ALG Connect: Atmospheric River-Fed Storm Pummels California with Flooding Rain

Atmospheric River-Fed Storm Pummels California with Flooding Rain

A Pacific storm that's tapping into an atmospheric river is pummeling California with heavy rainfall that has already produced flooding, rockslides and mudslides in parts of the state.

Moderate to heavy rain is ongoing in central California with snow falling at much higher elevations than the past several storms. More than four inches of rainfall has occurred in the Santa Cruz Mountains and in the southern Sierra since Thursday.

Flooding has already been a major issue in at least two towns so far - Springville and Kernville - where rapid snowmelt and heavy rain has led to flooding and flash flood emergencies.

NOAA's Weather Prediction Center has issued its highest threat level for excessive rainfall: A high risk of flooding has been issued for a part of the Central California coast and in the southern Sierra Nevada for the period between Friday and Saturday morning. NOAA said that "potentially unprecedented flooding" was possible "in areas where reservoirs are already near full".

Rain is falling on top of recent snow: This storm is not only 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 over the highest elevations, but rain could also fall at elevations up to 7,000 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 National Weather Service has issued flood watches for much of California, including the Bay Area, Sacramento, the mountains in Southern California, and the lower elevations of the Sierra.

There are several concerns with the rain-on-snow scenario.

Much of the snow cover in lower elevations could melt as the warmer air and rain moves in. That water released could flood smaller rivers, creeks and streams.

In lower elevations without snowpack, the combination of additional soaking rain on the already soggy ground will increase not only the threat of flash flooding, but also landslides and debris flows, particularly in areas burned recently by wildfires. Some rivers are forecast to rise into minor or moderate (locally major) flood stage in the Central Valley and closer to the coast north and south of the Bay Area.

Some areas of locally heavy rain and mountain snow could linger in parts of Northern California into the weekend.

After that, another storm, likely tapping another atmospheric river, is expected to push into the state later Monday into Tuesday. There is still uncertainty about the details of this early week system, typical of storms several days away, but this storm has the potential to produce additional flooding rainfall in many of the same areas, as well as strong winds.

We recognize the severe impact that current conditions are having on our customers, and we are committed to restoring service levels as quickly as possible. As a result of these weather issues, the ALG Client Service Team will send daily updates as we monitor the progress and track job level impact.