Moisture-packed storm to slam West Coast into midweek
AccuWeather forecasters are closely monitoring a storm that will unleash heavy rain and mountain snow across the West Coast of the United States into the middle of the week. Impacts ranging from flooding to high winds and major travel disruptions could be widespread -- and this will just be the start of what is shaping up to be a stormy end to 2022.
The storm that AccuWeather meteorologists are monitoring will arrive on the heels of several waves of rain, snow and ice across the Pacific Northwest into the start of the new year.
On Monday, the most potent area of low pressure in recent days arrived and it will continue to spread heavy rain inland across the Northwest as well as southward on Tuesday, extending into the San Francisco Bay area, according to AccuWeather Meteorologist Andrew Johnson-Levine.
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"The source of this unsettled weather is an atmospheric river or a long plume of deep moisture that stretches for many hundreds of miles across the Pacific. As this moist air reaches the West Coast and is forced upward by the steep terrain, steady and heavy rain is the result," Johnson-Levine explained.
Rain will continue to spread southward heading into the middle part of the week, with places as far south as Los Angeles and San Diego likely to be drenched late Tuesday into Wednesday.
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"San Diego has only received around half of their typical December rainfall so far, so any rain there will be beneficial," Johnson-Levine said.
Other portions of California could also use more rain in the forecast, with severe to exceptional drought ongoing across a majority of the Golden State, according to the latest outlook from the United States Drought Monitor. Moderate drought to abnormally dry conditions extend into the Interstate-5 corridor of the Pacific Northwest as well as the northern Rockies.
"This rain will be a big help, but will not fully end the drought in the state," Johnson-Levine said.
Although the rain will certainly help to chip away at these long-term precipitation deficits, the heavy rain may fall too quickly and result in flooding concerns, forecasters say.
"Rain is expected to become quite heavy at times, especially in the northern half of California. With rain falling so intensely, this is the region where localized flooding and mudslides will be most likely," Johnson-Levine said.
Rainfall amounts of 2-4 inches will be widespread along the coast from Washington to Oregon and Northern California. One to 2 inches of rain could fall as far south as San Luis Obispo, California, and perhaps to Point Conception. In Southern California, rain amounts generally under an inch are expected, but motorists and air travelers should anticipate delays even with these lower totals.
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Road conditions along stretches of Interstates 5, 10, 15 and 80 are likely to be slick as a result of the rain mixing with oil residue. Ponding of water on the roadways will further increase the risk of vehicles hydroplaning while traveling at highway speeds.
Along with the rainy weather will arrive wind gusts on the order of 40-60 mph, which can lead to downed trees and power lines, especially as the ground becomes increasingly saturated. Western Washington, Oregon and Northern California will be most at risk for experiencing these high winds.
In terms of wintry precipitation with the storm, AccuWeather meteorologists say heavy snowfall can be anticipated across the Sierra Nevada, with over a foot possible across the highest peaks by the middle of the week. This includes Donner Pass, where motorists should be prepared to face deteriorating road conditions from late Monday night to Wednesday.
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A burst of accumulating snow is also likely across the Washington and Oregon Cascades, as well as the northern Rockies. Snow may also mix with rain across the higher elevations of Nevada.
"By Thursday of this week, the storm system responsible for the heavy rain will shift inland, taking the precipitation with it. But with the atmospheric river aimed right at the West Coast, more rounds of rain are very likely to follow as we head into January," Johnson-Levine said.
Prior to the start of 2023, as many as two more rounds of stormy weather may sweep across the West Coast.
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