Atmospheric river to soak Southern California, Southwest

Alex Sosnowski

AccuWeather senior meteorologist

Following a delayed start to the wet season in fire-ravaged California, the switch has been turned on as a series of storms continues to douse the Golden State.

Soon after a storm over Northern and Central California winds down, a new plume of moisture, or an atmospheric river, a plume of tropical moisture in the atmosphere that can result in heavy rainfall or snowfall in a narrow swath, will unleash heavy rain and high country snow over Southern California to the interior Southwest at midweek.

North-central Montana is expected to continue to see relatively mild weather through the week, although a winter weather advisory on the northern Rocky Mountain Front and across the Great Divide was in place through noon today. The advisory called for blowing and drifting snow and reduced visibility and some new snow in the West Glacier area.

The forecast for Liberty, Chouteau, Hill and Blaine counties calls for highs in the 30s to 40s most of the week, with chillier temperatures in the upper 20s and 30s Thursday with a slight chance of snow then partly sunny that day in Liberty and Hill counties.

Lows are expected to be in the teens to 20s in those counties through the week.

The storm in California and the interior Southwest is not expected to unleash the equivalent of the late-November blast of rain and snow; however, it is likely to cause travel disruptions in many locations and perhaps more serious problems in others.

Snow levels are forecast to be rather high in Southern California, so motorists should not have to contend with winter conditions over Tejon and Cajon passes this time around.

Snowfall with the storm from Tuesday night to Wednesday night will generally be limited to elevations above 6,500 feet over much of Southern California and Arizona but can dip to 5,500 feet over the southern part of the Sierra Nevada, where 1-2 feet of snow is in store.

High snow levels can present a problem following a heavy snowfall, AccuWeather Meteorologist Randy Adkins said.

“The higher snow levels throughout the region will mean that rain will eat into some of the snowfall over the intermediate elevations in Southern California and Arizona in particular,” Adkins said.

The combination of rain, rising temperatures and melting snow can lead to flooding along some the small streams and short-run rivers flowing out of the mountains.

A general rainfall of 1-2 inches is forecast to fall in about 24 hours over coastal Southern California with 0.50 of an inch to 1 inch over the deserts.

Heavier rainfall is expected over the mountains. An AccuWeather Local StormMax™ of 4 inches can occur in the south- and west-facing slopes of the San Gabriel Mountains.

This is enough rain can to cause localized flash flooding in desert and urban areas.

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Havre Daily News staff contributed to this report.