Las Vegas, one of the driest major cities in the country, has already recorded nearly four times its normal October rainfall, and another soaking is on the way. AccuWeather Senior Meteorologist Adam Douty said the city had picked up 1.19 inches through Tuesday, and that a slow-moving storm arriving late Wednesday into Thursday "will only add to that departure from average," according to AccuWeather.

The bigger flood threat sits farther east. The Weather Prediction Center's Day 2 Excessive Rainfall Outlook, issued at 4:03 a.m. EDT Wednesday and valid from 8 a.m. EDT Thursday to 8 a.m. EDT Friday, places portions of southwest Colorado in a Moderate Risk, level 3 of 4, with a Slight Risk, level 2 of 4, across much of the Four Corners, including higher-end Slight Risk areas in Arizona, New Mexico and Utah.

For people from Las Vegas to Flagstaff, Silver City and Durango, the practical question is simple: where water will move fast and with little warning. That means dry washes, arroyos, slot canyons, burn scars and low-water crossings, which can turn dangerous within minutes.

A Second Soaking After Rachel

This storm follows a wet start to the month. The remnants of Tropical Storm Rachel dropped 1.08 inches on Las Vegas into Sunday morning, Oct. 11, tying the city's all-time daily rainfall record for October, a total last reached in 1947, the National Weather Service said, according to The Business Journal. Videos showed water rushing down a street near the Strip.

The new system is not Rachel. It is a deep upper-level low moving from California into the Great Basin and pulling tropical moisture northeast, with a small contribution from the remnants of Trudy, according to AccuWeather. WPC forecasters noted precipitable water of 1 to 1.5 inches, very high for October, and said hourly rain rates of up to 2.5 inches are possible with "at least locally significant impacts."

The National Weather Service office in Las Vegas has said the heaviest rain and mountain snow above 9,000 feet should arrive late Wednesday into Thursday, followed by drier, cooler weather by the weekend. WPC's Thursday outlook text did not single out the Las Vegas Valley, so residents there should watch for any watches or warnings from their local office.

Flood Watches Stretch From the Grand Canyon to the Gila

The NWS office in Flagstaff issued a Flood Watch at 5:53 a.m. MST Wednesday that runs through 11 p.m. MST Thursday for much of northern and central Arizona, including Grand Canyon Country, the Mogollon Rim, the White Mountains, Oak Creek and Sycamore Canyons, and parts of Apache, Coconino, Gila, Navajo and Yavapai counties. The watch warns that creeks, normally dry washes, swimming holes and recently burned areas could flood.

In southwest New Mexico, the NWS office in El Paso issued a Flood Watch covering the Silver City area, the Black Range and the Upper Gila River Valley in Grant, Hidalgo and Sierra counties, with scattered to numerous storms expected Thursday afternoon into Friday morning. Third-party alert listings show the watch running until 6 a.m. MDT Friday, though readers should confirm the end time directly with the El Paso office.

Near Phoenix, foothills and higher terrain north and east of the city could see 1.5 to 2.5 inches, while lower deserts are expected to get less than an inch, according to NWS forecasts summarized by The Watchers. In southwest Colorado, WPC coordinated its Moderate Risk with the NWS office in Grand Junction, citing freezing levels near mountain peaks that would keep most precipitation falling as rain.

Desert Ground Turns Rain Into Runoff

It seems backward, but the driest places often flood the fastest — and the reasons are built into the ground. Desert soils are frequently baked hard or capped by a crust that sheds water instead of absorbing it, and sparse vegetation does little to slow runoff. In cities, pavement and rooftops send water straight into streets and channels.

That runoff collects in washes and arroyos that sit dry most of the year, which is why AccuWeather warned that dry streambeds can quickly become fast-moving water in Las Vegas and Phoenix. A storm miles away can send a wall of water down a canyon under clear skies, a key danger for hikers in slot canyons and drivers at dips in desert roads.

Wildfire burn scars raise the stakes further. Burned soils repel water and lose the plants that hold slopes together, so heavy rain can trigger mudslides and debris flows, a concern AccuWeather flagged for recently burned areas of Southern California and the interior Southwest.

Staying Safe Around Washes and Low Crossings

The most important rule is the National Weather Service's "Turn Around, Don't Drown" guidance. Never drive or walk into moving water, even if it looks shallow. About 6 inches of fast water can knock an adult down, and about 12 inches can carry away a small car. Barricades at flooded crossings are there for a reason.

Hikers should check forecasts and park alerts before entering slot canyons or narrow drainages and leave immediately if water rises, debris appears, or the sound of rushing water grows. Residents near washes should know their route to higher ground, keep children and pets away from channels, and have a way to receive Flash Flood Warnings on their phones.

Rising water, muddy or debris-filled flow, a sudden roar upstream, and water over a roadway all call for immediate action. If you are trapped by rising water, get to higher ground and call 911. Standard homeowners policies generally do not cover flood damage, so property owners should photograph belongings and review flood coverage before the next storm.

The rain carries one benefit — though only a partial one. AccuWeather noted it should help drought-stricken areas and reduce wildfire risk, though it will not reverse low reservoir levels, with Lake Mead still falling.

WPC will update its outlook several times daily, and local NWS offices will issue warnings as storms develop. Nature World News will update this story as watches change. The bottom line: the heaviest risk falls Thursday and Thursday night across the Four Corners, with southwest Colorado most at risk, and the safest move anywhere in the desert Southwest is to stay out of washes and turn around at flooded roads.

What Readers Want to Know

Where is the flash flood risk highest?

Southwest Colorado, under WPC's Moderate Risk for Thursday and Thursday night, with Slight Risk across much of Arizona, New Mexico, Utah, and the Four Corners.

Is Las Vegas under a flood watch?

WPC's Thursday outlook text did not single out the Las Vegas Valley, but NWS Las Vegas expects the heaviest rain late Wednesday into Thursday. Check weather.gov/vef for current alerts.

Which areas have flood watches?

Much of northern and central Arizona through 11 p.m. MST Thursday, and parts of southwest New Mexico including the Silver City area from Thursday afternoon into Friday morning.

Why do desert cities flood so quickly?

Hard, dry soils and pavement shed water rather than absorbing it, sending runoff into washes and streets within minutes.

How much rain has Las Vegas had this month?

1.19 inches through Tuesday, nearly four times the October average, including a record-tying 1.08 inches from Rachel's remnants on Oct. 11.

What should I do if water covers the road?

Turn around. Never drive or walk through moving water, and call 911 if you are trapped.

When will conditions improve?

Forecasters expect drier, cooler weather by the weekend, though scattered showers may linger over the interior West.