931 AGUS76 KRSA 301612 HMDRSA Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 830 AM PST Tue Dec 30 2025 ...NORTHWARD MOVING CUTOFF TO BEGIN SPREADING PRECIPITATION ACROSS SOUTHERN CALIFORNIA TOMORROW... ...UPPER TROUGH TO SWING IN FROM GULF OF ALASKA ON THURSDAY INTO FRIDAY... .METEOROLOGICAL CONDITIONS (TUE AM - MON AM)... Current satellite imagery shows a cutoff low slowly moving northward, spreading a large layer of cirrus clouds across southern California. As this low continues it's northward trajectory into Wednesday, precipitation chances increase across southern California. Given the current offshore flow and and dry surface conditions, precipitation onset is slightly in question due to uncertainty in how long it will take columns to saturate. Precipitation will continue spreading northward throughout the day on Wednesday and reach northern California basins overnight Wednesday into Thursday. As this cutoff pinwheels off the California Coast, an upper level trough will dig from the gulf of Alaska, phasing with the weakening cutoff and remnant moisture. This system will likely bring stronger atmospheric river conditions (IVT >500 units) to coastal locations in Northern California. While we have seen some convergence between the models on timing and magnitude of precipitation, there is still some disagreement and relatively high uncertainty for the forecast area. Across northern California, the GFS and EC are not nearly as different compared to yesterday morning with the EC deterministic slightly higher than the GFS. Yesterday morning's forecast had basin average differences >2 inches around northern California. EC ensemble distribution is still quite a bit higher than the GEFS. The largest differences between the deterministic models looks to be over SoCal in association with the northward propagating cutoff and moisture plume in the early portions of the window. Looking at the Ventura River Basin, in particular, the EC has over 4 inches of basin average precipitation compared to the GFS with a much higher ensemble distribution. Morning forecasts primarily followed the NBM and WPC, resulting in increases across much of northern Calfiornia as guidance converged on accumulations across the region. The highest accumulations of >6 inches over the next 6-days target the higher elevations of Cape Mendocino, the Shasta Drainage, and northern Sierra (Yuba and Feather Basins). The reset of the North Coast and and Sierra Nevada Mountains are forecast to receive 2 to 5 inches with the Big Sur Coast and Transverse Mountains receiving 2 to 6. Freezing levels will start the forecast period over 12k feet under the weakening ridge and gradually lower throughout the 6-day window as troughing begins to move in. The north could lower to ~5k feet by Sunday, ~6k feet over the southern Sierra, 6.5-7k feet over southern California. Please Note: This product may NOT be routinely updated. Please refer to the following product issued by the CNRFC www.cnrfc.noaa.gov/Daily-Briefing for a graphical summary of weather and hydrologic conditions. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php CH $$