A trio of tropical systems is driving significant Southwest flood threat

Remnants of two recent East Pacific hurricanes – Priscilla and Octave – plus another developing storm off the Mexican coast are setting up a long-lived channel of deep tropical moisture that is targeting the Desert Southwest. The result will be several days of flood risk, including what could be a major threat across Arizona to end the workweek.

The National Weather Service has placed a moderate (Level 3 out of 4) flood risk in that state on Friday, including the cities of Phoenix and Flagstaff. There are also flood watches issued for portions of southeast California, eastern Nevada, much of Arizona, southern Utah and into southwest Colorado.

“Significant impacts are possible given the prolonged heavy rainfall potential,” the NWS warned, pointing at “parallels to early October 2018, when remnants of Hurricane Rosa moved into the Great Basin and prompted extensive flash flood impacts across central Arizona.”

Rainfall and flood potential begins to really ramp up Thursday in the United States and Mexico as Priscilla and the other systems move closer. Next week, rain risks then linger through at least early or midweek. By then, widespread zones of 2 to 5 inches of rain are probable, with more than a half-foot possibly falling in some locations.

The East Coast is also staring down a flood and storm threat this weekend into early next week thanks to a nontropical coastal low that will plague that region.

While there is much uncertainty as to where and when the heaviest will fall there, some locations near the shore could pick up as much as a half-foot of rain.

The early signs of deluges to come are already in Arizona and New Mexico. Showers, some heavy, covered portions of both states early Thursday. Priscilla has additionally caused tidal and inland flooding across recent days in western Mexico.

A handful of flash flood warnings in New Mexico were issued Wednesday with reports of high water over roadways south of Roswell.

Showers and thunderstorms should become numerous Thursday from southeast California through large chunks of the Four Corners states of Utah, Arizona and also into western Colorado and New Mexico. With the main slug of moisture still nearer to Baja California, Thursday’s rain is somewhat hit-or-miss but could surpass 1 inch, especially in the western half of the region.

Rain is increasingly widespread Friday when a large swath of 1 to 2 inches or more could fall from Arizona toward the Four Corners with some locations perhaps picking up twice that much rain. A similar story is expected for Saturday: The zone of heaviest rainfall risk focuses from central Arizona to southwest Colorado.

From Sunday through into Monday, a sinking front should push the heaviest rainfall south, focused on both sides of the international border. Scattered showers linger longer and may shift back north with an approaching storm system from the West Coast.

In an arid region, any sustained high humidity is noteworthy. This round is poised to break October records.

Precipitable water – a measure of moisture in the air – is forecast to reach more than 300% of normal from eastern California and southeast Nevada eastward through the Rockies per weather modeling. “Grand Junction has never had precipitable water over an inch (25mm) so late in the year, but looks like it will be near that level on both Fri & Sat,” Colorado climatologist Russ Schumacher wrote in a post on Bluesky. “Six standard deviations above average!”

The primary cause is the multiple areas of tropical storminess off the Mexican coastline on Thursday. The moisture is also being pulled northward by strong high pressure centered near the southwest Texas and Mexico border region and a well-positioned jet stream near the West Coast.

Most of the Desert Southwest has suffered expanding drought throughout 2025, despite focused bursts of rainfall during the summer monsoon.

Arizona is entirely covered by at least moderate (Level 1 out of 4) drought, according to the U.S. Drought Monitor, while extreme (Level 3 out of 4) drought covers a quarter of the state.

A majority of locations across the Four Corners region have picked up about 35 % to 70 % of average precipitation this year.

Dryness may both ease and exacerbate flood concerns. In areas where rain does not fall heavily, flood risk may be lower than normal. Where it comes down rapidly, the rock-hard soil will have trouble soaking rain up, and runoff quickly turns excessive.

A storm system from the north Pacific Ocean moving into the Pacific Northwest and eventually California will ultimately bump the tropical moisture plume out of the southwest U.S. next week.

While this storm could bring additional unsettled conditions, it shouldn’t offer much more precipitation, and drier conditions should follow.