms have popped all the way back to Nevada and the San Diego County mountains of California.

For the next few days, the monsoon thunderstorms will mostly be focused across the high ground of Arizona and New Mexico. However, the large ridge that will initially be centered more across the central and eastern United States will migrate more toward the Southwest by the weekend and next week.

As the shift happens, the flow aloft will turn more out of the southeast, which will tap the copious moisture across northern Mexico. This will spread the moisture back into the high terrain of Southern California and Nevada. Look at how much the precipitable water (which measures the amount of water in the atmosphere) changes from late Friday...

Western Blog - Precipitable Water 7/16

to Monday...

Western Blog - Precipitable Water Monday 7/16

Notice how much to the north and west the significant moisture has expanded by Monday. The flow aloft becoming more southeasterly will bring more moisture into the mid-levels of the atmosphere, creating an environment that is more conducive to supporting thunderstorms.

This increase in moisture will allow the showers and thunderstorms to develop over a wider area and impact areas outside of the mountains, so places like Phoenix and Las Vegas may start picking up the occasional drenching thunderstorm with heavy downpours and gusty winds.

Dust storms will also become a possibility as some of the storms wander into the deserts.

As the moisture increases, it will become somewhat more humid in the Desert Southwest, which will keep the air temperatures from getting quite as high. Even so, it will be a trade-off at times with the increasing humidity.

While the heat has been in blazing in the Southwest, the overall pattern will be pretty average across the Northwest this week since the upper-level flow will be what we call “zonal.” A zonal flow means the flow is generally west to east with no bit troughs or ridges slowing things down. This is how the flow across the country looks on Thursday.

Western Blog - Upper Level Pattern Thursday 7/16

A potent disturbance could bring more clouds and a few showers to western Washington late this week, but areas east of the Cascades likely would not see much from that. Even areas that do see showers probably won't be able to squeeze out much rain from this.

Next week, though, looks to be a different story. The massive ridge that will bring heat and humidity to much of the country east of the Rockies later this week will start to back west toward the Four Corners region. Look at how different the upper-level pattern looks by Monday:

Upper-Level Pattern on Monday 7/16

As that ridge builds into the Northwest, there will be a sizable bump in temperatures across the Interior Northwest, from east of the Cascades over through Idaho and Montana.

At this point, it doesn’t look like a setup to bring any major heat into Seattle and Portland, but there will be a jump in temperature to at least somewhat above-average levels by next week. I'll take a closer look at that in my next blog post coming up on Friday.

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

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