Flash Flood Threat in Arizona, California, New Mexico as Remnants from Tropical Depression Sixteen-E Spread Heavy Rain into Desert Southwest (FORECAST)

Jon Erdman
Published: September 22,2015

Remnants of a tropical depression may deliver another round of locally heavy rain in parts of the Southwest for one more day, Tuesday.
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. Tropical Depression Sixteen-E then dissipated Monday afternoon over northwestern Mexico. Moisture from the remnants of the depression will contribute to locally heavy 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 the remnants of Tropical Depression Sixteen-E, as we mentioned above.
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 near the northern Baja peninsula coast and upper-level high pressure centered over the Rio Grande Valley of Texas is acting like an atmospheric egg-beater, piping deep moisture northward into the Desert Southwest.
These summer surges of moisture in the wet phase of the North American monsoon are typical in the Desert Southwest. But deep moisture alone doesn't guarantee 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

Flash flood watches continue for portions of the Southwest, including parts of southern California, Arizona and western New Mexico.
These areas, comprised mostly of deserts and mountains, are most at risk of scattered thunderstorms with brief, heavy 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: Thunderstorm chances will continue into Tuesday from southern California through central Arizona (including Greater Phoenix) into New Mexico and southern Colorado.
• Flooding rainfall possible: Central, eastern Arizona, and southwest New Mexico, where local rainfall totals of 1 inch or more are possible.

Rainfall totals have already begun to add up in southern Arizona:

• Dan Saddle, northeast of Tucson, recorded 4.61 inches through 7 p.m. Monday.
• Park Tank, near Redington Pass, reported 3.94 inches through 7 p.m. Monday
• White Tail, near Palisades, measured 2.87 inches through 7 p.m. Monday.
• Nogales, along the Mexican border, reported 1.89 inches of rain as of 6:54 p.m. MST Monday. 
• Davis-Monthan Air Force Base in Tucson measured just over one inch of rain through 7 p.m. Monday.

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