How The Once-In-A-Decade 'Bomb Cyclone' Will Impact Southern California

SOUTHERN CALIFORNIA — As a "bomb cyclone" threatens flooding in Northern California and Oregon this week, forecasters in the southern part of the state are keeping a close eye on the once-in-a-decade system, which is expected to bring the first significant rainfall of the season to the region.

The powerful low-pressure system is expected to intensify on Tuesday as part of the rapidly decreasing pressure of “bombogenesis” — where bomb cyclone gets its name. In fact, it's expected to triple the criteria needed to be considered a bomb cyclone, bringing the heavy precipitation of an atmospheric river and strong winds in a storm so intense it occurs only "about once every ten years," the NWS in Medford, Oregon said.

We have initiated Key Messages for the powerful Pacific storm system set to produce a plethora of weather hazards across the Pacific Northwest over the next few days. Some of these hazards include excessive rainfall, flooding, heavy mountain snow, damaging winds, & pounding surf. pic.twitter.com/Jn5EjEOP7E
— NWS Weather Prediction Center (@NWSWPC) November 19, 2024

More than a month's worth of rain is excepted to fall in the northern Bay Area over the next few days, when serious flooding is expected. There's a chance Santa Rosa could see over 10 inches of rain. Heavy snow is expected in the Sierra Nevada, where the NWS issued a Winer Storm Warning.

Southern California isn't expected to get hit as hard. While timing and amounts are still up in the air and forecasts are expected to change daily, "this new pattern will likely bring the first significant rainfall of the season to the area," the NWS said.

A chance of rain begins for the area starting on Saturday and could continue through Monday.

In Los Angeles County, which is expected to see the see some of the greatest impact from the storm among SoCal counties, 0.4 to 0.6 inches of rain could fall. And there's a near-100% chance of rain. But forecasters say at this point, computer models show varying amounts of rain — some models show double that amount.

The NWS expects the amount of rain to diminish as the storm travels south toward San Diego.

"In general, regardless of how much rain falls, more is expected on the coastal mountain slopes than at the coast with more north and less south," the NWS said.

Lytle Creek and areas of the San Gabriel Mountains west of the Cajon Pass are also expected to be particularly rainy Saturday and Sunday. At that location, there's a 75% chance of one-tenth of an inch or more of rain, 10% chance of 1.5 inches or more and 2% chance of 3 inches or more, according to the NWS.

Highs will be well below normal across the region over the weekend, in the upper 50s to mid 60s, the NWW said.

Snow levels are expected to remain above 8,000 feet through Sunday, but could lower to 6,500 to 7,500 feet on Monday, the NWS said.

But before any rain hits SoCal, the region should expect fair skies and dry weather as part of a warming trend through Thursday.

Max temperatures Tuesday were expected to be 3 to 6 degrees below normal. By Thursday, highs will be 3 to 6 degrees above normal — in the 70s to lower 80s in inland areas and valleys and mid 60s to low 70s at the beaches.