A major storm and atmospheric river are poised to impact California today and Thursday with heavy to excessive rainfall, flooding with debris flows and landslides near recent burn scar areas, heavy mountain snow and high winds.
The NWS Weather Prediction Center (WPC) has issued a Moderate Risk of excessive rainfall over parts of the California Coast from Wednesday into Thursday morning, December 4 into 5, 2023.
The associated heavy rain will create numerous areas of flash flooding, NWS Ziegenfelder noted.1
Furthermore, many streams may flood, potentially affecting larger rivers.
Additionally, heavy snow will return to the Sierra Nevada Mountains on Wednesday.
Late Wednesday afternoon, coastal rain and higher-elevation snow will also return to the Pacific Northwest.
In addition, a few patches of rain/freezing rain will develop over interior Washington State overnight Wednesday into Thursday morning.
On Thursday, rain and higher-elevation snow continue over California. However, the threat of excessive rainfall will decrease slightly. Therefore, the WPC has issued a Slight Risk of excessive rainfall on Thursday into Friday morning.
The associated heavy rain will create mainly localized areas of flash flooding, with urban areas, roads, small streams, and burn scars the most vulnerable.
While the heavy snow will continue over the Sierra Nevada Mountains, overnight Thursday, the snow and rain will mostly end over California.
The coastal rain will continue over the Pacific Northwest into Friday morning, January 6.
During the late afternoon Wednesday, snow will move into the Great Basin and parts of the Southwest and the Central Rockies by Friday morning. The snow will result in reduced visibility and hazardous driving conditions.
The event comes just several days after a powerful atmospheric river drenched California, leaving at least 2 people dead and more than 300 000 customers without power.2
An atmospheric river is a long, narrow band of highly concentrated atmospheric moisture that can transport huge amounts of water vapor from the tropics to more temperate regions of the globe.
These powerful weather systems are typically several thousand kilometers in length and just a few hundred kilometers in width, and they are capable of delivering prodigious amounts of rain or snow over a relatively short period of time.
Atmospheric rivers are a common weather phenomenon in California and can occur at any time of year. They are often responsible for heavy rainfall and flooding in the state, and can have significant impacts on both the environment and human activities.
1 Short Range Forecast Discussion – NWS Weather Prediction Center College Park MD – 254 AM EST Wed Jan 04 2023
2 Powerful atmospheric river drenches California, leaves at least 2 people dead and more than 300 000 customers without power, U.S. – The Watchers – January 2, 2023
