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Southwest monsoon continues to bring dangerous flooding, lightning risk
As an annual weather pattern known as the North American monsoon continues to bring much-needed rain to the southwestern United States, risks associated with flash flooding and lightning strikes will persist through this weekend and beyond.
In some places, the rain Wednesday 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 on 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.
As is often the case with the North American 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 locations are likely to be targeted.
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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. 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.
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This image shows the speed of a flash flood in the Flagstaff, Arizona, area on July 13. (Image/City of Flagstaff) |
The flow of moisture from the Gulf of Mexico and the Gulf of California that produces the shower and thunderstorm activity will be persistent, "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 showers and thunderstorms are forecast to continue over the interior Southwest through this week, but thunderstorms and sporadic downpours could expand during the weekend.
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"Enough moisture may spread northwestward to trigger spotty thunderstorm activity in Nevada, Oregon, Idaho, Montana and even to the high deserts and Sierra Nevada mountains in California on Sunday and Monday," AccuWeather Senior Meteorologist Dave Houk said.
On Tuesday, downtown Los Angeles managed to pick up 0.10 of an inch of rain, which was the wettest single day since the middle of March for the city.
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The storms that will bring the risk of heavy rainfall for some communities could also produce lightning strikes that can ignite new wildfires, especially where little 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 concern for the expanding areas of thunderstorms this weekend," Houk added.
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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 and 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 million 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.
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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 Thursday, July 15, 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.
Early, extreme heat 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.
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