CW3E AR Update: 6 March 2023 Outlook

March 6, 2023

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Warm Atmospheric River to Bring Heavy Precipitation, Potential Flooding to California Later This Week

An atmospheric river (AR) is forecast to make landfall in California later Thursday, bringing with it considerable rainfall to the region through Saturday
IVT > 750 kg m-1 s-1 is forecast during AR landfall along the coast of California, bringing AR 3/AR 4 conditions (based on the Ralph et al. 2019 AR Scale) to Central and Northern California
This AR is forecast to be a warm storm, with a broad region of 850-hPa temperatures > 0° C forecast over California during this storm period
The 00Z ECMWF 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
Models are showing low-to-moderate confidence in a second AR landfall in California early next week
The NWS Weather Prediction Center (WPC) has forecast 48-hour precipitation totals > 3 inches over the Coast Ranges of Northern and Central California and > 5 inches over the Sierra Nevada in association with the first AR
The 00Z ECMWF is forecasting considerably more precipitation (5.06 in) for the Russian River Watershed as compared to the 00Z GFS (1.32 in), highlighting significant model-to-model uncertainty with this event
GEFS ensemble has forecast freezing levels to rise rapidly, to nearly 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