Saturday, September 19, 2015

Desert Southwest Moisture Surge May Bring More Rain, Flooding to Southern California, Arizona, Nevada, Utah (FORECAST)

Jon Erdman
Published: September 18,2015

This week we've already seen 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.
Unfortunately, the atmospheric pattern appears poised to deliver another round of heavy rain to the Southwest in the week ahead.

Moist Surge Pattern Ahead

 

 

 

 

 

 

 

 

The Setup: A Tropical Tap

The Southern California event earlier in the week featured remnant moisture from what was once Hurricane Linda.
This time, there may not be a hurricane, tropical storm, or even a tropical depression off Mexico's Baja California peninsula. However, a batch of deeper moisture is hovering over the far eastern Pacific Ocean and the west coast of Mexico.
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.
Starting this weekend, 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 should eventually get kicked northeastward across Southern California and the Desert Southwest next week, providing added instability and lift in the atmosphere for clusters of thunderstorms.

Southwest Rainfall Outlook

City Five-Day Forecasts

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Flood Threat Outlook, Uncertainties

Typical of any forecast several days out, it is not possible to pinpoint where clusters of thunderstorms will form, and, thus, where areas of local flash flooding may occur.
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:
  • Threats: 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.
  • Timing: Increased thunderstorm coverage may begin as soon as Sunday afternoon, but more likely Monday through Wednesday.
  • 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 of next week, 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 in the week ahead, 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

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