California Braces for Floods as Atmospheric River System Brings Soaking Rain and Snowmelt

Following weeks of heavy snowfall, parts of California now face a new threat of floods from a Pacific storm tapping into an atmospheric river. Here's how the forecast is expected to play out and what the biggest concerns are:

The first system will dump snow through Wednesday, including in the Siskiyous and Sierra of Northern California, with snow expected to fall over elevations as low as 1,000 feet. The National Weather Service predicts 1 to 2 feet of snow could fall in the northern Sierra by Wednesday.

The next storm is set to arrive Thursday. It will be a warmer Pacific storm, meaning that rain will fall at higher elevations than the recent string of colder storms, which could complicate the flood potential. This storm will also be fuelled by an atmospheric river that will stretch over 2,000 miles long from near Hawaii, with a moderate to strong intensity.

Rain is expected to fall on top of recent snow, not only posing a heavy rain threat for some lower elevations, but also in some higher elevations where snow is on the ground. Multiple feet of additional heavy snow are expected in Northern California above 8,000 feet in elevation, while rain could also fall at elevations up to 8,000 feet there. Several inches of rain are also expected in the Sierra foothills and other low elevations in Northern and Central California, from near the Bay Area down the Big Sur coast.

The rain-on-snow scenario could cause several concerns. For instance, much of the snow cover in lower elevations could melt as the warmer air and rain move in, leading to flooding in smaller rivers, creeks and streams. Falling rain could add weight to snow already stressing roofs and structures in the high country. Additionally, landslides and debris flows could occur in lower elevations without snowpack, particularly in areas burned recently by wildfires.

The National Weather Service has issued flood watches for much of Northern and Central California, including the Bay Area, Sacramento and the Sierra foothills. To minimize the stress from the weight of snow and any rain that could fall, clear as much snow off your roof as possible if it can be done safely. In lower elevations, clear any gutters, curbs and other drainage areas of debris to allow water to drain away. If you live near an area recently burned by a wildfire, be ready to evacuate due to debris flows on a moment's notice