Tropical Storm Rachel weakened into a post-tropical cyclone Saturday night as it approached and moved onshore in Mexico's northern Baja California Peninsula late Saturday night. Its forecast track takes the landfall point just south of the U.S. border, but the storm's impacts spread far north: San Diego is under a tropical storm watch, the National Weather Service expects peak wind gusts of 25 to 40 mph at lower elevations and 50 to 70 mph in the mountains early Sunday, and a Level 3 of 4 flooding rainfall threat is in effect from Saturday night through Sunday across Southern California, southern Nevada, southwest Utah and northwest Arizona.

This is only the second time in San Diego's history that the city has been under a tropical storm watch, and the first since Hurricane Hilary in August 2023 — the year such forecast alerts first began being issued for the region. According to CNN, citing the NWS San Diego office, tropical weather systems affected Southern California infrequently before 2023, but none triggered watches or warnings. Per FOX Weather, the FOX Forecast Center warns that months of rainfall could fall in just a few hours: desert locations could pick up two months' worth of rain, with up to 2 inches at lower elevations and as much as 5 inches at higher elevations, while Las Vegas could see up to 2 inches — triple its usual October total.

The Weather Prediction Center has placed a Level 3 of 4 flash flood threat — meaning numerous flash flood events with significant impacts possible — on areas including San Diego and Las Vegas, per CNN. Across Arizona, flood watches run from 11 a.m. Oct. 10 through 5 a.m. Oct. 12 for Mojave, Yavapai, western Coconino, northern Gila and far southern Navajo counties, with a moderate 40% flash flood risk for western Mohave County, according to The Arizona Republic. Wind Advisories cover nearly the entire state, with sustained winds up to 35 mph and gusts to 55 mph, and Winter Storm Warnings are in place across western Montana, per FOX Weather, as the storm's moisture fuels the Lower 48's first significant fall snowstorm with 6 inches to a foot possible.

Coastal flooding is the most visible face of the storm. A coastal flood warning runs from the Malibu coast and Los Angeles County beaches to coastal San Diego County until Sunday afternoon, and in Newport Beach, powerful waves have slammed the Balboa Peninsula, flooding streets Friday and drawing crowds to watch the rough surf. The National Hurricane Center says water levels are running as much as 1.5 feet above predictions and warns of life-threatening surf and rip currents along Southern California and northern Baja. The danger is amplified by the near-record strong super El Niño, which has raised West Coast water levels — in some areas, homes have already been lost to the sea.

The heaviest flash flood threat falls on terrain poorly built to handle it. FOX Weather flags Riverside County in southeast California, northwest Arizona, southern Nevada and the slot canyon regions of southwest Utah as areas of greatest concern, since desert ground cannot rapidly absorb intense bursts of rainfall. San Diego and Las Vegas could each receive an inch or more — about double San Diego's usual October rainfall. FOX Weather also warns people not to drive through flooded low-water crossings, and Arizona forecasters note blowing dust from gusty winds as an added hazard. Remnants of the system could keep hazardous coastal conditions going next week even after Rachel dissipates over the Southwest, per the Hurricane Center.

Rachel's remnants spread rain into southern Nevada and Las Vegas on Sunday and across the Desert Southwest, with wind gusts of 45 to 55 mph expected inland across parts of Arizona, southern Nevada and Utah on Sunday. In Phoenix, The Arizona Republic forecasts a quarter to 1.5 inches of rain through Tuesday, scattered showers Monday and Tuesday, and a marked cool-down: highs dip from the lower 90s Saturday to the 80s Sunday, with temperatures possibly reaching the upper 70s by Tuesday and Wednesday before returning near 90. Across Arizona, the storm's best rain chances run through Oct. 13, after which a broader low-pressure trough could generate another round of showers and storms early next week.