Atmospheric river to bring heavy precipitation, potential flooding to California

A new atmospheric river (AR) event is forecast to impact California on Thursday, March 9, 2023, bringing with it considerable rainfall through Saturday, March 11. Forecast models are predicting that the incoming AR will reach AR4 (on a scale of 1 – 5) ranking in the Bay Area, with an 80% chance of being at least an AR3. A second AR landfall is possible early next week.

• The combination of high freezing levels, heavy precipitation, and existing snowpack conditions will likely result in a substantial amount of rain-on-snow, thereby increasing runoff and the potential for flooding. The GEFS ensemble has forecast this will lead to significant runoff and flooding concerns in the affected areas.

The Bay Area is preparing for heavy precipitation and potential flooding as a warm atmospheric river is set to make landfall in California later this week. Forecast models are predicting that the incoming AR will reach AR4 ranking in the Bay Area, with an 80% chance of being at least an AR3.

According to the National Weather Service (NWS), the AR is expected to bring considerable rainfall to the region from Thursday through Saturday. The NWS Weather Prediction Center has forecast 48-hour precipitation totals of over 76 mm (3 inches) over the Coast Ranges of Northern and Central California and over 127 mm (5 inches) over the Sierra Nevada in association with the first AR.

The AR is forecast to be a warm storm, with a broad region of 850-hPa temperatures over 0° C (32°F) predicted over California during the storm period. This will cause freezing levels to rise rapidly, to nearly 1 829 meters (6 000 feet) across Northern California and the Sierra Nevada ahead of this AR.

The combination of high freezing levels, heavy precipitation, and existing snowpack conditions will likely result in a substantial amount of rain-on-snow, thereby increasing runoff and the potential for flooding. The GEFS ensemble has forecast this will lead to significant runoff and flooding concerns in the affected areas.

Forecast models are showing low-to-moderate confidence in a second AR landfall in California early next week. The 00Z March 6 ECMWF model is forecasting a stronger AR at coastal locations in Central California as compared to the GFS, while the GFS is forecasting a slightly longer duration of AR conditions along the coast.

The ECMWF model is also forecasting considerably more precipitation (128.5 mm or 5.06 inches) for the Russian River Watershed as compared to the 00Z GFS (33.5 mm or 1.32 inches), highlighting significant model-to-model uncertainty with this event.

Temperatures are expected to be unseasonably cool over much of the West on Wednesday and Thursday. March 8 and 9.

In the Northern/Central Rockies, high temperatures are forecast to range from -6 to -1 °C (20s and 30s °F), while in the Great Basin, temperatures will range from -1 to 4 °C (30s and 40s °F). The Pacific Northwest can expect temperatures in the range of 4 to 9 °C (40s °F), while much of California will experience highs in the range of 9 to 15 °C (50s °F).

Portions of the Southwest and Southern Rockies will be the warmest areas in the West during this time, with highs forecast to be above average in the range of 16 to 26 °C (60s and 70s °F).

Residents are urged to take necessary precautions to stay safe, including monitoring weather reports and evacuation orders, securing loose objects and outdoor equipment, and avoiding flood-prone areas.

References:

¹ AR Outlook – CW3E AR – March 6, 2023

² Short Range Forecast Discussion – NWS Weather Prediction Center College Park MD – 300 AM EST Wed Mar 08 2023

Featured image credit: GFS/TropicalTidbits