Rachel could bring up to 5 inches of rain and flash flooding to parts of the Southwest this weekend

The National Hurricane Center’s rainfall forecast for Tropical Storm Rachel calls for 1 to 3 inches across the Southwest United States this weekend, with local totals up to 5 inches, enough to raise the risk of flash and urban flooding. Rain is forecast to spread inland from Saturday night, Oct. 10, as the storm’s remnants cross the desert on Sunday.

The coast gets the wind and surf. For Arizona, Nevada, Utah and inland California, the danger is water running off dry ground and into washes, slot canyons and burn scars.

The 5-Inch Figure in NHC’s Rainfall Forecast

The 5-inch number comes from the hurricane center’s Intermediate Advisory 50A, issued at 11 a.m. PDT on Oct. 9, and it was repeated in the 8 p.m. PDT Advisory 52. Both give 1 to 3 inches for the Southwest, locally up to 5 inches, and 1 to 3 inches for parts of Baja California through the weekend. The words “up to” matter: 5 inches is a local maximum, not a regional average.

In its Discussion Number 52, the hurricane center ranked rainfall as its third key message, saying moisture from Rachel will increase across the Southwest and northern Baja through the weekend, creating a risk of significant flash and urban flooding, in forecaster Adams’ discussion. Rachel itself was 45 mph and 720 miles south-southwest of San Diego at that hour, moving northeast at 9 mph.

The same discussion explains how the rain reaches the desert after the storm’s wind is gone. The low-level center is likely to dissipate over the Peninsular Ranges of northern Baja and southern California, while the mid-level center keeps moving into the southwestern United States, carrying the tropical moisture with it. NHC forecasts rapid weakening once the storm moves inland on Sunday, and it expects strong, gusty winds over interior Southwest areas, especially higher terrain, from the weekend into early next week.

WPC Moderate Risk of Excessive Rainfall for Saturday and Sunday

The National Weather Service’s Weather Prediction Center turns the tropical forecast into a flooding risk by region. In its excessive rainfall discussion, issued 8:59 p.m. EDT on Oct. 9 and updated at 0102 UTC on Oct. 10, forecasters have a Moderate Risk for Saturday, Oct. 10 into Sunday morning covering portions of southern California, northwest Arizona, southern Nevada and southwest Utah.

A 21Z update raised the Moderate Risk over the Peninsular Ranges as tropically enhanced rain from Rachel moves across southern California, with 1 to 3 inches possible mainly in the southern California mountains. A Slight Risk was added for southeast California, southern Nevada and northwest Arizona for Saturday night.

Sunday, Oct. 11 carries the larger signal. The Moderate Risk for that day follows the expected low track from southern California to northeast Utah and southwest Wyoming, where WPC expects the heaviest totals, and earlier versions of the discussion put the greatest concern from Riverside County into northwest Arizona, southern Nevada including Las Vegas, and the slot canyon country of southwest Utah.

Track Uncertainty in NWS Las Vegas and Phoenix Forecasts

The local offices were more cautious about amounts. NWS Las Vegas, in a forecast discussion issued at 3:32 p.m. PDT on Oct. 8, said the forecast track had shifted east, so the brunt of Rachel’s remnants was expected over western Arizona, with less confidence in totals west of the Colorado River. It flagged gusts of 40 to 50 mph or greater across the lower Colorado River Valley and western Arizona on Sunday, with the remnants racing across northern Arizona and southern Utah that evening.

NWS Phoenix, in its discussion of 4:20 p.m. MST on Oct. 8, put rain chances at about 50 to 60 percent or higher area-wide from Saturday night through Sunday, and described amounts as highly uncertain and dependent on Rachel’s final track. Both discussions predate the 8 p.m. PDT Oct. 9 NHC package, so their rain and track details were written before the latest warnings went up in Baja.

Monday and Tuesday Rain on Slot Canyons and Burn Scars

The threat does not end when Rachel’s center dissipates. WPC’s discussion carries a Slight Risk of excessive rainfall for portions of Arizona into Utah from Monday, Oct. 12 into early Wednesday, with a new Day 5 Slight Risk for Tuesday. Remnant moisture from Rachel, combined with an upper trough, could produce hourly rain of up to about 1 to 1.25 inches, which WPC says could cause problems in slot canyons, burn scars, arroyos and urban areas.

Set against NHC’s local maximum of 5 inches, an hour at 1.25 inches amounts to a quarter of the entire event total, which is why slot canyons, arroyos and burn scars appear by name in WPC’s wording rather than the broad region alone.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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