An unusual weather pattern is taking shape over the Southwest, which will persist into the weekend.

A broad trough will drop southward across the West on Thursday and Thursday night, which will lead to showers and thunderstorms developing over the higher terrain of the Sierra, central and southern Nevada and northern Arizona. There will be some heavy downpours with some of the showers and storms.

Las Vegas will be one of the areas of most concern into Thursday night and Friday, when numerous showers and thunderstorms may move through. Las Vegas only averages 0.12 of an inch of rain for the entire month of May, so this is definitely an unusual pattern for the area.

Flash flooding will be a big concern in areas where these downpours develop during what is typically a very dry time of year. This happened with the showers and storms that formed due to the upper-level low earlier in the week and could happen on a more widespread scale this time around.

Dust storms could also develop ahead of any stronger thunderstorm in open desert areas.

The upper low will slide southward along the California coast into Friday, which will lead to showers and thunderstorms developing across Central and Southern California. Some of the hills around Los Angeles could get flooding downpours, as was the case earlier in the week.

The 500-mb chart valid late Friday night, showing the upper-level low moving along the Southern California coast. It will move eastward from here into Arizona and New Mexico.

As the low continues to push southeastward, the focus for showers and thunderstorms will likely be farther south on Saturday, developing as far south as the San Diego County mountains. Could there be a shower back as far west as San Diego? Sure, although the higher probability for a flooding downpour definitely looks to be in the mountains.

By Sunday and Monday, the low will move through Arizona and New Mexico. This will increase the chance for a shower or thunderstorm in places like Phoenix. The city averages just 0.11 of an inch of rain for the month, so a lone downpour could exceed the monthly average.

We've already discussed the wacky impact this system will have on temperatures, as afternoon highs in the desert Southwest will be lower than some parts of the Pacific Northwest that are almost always cooler.

The upper-level low will weaken and move eastward into early next week. As it does so, the shower and thunderstorm coverage will start to diminish across the Southwest, although it will take the longest for that to happen across the higher terrain, where showers and storms could still pop up through Tuesday.

As temperatures fall in Seattle and Portland early next week and Phoenix returns to afternoons in the 90s, the natural balance that we're used to will be restored.

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