Damaging Thunderstorms and Flooding Rain Likely Across the Desert Southwest as Monsoonal Moisture Returns

July 31,2016
Now that we are in the monsoon season across the Southwest, thunderstorms, heavy rain and flash flooding frequently occur, but the weather pattern has been relatively dry much of July. Some big changes are already underway, however, and residents across parts of the Southwest need to be prepared.
Due to a number of factors, moisture has been quickly returning to the Southwest, and that will result in a continued increase in thunderstorm activity over the next several days.
Some thunderstorms are likely to produce strong winds, large hail, heavy rain and flash flooding.

Radar, Watches, and Warnings

Thunderstorms developed late Friday and Friday evening across the Southwest and produced dust storms, wind damage and locally heavy rain, especially in Arizona.

• Dust storms, with visibilities near zero occurred in the Arizona counties of La Paz, Pinal, Yuma, Yavapai and in Riverside County, California.
• A wind gust of 89 mph was reported at Gila Bend, Arizona.
• Wind gusts to 70 mph in Phoenix, Arizona, knocked down power lines and over 25, 000 customers were without power for a time, according to Arizona Public Service.
• There was a report of 1.02 inches of rain just north of Scottsdale, Arizona.

Threat of Thunderstorms and Heavy Rain Continues

There was a dramatic increase in moisture on Saturday, and this will continue into Sunday evening, so thunderstorms will be more likely to produce localized heavy rain and flash flooding.
The area with the best chances to see high impact thunderstorms will be across the southern half of Arizona, but thunderstorms will also erupt over parts of southern California, southern Nevada and southern Utah.
In addition to heavy rain, other threats will include large hail, damaging winds and dangerous lightning.
Strong winds could produce dust storms (haboobs) with visibilities down to zero.
(FORECAST: Phoenix | Tucson, Arizona | Las Vegas | Needles, California)

Southwest rainfall forecast through Wednesday

Monsoonal moisture will continue to be in place across the Southwest into midweek, so the threat of showers and thunderstorms will continue through at least Wednesday. Areas that are likely to be affected will include much of Arizona, southern Utah, southern Colorado and western New Mexico.
Potential impacts will continue to include locally heavy rain, localized flash flooding, large hail, damaging winds, dust storms and dangerous lightning.
The National Weather Service Forecast Office in Phoenix, Arizona, has issued a checklist for residents and visitors to the area.

• Make sure that your vehicle wiper blades are in good shape (no cracks or blades hanging off their holders).
• Have a fully charged cell phone handy.
• Allow extra time to get to your destination.
• Have patience on the road.
• Keep your attention focused and respect other motorists.

Setup for Southwest Rain and Thunderstorms

Monsoonal moisture has been spotty across the Southwest over the past month, due to an expansive ridge of high pressure that has blocked moisture from flowing into the region. Some changes are occurring as the high shrinks and slides eastward.
This has allowed moisture, coming up from the south and southeast, to surge up into the Southwest U.S.

Moisture has pushed into the Southwest resulting in increased t-storm activity.

As we move into the week ahead, monsoonal moisture will be in place, and a series of disturbances will move from east to west across the region. This pattern should last through at least Wednesday.
Rainfall totals across parts of the Southwest, especially in Arizona, could range from 1 to 3 inches (with locally higher amounts possible). Be careful out there, and remember to not drive into flooded roadways.
Computer model guidance is suggesting that moisture will be on the decline later in the week.
MORE: Arizona Dust Storm