An atmospheric river is set to move toward Southern California and has potential for a record-breaking plume of moisture toward the region beginning Tuesday throughout the end of the week. With the potential for rainfall rates of up to one inch per hour for some locations, there is a moderate risk for flash flooding north and west of Los Angeles on Wednesday.
All of the activity will begin on Tuesday for the West Coast. Rain showers will sneak into coastal California during the morning hours. Then during the afternoon and evening, a more widespread, light to moderate rain will work into most of the remainder of the state with the exception of the desert region toward Arizona while snow begins to impact much of the Sierra Nevada. Then overnight Tuesday, the snow will persist in the Sierra while rounds of rain continue to pummel much of the Golden State. A few scattered showers may also work into southern Oregon and southern Nevada.
Wednesday will be the big event. Not only is the heaviest of precipitation set to slam the state but the risk for flash flooding and landslides increases, especially in the burn scar areas of Southern California. This is due to the maximization of anomalous moisture associated with the atmospheric river moving into the region, especially the central part of the state. So on Wednesday, a steady light to moderate rain with embedded pockets of heavier rain is forecast to move in. It will be more scattered at the northern and southern ends of the state while the central half gets slammed. Then as we get into Wednesday night, the atmospheric river will begin to slowly drift to the south, so that will put cities like Los Angeles and San Diego for flooding rains. A widespread, steady rain is forecast to impact the entire state Wednesday night. Showers will also expand into much of Washington, Oregon, Nevada, and portions of Idaho and Utah. Meanwhile in the higher elevations, snow will be falling in the Sierra Nevada and portions of the Cascades.
By Thursday, snow levels will drop in the Sierra as cold air works down from the atmosphere. Snow will also continue or expand into the San Gabriel Mountains, the Cascades, and the Bitterroots. Flooding rains will continue for the remainder of California while lighter rains fall across the Northwest and the similar areas as Wednesday night. Thursday night, rain will clear out of California from south to north. Showers will continue throughout much of the night in Southern California while Northern California will dry out in the evening. Snow will persist, specifically in the Sierra Nevada. Snow is also forecast to continue in the Cascades and now portions of the Rocky Mountains. Rain showers will slide into northern and western Arizona.
Then on Friday, the first storm will bring wet and snowy weather depending on elevation to much of the Four Corners States. This will happen at the same time as a new storm moves into the Northwest, allowing for snow to continue in the Cascades. Snow will also return to the mountains of Northern California. In the lower elevations, it will be rain that falls from coastal Washington, Oregon, and Northern California. Overnight Friday, there won’t be too many changes. Much of California will be dry with the exception of snow in the northern section of the state as well as in parts of the Sierra Nevada. Snow will also continue to fall in the Cascades of Washington and Oregon plus inland portions of the state and the Bitterroots in Idaho and Montana.
By the weekend, more storms will move in, but they won’t be as significant as this minor one, keeping the Pacific Northwest wet while California experiences a break from the precipitation. Through Saturday, up to 100 inches of much-needed snow is forecast to fall in the peaks of the central and southern Sierra Nevada.