Flood Watches In Effect For Southwest

Priscilla and Raymond are expected to bring significant rainfall to the Southwest over the weekend and into early next week.

The Big Picture

These two storms will bring two bursts of moisture to parts of the Southwest through early next week. Each burst will bring a threat of flooding from southern Arizona to the Four Corners.

Rainfall in this area could be devastating, as several months' worth of rainfall is expected for areas not used to seeing this much rainfall this quickly.

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Phoenix's average rainfall for the month of October is about 0.56 inches. They saw about an inch of rainfall across Friday and Saturday and and could see an addition 0.5-1.5 inches of rainfall through Sunday evening, which would equal several months of rainfall in a couple of days.

Las Vegas saw 0.92 inches of rainfall on Friday, which was the fourth wettest day ever recorded in October. This rainfall total shattered the previous daily rainfall record of 0.16 set back in 2012. Daily records also fell across parts of Arizona on Friday including Flagstaff, Winslow and Prescott.

Jacks Canyon, located east-southeast of Flagstaff, Arizona, saw a more than 5-foot rise in water levels late Friday morning due to around one inch of rain.

Between the two storms, 2-4 inches of rain is possible in some spots, with localized areas seeing up to 6 inches.

Timing The Rainfall And Flood Threat

Through early Sunday: Firstly, Prescilla's moisture will enhance the flood threat for the Southwest.

Moisture from Priscilla is already ongoing in the Southwest, where flood watches are in effect in cities including Phoenix and Tucson, Arizona; and Durango, Colorado; and Lordsburg, New Mexico.

Sunday - Tuesday: The second burst of moisture will arrive from Raymond's remnants right on the heels of Priscilla.

Raymond will take a track just to the east of Priscilla's, and it could also bring another round of rainfall to the Southwest early next week.

While the heaviest rainfall is expected to be slightly further east than Priscilla’s rainfall, should any additional rain fall, it will not take much to cause additional flooding.

NOAA suggests the flood threat will remain a 2-out-of-4 risk, but that the threat will remain elevated at this level through early Tuesday in parts of the Southwest.

Rainfall Could Be Helpful

The Southwest is actually in drought and could use the rain. Arizona and Utah are entirely under drought, according to the latest from the US Drought Monitor.

Phoenix and Flagstaff, Arizona, as well as Salt Lake City and Cedar City, Utah, are all running over an inch below average for this time of the year.

While rainfall from these storms could be helpful, this much in such a short period of time is not as helpful for relieving drought as people would think because most rainfall can't be absorbed by the soil.

Why Pacific Hurricanes Matter In The Southwest

When the region is called the Desert Southwest, it is no surprise that rainfall is scarce.

Phoenix, for example, has a yearly average rainfall of 7.22 inches, with no monthly average even reaching 1 inch.

But states like California and Arizona do experience bursts of moisture from the tropics several times during a year.

They can often bring several inches of rainfall. That may not seem like a lot to someone in the Southeast, but to the Southwest, this is a big deal.

Desert soil isn’t able to hold moisture as well as other regions, so it doesn’t take much rainfall before runoff and flooding begin. Dangerous land and rock slides are often not far behind.

And recent burn scars are even more susceptible to these dangerous debris flows and landslides.

Rob Shackelford is a meteorologist and climate scientist at weather.com. He received his undergraduate and master’s degrees from the University of Georgia studying meteorology and experimenting with alternative hurricane forecasting tools.