Monsoon to expand into California, Northwest with lightning and flood risk

As an annual weather pattern known as the North American monsoon continues to bring much-needed rain to the southwestern United States, moisture is forecast to expand over the West and bring the risk of lightning-induced wildfires and isolated flash flooding to more of the region this weekend and early next week.

As is often the case with the monsoon, on a given day some locations can be hit hard with an inch of rain and surrounding locations may barely get a drop. It is nearly impossible to predict well in advance which specific locations are likely to be targeted by the showers and thunderstorms associated with the pattern.

However, there are indications that moisture that has been largely confined to Arizona, New Mexico, southern Utah and southwestern Colorado this week is about to expand.

"Folks in Southern California need to be ready for the first big surge of monsoon moisture of the year as the pattern is poised to produce thunderstorms over the deserts and mountains beginning this weekend," said Ken Clark, a western weather expert who spent more than 40 decades with AccuWeather before retiring in 2018.

The combination of an eastward shift in the big high pressure area over the interior West and the approach of a storm off the British Columbia coast is forecast to create a plume of moisture that will expand westward and northward this weekend to next week.

"In Southern California, prevailing southeasterly breezes will bring in storms with gusty winds and downpours that can lead to flash flooding and small hail," Clark said.

The storms may not only be limited to desert locations such as Las Vegas, Yuma, Arizona, and Palm Springs, California, but may also visit parts of the San Diego and Los Angeles metro areas.

The same conditions are likely to expand northward across Nevada and California from Sunday to Monday and from Oregon and Idaho to Washington and Montana from Monday to Tuesday.

This means that cities such as Reno, Nevada; Sacramento and Redding, California; Medford and Pendleton, Oregon; Yakima and Spokane, Washington; Boise, Idaho; and Missoula, Montana, could be affected by thunderstorms in the pattern.

A small amount of moisture related to the monsoon in Southern California allowed downtown Los Angeles to pick up 0.10 of an inch of rain this past Tuesday. The rainfall resulted in the wettest single day since the middle of March for the city.

The combination of the rugged terrain and rocky soil conditions are conducive to rapid runoff from (even moderate) showers. This can send water rushing through city streets and remote canyons with little notice.

The runoff can be especially bad in recent burn scar locations.

This image shows the speed of a flash flood in the Flagstaff, Arizona, area on July 13, 2021. (Image/City of Flagstaff)

In the Southwest, normally dry stream beds, known locally as arroyos, can rapidly fill with water and become raging torrents.

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People living in or traveling through the region, including hikers and campers, should be alert for changing weather conditions. A downpour miles away and upstream from a location can lead to flash flooding even where no rain may be actively falling.

The flow of moisture from the Gulf of Mexico and the Gulf of California that produces the shower and thunderstorm activity will be persistent into next week in the Southwest, "although the intensity and overall coverage of the rain is likely to vary somewhat from day to day," AccuWeather Senior Meteorologist Dave Bowers explained.

The storms that can bring the risk of flooding rainfall for a small number of communities will also produce lightning strikes that may ignite new wildfires, especially where little rain or no rain manages to fall. Gusty winds in the proximity of the storms can quickly cause a small brush fire to grow into a raging wildfire.

"Lightning strikes with little or no rainfall are a big concern for the expanding areas of thunderstorms through next week," AccuWeather Senior Meteorologist Dave Houk said.

This photo provided by the Oregon Department of Forestry shows active fire along a ridge at the Grandview Fire near Sisters, Ore., Sunday, July 11, 2021. (Oregon Department of Forestry via AP)

During the period from 2008 to 2012, an average of 9,000 wildfires were started by lightning strikes. These fires tended to be much larger in size when compared to those caused by human interaction.

As of July 15, there have been 34,411 wildfires in 2021 that have consumed 2,255,218 acres, according to the National Interagency Fire Association. Even though the number of acres burned to date is below the average of 3,208,996 acres, the number of fires is above the average of 30,769 incidents.

A vast area of ongoing and building drought has been largely responsible for the number of wildfires in the western U.S. this year. The drought has its roots into the winter of 2020-2021, thanks to below-average precipitation in much of the region.

The drought continues to worsen over much of the West with the latest information from the United States Drought Monitor indicating the percentage of extreme drought had increased to 64% as of Wednesday, July 14, 2021.

Even though the monsoon will provide some rainfall and is likely to trigger flash flood incidents, much more rain is needed than the annual phenomenon can provide to significantly turn the drought situation around.

Extreme heat that's arrived early in the region has caused a much greater amount of high country snow to melt earlier than usual. Runoff from this moisture source is essential for keeping streams and rivers flowing through the end of the summer season. When this runoff diminishes earlier than usual, water levels on reservoirs and rivers are even more prone to dropping off sooner and dipping to record-low levels.

In parts of the Southwest, the rain Wednesday, July 14, was intense enough to lead to flash flooding, including parts of the Flagstaff and Tucson, Arizona, areas. As much as 2 inches of rain fell in 24 hours based on radar in remote areas Wednesday.

Wednesday was the most active day of the monsoon so far this year for the Phoenix metro area as a whole, according to the National Weather Service (NWS). While only 0.10 of an inch of rain fell at Phoenix Sky Harbor International Airport, and there were no major flooding problems locally, all of the reporting stations in the region reported rainfall. The last time this happened was in September 2019. Rainfall of this extent typically only occurs one to three times during the monsoon season, the NWS office in Phoenix said.

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