Southwest US braces for flooding rain, severe thunderstorms into late week
A large, developing storm system will bring rounds of heavy rain and thunderstorms to the Southwest from midweek into late week, raising the risk of flash flooding, damaging winds and travel disruptions.
Rain and mountain snow are on the way to the West Coast as moisture comes ashore from the Pacific.
A large storm will slowly spin into the Southwest during the middle and latter part of this week, connected to the jet stream and an uptick in tropical moisture. The storm will bring beneficial rain to drought-stricken areas and help lower wildfire threats, but it will also raise the risk of flash flooding, mudslides and severe thunderstorms.
Multiple lobes of rain will be associated with the massive setup, with rain reaching well inland from the Pacific coast to parts of the Rockies. A small amount of tropical moisture from Trudy may also become involved over the interior Southwest.
Rounds of showers were already pushing well inland over Oregon and Northern California on Tuesday. Showers will be in the San Francisco area through Thursday.
"The timing of the heaviest rain in Southern California is expected to be from Wednesday evening to the first thing Thursday morning," AccuWeather Senior Meteorologist Adam Douty said. "The storm will bring a general 0.50 to 1 inch of rain to low-elevation coastal areas of Southern California with heavier rain, averaging 1-2 inches on the south- and west-facing slopes of the hills and mountains in the region."
Mudslides and other debris flows can occur in any steep terrain, but areas that have recently experienced wildfires face a much higher risk.
The wettest period around Las Vegas and Phoenix is likely to be from later Wednesday to Thursday when multiple downpours are possible. Downpours in urban areas and deserts can lead to dangerous flash flooding. Normally dry streambeds and drainage basins can rapidly become raging rivers.
"Las Vegas has already picked up nearly four times their historical average rainfall for October with 1.19 inches falling through Tuesday," Douty said. "This storm will only add to that departure from average."
The showers and thunderstorms are likely to push eastward into parts of New Mexico and Colorado from Wednesday to Thursday as well, but they may also persist on Friday.
At least sporadic showers and storms will linger over the interior West into the weekend.
The same setup will bring a risk of severe weather from Wednesday to Thursday with the greatest risk of storms packing high winds and hail, along with flash flooding, being over the interior Southwest.
The main threat of severe weather will focus on Arizona on Wednesday.
On Thursday, the threat will expand northward and eastward, reaching a large part of Utah, western New Mexico and eastern Nevada, while continuing in central and eastern Arizona.
Meanwhile, along the coast of Southern California, a few embedded thunderstorms will pose the risk of producing a couple of brief waterspouts and tornadoes from Wednesday afternoon to early Thursday.
Since part of the storm setup involves a large dip in the jet stream, which is associated with chilly air in the upper levels of the atmosphere, some snow will fall over the ridges and peaks of the Sierra Nevada. While some wet snowflakes can mix in from time to time, no snow accumulation is anticipated over the passes.
The large storm with its vast areas of rain will bring another round of runoff into the Colorado River basin. It will take much more rain than this storm can provide to ease the historically low water levels, but the impending rain will help somewhat in the short term.
Water levels have risen about 5 feet from its lowest level on Sept. 16, on Lake Powell, while water levels continue to fall farther south along the Colorado River basin at Lake Mead.
It will take a winter full of storms with heavy mountain snow to significantly raise water levels during the subsequent spring thaw due to vast areas of extreme to exceptional drought in the region.
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