Moderate to heavy rain and mountain snow late today through Wed with possible flooding

March 5, 2019 – Periods of moderate to heavy rain and mountains snow develop this afternoon and evening and continue through Wednesday. Mountain travel will be impacted and additional rain will increase flood potential. Showers continue Thursday into the weekend.

Discussion

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Cloudy skies cover the region early this morning as baroclinic cirrus ahead of the offshore system spreads overhead the moist lower levels. Current temperatures remain relatively mild and range from the upper 20s and lower 30s in the mountains to the mid 40s to lower 50s across the Central Valley.

Focus turns to offshore extra-tropical cyclone with warm conveyor belt ahead of it entraining deeper moisture northeastward from ITCZ. GEFS/GFS Integrated Water Vapor Transport shows some of this moisture getting entrained into the storm, but bulk of higher PW air will be shunted into central and southern portions of CA. Precipitation spreads over the area from the southwest today and will become moderate to heavy tonight into Wednesday.

1 to 4 feet, locally higher, of new snow expected in the mountains of Western Plumas county and the northern Sierra Nevada above 5500 to 6000 feet. Winter storm warning in effect to cover this. Gusty wind will accompany the frontal passage Wednesday, but current guidance suggests wind speeds will remain mostly below advisory criteria in the Central Valley. Instability progs suggest potential for afternoon thunderstorms on Wednesday, with highest CAPE values depicted mainly over the Central Valley and Coastal Range.

Main trough with embedded short waves will keep light to moderate showery precip going Thursday into Friday. Rainfall totals through Friday look to be around 1 to 3 inches for the Central Valley and 2 to 6 inches for the foothills and mountains. With saturated soils, and streams, creeks and main stem rivers running high, additional rain will increase flooding possibility. An areal flood watch is in effect.

Extended Discussion (Saturday through Tuesday)

The next system within the wave train of disturbances will lurk off the coast on Saturday morning. As shown in yesterday`s guidance, this particular upper trough is forecast to dig along the coast before establishing a position off of northern Baja California by Monday morning. This evolution does not usually bode well for organized precipitation across Northern California. Some uncertainty remains in the exact track, particularly where the upper low begins to dive southward. A vast majority of the 00Z GEFS members are dry, but a few show some precipitation over the weekend. Did keep low end probabilities through Monday, mainly toward the foothills and mountains where orographics comes into play. As the closed low takes a track toward the Desert Southwest, dry northerly winds will encompass the region. Amplified flow should re-enter the picture by next Tuesday (March 12) although models keep any appreciable impacts over the Pacific Northwest. Regarding the temperature forecast, readings will likely stay a bit below average for early March with Valley highs in the mid 50s to low 60s and higher elevations staying in the 30s and 40s.