Flash Flood Watches in Arizona, California, New Mexico as Tropical Depression Sixteen-E Spreads Heavy Rain into Desert Southwest (FORECAST)

Jon Erdman
Published: September 21,2015

A tropical depression just 160 miles from the U.S. border is poised to deliver another round of heavy rain to the Southwest over the next couple of days.
Tropical Depression Sixteen-E made its second and final landfall in northwestern Mexico Monday morning, about 160 miles south-southwest of the bend in the Arizona-Mexico border. Moisture from the depression will contribute to widespread rainfall, some of it heavy enough to trigger flash flooding.
The flood threat follows the deadliest flash flood in Utah history last week, followed by one of the heaviest September rain events on record in Los Angeles and San Diego.
(MORE: Utah Deadly Flood | Strange S. Calif. Sep. Rain)
While much of the region west of the Rockies – California, in particular –  is desperate for a wet and snowy fall, winter and spring to battle a multi-year drought, heavy rain falling over urban areas, rocky terrain or in the desert can be dangerous.

Moist Surge Pattern Ahead

 

 

 

 

 

 

 

 

 

 

 

The Setup: A Tropical Tap

The Southern California event early last week featured remnant moisture from what was once Hurricane Linda.
This time, the moisture is courtesy of Tropical Depression Sixteen-E, which made landfall on Mexico's Baja California peninsula early Monday, crossed the Gulf of California, and made a second landfall a few hours later in the state of Sonora.
This moisture has already contributed to thunderstorms with very heavy rainfall over the past couple of days in northwestern Mexico. Mazatlan reported 122.5 millimeters (just under 5 inches) of rain in a 24-hour period Wednesday into Thursday morning, and noroeste.com reported flooding of some neighborhoods in Culiacán.
Now, the combination of upper-atmospheric low pressure several hundred miles southwest of Southern California and upper-level high pressure centered over the Rio Grande Valley of Texas is acting like an atmospheric egg-beater, piping deep moisture northward toward the Desert Southwest.
These summer surges of moisture in the wet phase of the North American monsoon are typical in the Desert Southwest. But some forecast guidance suggests the moisture content of the atmosphere early in the week ahead may approach record levels for the time of year in some parts of the Southwest.
Of course, it's not just deep moisture that guarantees heavy rainfall.
In this setup's case, the upper-level low along with the remnants of Tropical Depression Sixteen-E will track across Southern California and the Desert Southwest early this week, providing added instability and lift in the atmosphere for clusters of thunderstorms.

Flash Flood Watches Issued

Current Radar

Southwest Rainfall Outlook

Flood Alerts

City Five-Day Forecasts

Flash flood watches have been issued for portions of the Southwest, including much of southern California as well as a large part of Arizona and western New Mexico.
These areas, comprised mostly of deserts and mountains, are most at risk of scattered thunderstorms with torrential rainfall. At this time, it appears the threat of torrential rainfall will stay south of hard-hit southern Utah, and should also stay south of Las Vegas.
Here's what to look out for:

• Primary Threat: Locally heavy rain and flash flooding in urbanized areas, as well as normally dry washes, arroyos and canyons. Debris flows are possible in recent burn areas.
• Timing: Rain started to move north into southern Arizona and southwestern New Mexico during the morning Monday. Thunderstorm chances increase Monday afternoon and evening from southern California through central Arizona (including Greater Phoenix) into western New Mexico. The risk spreads into the mountains of northern Arizona and northern New Mexico Monday night. Scattered thunderstorms will continue Tuesday in many of these areas.
• Flooding rainfall most likely: Southern Arizona and southwest New Mexico, where widespread rainfall totals of 1 to 4 inches are expected along with locally higher amounts.
• Locally heavy rainfall also possible: Southern California, western and northern Arizona, New Mexico, southwest Colorado.

Rainfall totals have already begun to add up in far southern Arizona. Nogales, along the Mexican border, reported 1.21 inches of rain as of 10:09 a.m. MST Monday, with heavy rain still falling at that time.
By Wednesday, the upper-level low should have been kicked far enough north and east to allow drier air to return to the Desert Southwest, diminishing the areal coverage of thunderstorms and ending the heavy rain threat in most areas.
A few thunderstorms will linger Wednesday in the southern Rockies, but by that time the primary risk of heavy rainfall will shift into the central and northern Plains states.
(FORECAST DETAILS: L.A. | San Diego | Las Vegas | Phoenix)
If you have plans to visit the Desert Southwest, including the national parks of southern Utah, Arizona and the deserts of southeast California this week, remain aware of the danger of flooding, particularly in slot canyons and near normally dry washes.
(MORE: Your Vehicle is Dangerous In a Flash Flood)
As we saw in the deadly Hildale, Utah, flash flood, it doesn't have to be raining over a location in the Desert Southwest for a dangerous flash flood to ensue. Staying aware of the weather situation even miles away can save your life.

MORE: Deadly Utah Flooding