Moisture from what remains of a hurricane will hang over the Southwest United States like a wet sponge this weekend, bringing a chance of significant heavy rainfall and flash flooding to some places.
Here’s what you need to know:
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There’s a chance for heavy rain across the Southwest through the weekend and into early next week, with the most severe conditions likely to come on Friday into Saturday.
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Flood watches have already been issued for parts of southeast California and eastern Nevada, much of Arizona, the southern half of Utah and the southwest corner of Colorado. Watches, which mean that hazardous conditions are possible, are often followed by warnings as the forecast becomes clearer.
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Rainfall totals in the wettest areas are likely to range from one to four inches, but up to a half foot is possible.
The moisture is coming from Priscilla, a storm in the Pacific that strengthened into a hurricane on Sunday before weakening as it moved north.
The storm was churning up seas on Thursday, bringing dangerous surf and rip currents to beaches along the Pacific coast of Baja California and portions of southwestern and west-central Mexico. But the main hazard with this system is the remnant moisture that’s being pulled northward.
The wave of moisture was bringing a chance of rain, thunderstorms and flooding across parts of California, Arizona, Nevada and Utah on Thursday. The threat will spread east on Friday afternoon through Saturday.
Some of the greatest risk is around Phoenix.
An area of Arizona that includes Phoenix falls within the bull’s-eye of a region most at risk for heavy rain and flash flooding.
Sean Benedict, a meteorologist with the Weather Service office in Phoenix, said the city faces a significant risk of flooding about every two years.
“It seems to always be attributed to a tropical system,” Mr. Benedict said.
Phoenix is projected to record one to two inches of rain through Saturday; that’s two to three times the amount of rain that usually falls in October.
Arizona has a dry desert climate, and Phoenix records only about seven inches of rain a year on average. During heavy rain, the greater Phoenix area floods easily, as the ground is less permeable compared with some other places, such as the East Coast, where richer soils are more easily able to absorb water.
“Typically, an inch of rain can cause flooding here,” Mr. Benedict said.
Flooding is likely in parking lots, on roadways and in washes, which are typically dry stream beds that can quickly fill with water during heavy rain.
Mr. Benedict warned that these washes can be dangerous because drivers underestimate the depth of the water and proceed to drive through them.
“Unfortunately that’s how we’ve had loss of life from flooding here,” he said.
The Weather Prediction Center said the moisture flow that’s expected to set up over Arizona on Friday is similar to the one that occurred in October 2018 when remnants of Hurricane Rosa brought extensive flash flooding to central Arizona. In that event, Phoenix recorded 2.36 inches on Oct. 2, the highest amount of rain ever measured in the city in a single day in October.
The heavy rain that fell during the catastrophic flooding in Texas in July was also associated with remnant tropical moisture.
The risk is widespread.
While south-central Arizona is the region of most concern, a corridor of moisture over the Four Corners region, where Utah, Colorado, Arizona and New Mexico meet, could bring heavy rain and flood risk on Friday into Saturday. Rainfall totals could range from one to four inches with six inches possible at the wettest location.
Rainfall amounts are expected to be less in the southeast corner of California and northeast Nevada where up to one inch of rain is likely.
A second surge of moisture associated with another system in the east Pacific, Tropical Storm Raymond, is expected to bring a continued chance of rain to the Southwest beginning Sunday and into Monday.
Allison Santorelli, a meteorologist with the Weather Prediction Center, said this “second round” is likely to be focused over southeast Arizona into southwest New Mexico, just southeast of the region that’s expected to see the heaviest rain in coming days.
Amy Graff is a Times reporter covering weather, wildfires and earthquakes.
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