Measurable rainfall in the first week of June has only been recorded 21 times since 1896, according to weather records. The rain would be welcome after a winter of below-normal precipitation.
A storm spinning off the coast of Baja California in Mexico on Saturday was poised to dive into the Southwest United States and drag with it remnants of post-tropical storm Alvin.
This system, which is uncommonly wet for this time of year, will bring a rare chance for thunderstorms and brief heavy downpours to the region, especially to southeast California, southwest New Mexico and southern Arizona, including Phoenix, Sunday into Monday.
The rain would be much welcome in an area with widespread drought conditions after a winter of below-normal precipitation.
“For this time of year this is quite unusual,” said Mark O’Malley, a meteorologist with the National Weather Service office in Phoenix.
The Weather Service’s official gauge for the Phoenix area is at Sky Harbor International Airport. It has recorded measurable rainfall in the first week of June on 21 occasions, with records going back to 1896.
“Normal rainfall is zero,” Mr. O’Malley said of the first week in June.
There’s a 75 percent chance the airport will record rain on Sunday afternoon or evening, with rainfall chances continuing into Monday.
A thunderstorm or two could move over the airport and bring half an inch of rain, or the downpour could hit 10 miles west of the airport, and there would hardly be any rain.
This unusual weather setup will bring a chance for rain and thunderstorms to most of the Southwest on Sunday into Monday, including southeastern California, southern Nevada, all of Arizona, western New Mexico, the Wasatch Mountains in Utah and portions of the Colorado Rocky Mountains.
“This is a fairly large swath of moisture, so I’d actually say, there’s not just a chance of rain, but rain is likely,” said Peter Mullinax, a meteorologist with the Weather Service’s Weather Prediction Center.
The chances are highest across southern Arizona, southwest New Mexico and southeast California, and the Weather Prediction Center has put this area under a marginal risk — level 1 out of 4 — for excessive rainfall that could lead to flash flooding on Sunday. A slice of Southern Arizona is at a higher slight risk, level 2 out of 4.
Mr. O’Malley said that minor flooding on roadways in the greater Phoenix area is possible.
Storms occasionally pass over the Southwest in late spring but they’re usually dry. Rain is more common during the monsoon season that starts June 15 and lasts into September.
“These storms come through and you’d never know, other than a little wind,” Mr. O’Malley said. “With this storm, you have that moisture that’s being pulled in from Alvin — that’s the big difference.”
Mr. Mullinax said there’s also a strong southerly wind component that’s escorting the tropical moisture northward into the Southwest.
Alvin formed over the Pacific Ocean off the coast of west Mexico on Thursday, sending pounding surf to west-central Mexico and southern Baja California. The system has since dissipated and was a post-tropical cyclone over the North Pacific Ocean on Saturday.
