Heavy rain, flooding from Pacific storms to focus on California leading up to Christmas
The train of storms, including a pineapple express that has focused on western Washington, will shift southward this weekend to concentrate on California through much of the week of Christmas. As the rounds of rain intensify, so will the risk of flooding and mudslides.
Pineapple express to visit Northern California starting this weekend
“The upcoming atmospheric river that will focus on Northern California from late Saturday night to Monday qualifies as a pineapple express,” AccuWeather Senior Meteorologist Brett Anderson said.
A pineapple express is a specific atmospheric river, or plume of moisture, that extends from near Hawaii to the West Coast of the United States.
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The anticipated heavy rain will follow a storm that will bring heavy rain and flooding to western Oregon to close out this week. Some rain from that storm will affect part of Northern California into Friday night.
The storm into the end of the week will deliver 4-8 inches of rain to the northwestern corner of California with a general 0.50 of an inch to part of the Sacramento Valley and 4 inches to portions of the Siskiyous and the northern Sierra Nevada.
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“The storm from Saturday night to Monday will bring more substantial rain to Northern California,” AccuWeather Lead Storm Warning Meteorologist Michael Stahlman said.
There is the potential for several inches of rain to fall in a few hours at the height of the atmospheric river from Sunday to Monday in the west-facing slopes of the coast ranges, Siskiyous and northern Sierra Nevada.
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“Pockets of 6-12 inches are forecast, especially in the lower and intermediate west-facing slopes of the northern Sierra Nevada,” Stahlman said. That amount of rain in a span of 12-48 hours is more than enough to unleash rapid small stream and short-run river flooding.
San Francisco’s rain is projected to range between 2 and 4 inches. Roads will become slick with ponding in low-lying areas. Where leaves block storm drains, more significant urban flooding is likely to occur.
New Pacific storm to form Christmas week, reach much of California
The atmospheric river is forecast to abate from Monday to Monday night as a new storm develops just off the West Coast of the United States during the middle of next week.
Rain will become more spread out over much of California during the week of Christmas.
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“Due in part to the atmospheric river at the start of the week, additional rain from Tuesday to Thursday in parts of Northern California, particularly in the west- and southwest-facing slopes of the mountains, will lead to a medium to high risk of flooding,” AccuWeather Storm Warning Meteorologist Julia Williams said.
It is possible that rain begins in parts of coastal Southern California as early as Tuesday.
Drenching downpours and urban flooding problems are predicted to occur in the Los Angeles and San Diego metro areas from Wednesday to Friday, which may spoil outdoor Christmas-related activities. At least some rain is likely to fall on these areas during Christmas Day. Some showers will reach the California deserts well inland. Showers may extend as far inland as Las Vegas and Phoenix.
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“As the rain expands into the southern part of California, there is a medium to high risk of flooding, especially in the south- and west-facing slopes of the Transverse Mountains in the region from Tuesday to Thursday,” Williams said.
Meanwhile, in western portions of Washington and Oregon, the weather will trend less rainy and snowy, when compared to recent weeks. However, it is unlikely that the week of Christmas will be totally free of rain and mountain snow in these areas.
Sierra Nevada snow upcoming
Snow levels will start off high this weekend but will fluctuate in the Sierra Nevada through the week of Christmas. Several feet of snow are expected on the ridges and peaks of the Sierra Nevada, which will be a boon for ski enthusiasts over the holidays and a future boost for runoff during the spring and early summer, as little to no snow has fallen thus far this season.
However, snow will periodically dip down to Donner Pass, California, along Interstate 80, and that will pose difficulties for travel. Motorists are encouraged to check the local AccuWeather forecast daily for updates on when the most intense snow will fall in the region.
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