California Prepares for Chance of Flooding and Mudslides

Forecasters have been warning for days that an atmospheric river is going to douse parts of California this week, bringing torrential rain and the threat of flash flooding and mudslides to the Golden State. The forecast is becoming clear as the storm approaches from the Pacific Ocean. Here are what the latest models are indicating.

Atmospheric River to Create Threat of Flooding and Mudslides in California

A quickly-moving storm system is expected to send inches of rain and feet of snow across California in the coming days. The system is shaping up to be the biggest storm of the winter season thus far, sending a plume of moisture across the state. While heavy rain and the resulting risk of flooding and mudslides will be the primary concerns along the coastal areas and valleys, feet of snow is expected to blanket the Sierra Nevada.

The moisture is much needed in California as the state grapples with increasing levels of drought. The majority of the state did not see any meaningful moisture as the wet season kicked off in early October. The dry conditions are largely responsible for the deadly wildfires that roared through Southern California in January.

However, all of the rain and snow expected this week will come with its own set of risks. The firehose of moisture associated with the atmospheric river is expected to move to the south along the state's coastline, directing the heaviest of the rain along the shoreline and over into the Coast Ranges and the west-facing slopes of the Sierra Nevada.

You can expect the rain to begin to fall late Wednesday and into Thursday. The heaviest bands of rain and snow will set up on Thursday and Friday.

How Much Rain to Expect

The forecast is calling for widespread rainfall totals of 2 to 4 inches for the lower terrains of the coastline. The west and southwest-facing slopes of the coastal mountains are bracing for rainfall amounts of 4 to 8 inches.

A good portion of the rain is expected to fall in a period of 12 hours or less. The fast nature of this system will translate to a heightened risk of flash flooding, mudslides, and debris flows. This threat will be the greatest in recent burn scar areas. This is because the wildfires burned through the vegetation, leaving nothing to prevent the water and mudslides from roaring down these scars. Additionally, the runoff could contain toxic materials as not all of the cleanup from the fires has been completed.

Places such as Santa Barbara will be in the bullseye of some of the most potent rain bands. The resort city along the coast is forecast to see the showers begin on Wednesday afternoon. About a half of an inch of rain is expected overnight and into Thursday with 1 to 2 inches in store the next day. It will be a chilly stretch of days in Santa Barbara with highs hovering in the mid to upper 50s. Overnight lows will bottom out at the 50-degree mark. Winds will also pick up on Thursday, landing at speeds of 10 to 20 mph from the south.

You will find a similar forecast down the road in Los Angeles. Like Santa Barbara, the rain will become steadier after the sun sets on Wednesday. After picking up about a quarter of an inch of moisture on Wednesday night, Thursday will deliver another 1 to 2 inches with an additional half of an inch expected overnight and into Friday. The showers will linger in Los Angeles through Friday morning before finally clearing out just in time for the weekend.

San Diego will be a bit warmer and not quite as wet. This part of Southern California will see rain showers that are more intermittent. The most rain is expected to fall overnight Thursday with a half of an inch in the forecast. Highs will top out in the upper 50s on Wednesday before reaching the low 60s on Thursday. It will remain gray and dreary in San Diego until the sun makes an appearance on Saturday.

Expected Snow Accumulation

Winter sports enthusiasts are likely wondering how much fresh snow will come out of this latest winter weather event. Meteorologists are predicting that 2 to 4 feet of snow will land across elevations of over 7,000 feet in the Sierra Nevada. Amounts totaling 6 feet may be recorded in the highest ridges of this range.

Snowfall of this magnitude will create disruptions on the roads. Motorists planning to use Donner Pass along Interstate 80 in California will want to check the road conditions before heading out through the end of the week. Snowfall rates may reach 2 to 4 inches per hour over this heavily traveled pass during the peak of the activity.

Disruptive snow will also be a concern to the south. Mammoth Lakes, California is under a winter storm warning beginning Wednesday at 10 pm and lasting through 10 pm local time Friday. Snow accumulations ranging between 1 to 3 feet are expected along U.S. 395. The Sierra crest is anticipating snowfall of 4 to 5 feet with 2 to 8 inches predicted for the lower elevations located to the east of U.S. 395.

Winds will be whipping around at gusts of up to 100 mph on the ridges of this range. Gusts hitting speeds of 50 mph are a possibility at the lower elevations, sending the flakes flying and creating whiteout conditions at times. The mercury will hover in the mid 30s for highs in Mammoth Lakes through the end of the week.

California Moisture Levels Falling Short of What is Needed

Last spring was exceptionally wet across much of the West Coast, helping to fill crucial water stores. However, the fall months were much drier than usual for a good part of California. The storm train finally fired up at the end of January, working to begin to restore the water levels.

But climate experts and state officials are warning that snowpack is running well below the historical average across the Sierra. It has also been bone-dry in Southern California. This part of the state has only picked up 5 to 35% of its normal amount of rainfall since the beginning of the rainy season in October.

The upcoming weather event will be instrumental in helping to bring rainfall and snowfall amounts closer to the normal range by this point in the season. However, more weather patterns like this will be needed heading into the early part of the spring if California hopes to land at normal moisture levels again.

The snow across the Sierra Nevada is particularly important as this snowpack is what melts and runs off to fill the state's crucial streams, rivers, and reservoirs. This water is then tapped during the drier months of the year.

Looking ahead, the western U.S. will begin to dry out after this atmospheric river dissipates. But the long-range forecast is predicting that more significant moisture producers will roam the western U.S. closer to the end of February.

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