Moisture From Tropical Depression Sixteen-E May Bring More Rain, Flooding to Southern California, Arizona, Nevada, Utah (FORECAST)

Jon Erdman
Published: September 20,2015

The atmospheric pattern appears poised to deliver another round of heavy rain to the Southwest early this week thanks to moisture from Tropical Depression Sixteen-E.
Last week featured the deadliest flash flood in Utah history, 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 is located off Mexico's Baja California peninsula as of Sunday night.
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.
Into Monday, the combination of upper-level low pressure several hundred miles southwest of Southern California and upper-level high pressure centered over the Rio Grande Valley of Texas will act 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.

Southwest Rainfall Outlook

Flood Alerts

City Five-Day Forecasts

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Flood Threat Outlook, Uncertainties

Typical of any forecast a few days out, it is not possible to pinpoint where clusters of thunderstorms will form, and, thus, where areas of local flash flooding may occur.
Flash flood watches have been issued for portions of the Southwest, including southeastern California as well as central and southern Arizona.
In this case, slight changes in the positions of the aforementioned upper-level high and low can make a big difference in where the deep moisture plume ends up.
At this time, our forecast guidance is somewhat uncertain how far west that plume will be pulled temporarily into Southern California. The farther west that plume is pulled, the higher the local heavy rain threat for the Southland, including L.A. and San Diego.
Here's what we know and what we're still uncertain of right now:

• Primary Threat: Locally heavy rain and flash flooding in both urbanized areas, as well as normally dry washes, arroyos and canyons. Debris flows are possible in recent burn areas.
• Additional Threats: A few strong to severe thunderstorms, with damaging wind gusts and hail, may develop.
• Timing: Increased thunderstorm coverage may begin as soon as Sunday night, but more likely Monday through Wednesday. The heaviest and most widespread rainfall is expected Monday night and Tuesday.
• Where most likely: The Lower Colorado River Valley, including much of Arizona, low deserts of southeast California, southern Nevada, southern Utah and western Colorado
• Where also possible: Southern California (mountains, possibly valleys also), northern Utah

By Thursday, the upper-level low should have been kicked far enough east to allow drier air to return to the Desert Southwest, diminishing the areal coverage of thunderstorms and ending the heavy rain threat.
(FORECAST DETAILS: L.A. | San Diego | Las Vegas | Phoenix)
If you have plans to visit the Desert Southwest, perhaps 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