California faces flash flooding threats as storm promises days of significant rain
Fresh on the heels of a storm system that brought rain to Northern California to kick off the week, a second, stronger storm is swirling toward the region, keeping periods of heavy rain in the forecast for much of the week and threatening flash flooding.
Light to moderate rain lingers across the northern half of the state on Tuesday from the previous storm, which brought 1-3 inches of rainfall across the coastal mountain ranges and around an inch to the San Francisco Bay Area. San Francisco reported 0.80 inches of rain Monday, making for its wettest day since March 21.
Additional rainfall totals of up to an inch are expected Tuesday in the coastal lowlands, with 1-2 inches of additional rain expected in the coastal mountains. While water levels will rise across creeks and streams in the region, major flooding is not expected.
Flash flooding concerns increase with the second storm as it drops south off the coast of the Pacific Northwest and heads into the offshore waters of the Bay Area on Wednesday. The storm carries an unusually high amount of moisture, renewing steady rain for Northern and Central California while now introducing moderate to heavy rain into Southern California.
The heaviest rainfall totals will stretch from just south of the Bay Area along the coast into the Los Angeles area on Wednesday. Meanwhile, heavy snow will pummel the higher elevations of the central and southern Sierra Nevada, with Winter Storm Warnings posted for 1-2 feet of snow above 8,000 feet through Wednesday.
West Coast snowfall forecast through Thursday. (FOX Weather)
The storm will continue to slowly drift south on Thursday toward Southern California, drying out the Bay Area but still soaking the Los Angeles area with several inches of rain, especially in the mountains. With unstable air tossed into the mix, some of the showers may intensify into thunderstorms carrying bursts of heavier rainfall.
Rainfall totals are forecast to reach or exceed 5-8 inches by Friday morning in the mountains, with isolated higher amounts, while 2-4 inches is likely across the coast and valleys of Southern California, including the Los Angeles area.
Southern California rainfall forecast through Friday. (FOX Weather)
"We’re working this rain up into terrain," FOX Weather Meteorologist Britta Merwin said. "You've got that orographic lift; you squeeze out all the moisture possible, and you get these gnarly rain totals up in the hills that can approach 7-8 inches."
Flood Watches stretch from San Luis Obispo along the coastal mountains south into Los Angeles.
Flood Watches in Southern California (FOX Weather)
"Southern California … although we do get handed some active weather in El Niño setups, they're not used to having 8 inches of rain in the mountains outside of Los Angeles," Merwin said. "Although it's a setup that we see plenty in Southern California, it’s an area that can’t handle that type of rain, so it always translates to a flood risk; it’s always feast or famine – you don’t get the in-between."
Wind gusts will reach 30-50 mph during the peak of the storm, with isolated gusts to 60 mph, especially over the mountains.
The overall rainfall amounts combined with the potential for bursts of thunderstorm-triggered heavier rain, especially over recent wildfire burn-scar areas, has prompted NOAA’s Weather Prediction Center (WPC) to put much of Southern California at a Level 3 out of 4 risk for flash flooding.
The flash-flood threat in California on Wednesday and Thursday. (FOX Weather)
"This (storm) has the potential to lead to some significant flooding concerns, including flash and urban flooding, with some possibility for debris/mud flows in the terrain areas or over burn scars like the South Fire burn scar in Ventura County," the WPC said in issuing its Level 3 risk.
The storm lingers into Friday before finally pushing inland late Friday near the Mexico border and heading into the Desert Southwest. However, long-range forecasts indicate additional rain is likely in California next week.