By Alex Sosnowski, AccuWeather senior meteorologist
July 19,2017, 9:47:23PM,EDT
The risk of isolated flash flooding and wildfire ignition and spread will continue across parts of the western United States through this weekend.A northward surge of high humidity associated with the North American monsoon will produce sporadic rainfall and may ease the threat of wildfires in some areas and assist firefighting efforts in others.
However, this same batch of moisture can produce thunderstorms with torrential rain in a small area and no rain with chaotic wind gusts in others.
The terrain and arid nature in much of the region sets the stage for both flash floods and the rapid spread of wildfires.
Nine people were killed and one is still missing following flash flooding at a popular swimming spot in central Arizona this past weekend, according to the Associated Press. A wall of water from a distant thunderstorm flowed rapidly down a placid stream and overwhelmed the bathers near the town of Payson.
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As the pattern continues through this weekend and beyond, flash flooding can strike not only rural areas and recent wildfire scars in the region. Portions of the major metro areas of Las Vegas, Phoenix and Salt Lake City could be affected.
Motorists should remain on the lookout for sudden flooding during downpours as well as a sudden drop in visibility from dust in areas where little or no rain falls.
Campers and hikers should keep abreast of their surroundings by scanning for distant thunderstorms and monitoring weather bulletins as they are issued.
The intense summer sun will continue to beat down on the landscape and dry out vegetation over much of the region.
While fluctuations in fire weather will occur on a daily basis, the risk of wildfire ignition and spread will remain.
Thunderstorms, by their namesake, generate lightning. When lightning strikes dry brush, a fire can erupt. While as many as 90 percent of wildland fires in the U.S are caused by humans, much of the remaining causes are from lightning strikes, according to the National Park Service.
"The greatest risk of lightning-induced wildfires will be on the leading edge of the moisture from the monsoon," according to AccuWeather Meteorologist Evan Duffey.
In this area, the storms generally lack a great deal of moisture and tend to yield very limited rainfall, but produce a significant amount of lightning strikes.
The cycle of spring growth, followed by summer heat and wildfires, are familiar to those who have spent years in the West.
"Following the ample rain and mountain snow from this winter, there is more brush available to act as a potential source of fuel for the fires in the lower and intermediate elevations," Duffey said.
"At least in high country, the fuels will remain wet much longer and more resistant to fire, thanks to lower temperatures and the snow cover that lingered well into the spring," Duffey said.
As of Wednesday, July 19, 2017, there were dozens of active wildfires in the western U.S., including at least 17 in Arizona, 18 in Nevada, 11 in Montana, 10 in New Mexico, nine in California, nine in Colorado, six in Idaho, four in Washington, two in Utah, one in Oregon and one in Wyoming, according to the Incident Information System.