Moderate to heavy rain and mountain snow Tue and Wed with strong wind and possible flooding

March 3, 2019 – Rain showers and mountain snow showers continue today into early Monday. Mainly dry Monday into early Tuesday. Periods of moderate to heavy precipitation late Tuesday and Wednesday, with wet weather continuing into Friday.

Discussion

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Cyclonic onshore flow with weak embedded vort maxes, and LFQ of 140 kt jet overhead, keeping scattered showers over interior NorCal today. Snow showers continuing in the mountains of Western Plumas and the northern Sierra Nevada where chain controls are in effect again. Snow levels are around 6000 feet and additional accumulations of 3 to 8 inches, locally up to a foot, are possible through early Monday. Mesoscale models showing some weak increased instability this afternoon over eastern portions of the S Sac and Nrn San Joaquin Valleys, extending into the Motherlode. CAPE values look significantly less than yesterday afternoon, so will hold off mention of isolated afternoon thunderstorms for these areas.

Channeled weak vort maxes keep showers in the forecast through early Monday then subsidence increases over the CWA. Monday into Tuesday should be mostly dry. With saturated grounds, some morning valley fog may be possible Tuesday, however expect it to be patchy as baroclinic cirrus from approaching Pacific storm increases overhead.

Satellite imagery shows next extra-tropical cyclone to impact interior NorCal centered near 32N 149W with associated front extending to near 140 W. To the south of this system is a moist subtropical plume extending NE from ITCZ. GEFS/GFS Integrated Water Vapor Transport showing bulk of this higher PW air will be shunted into central and southern portions of CA as the storm moves inland.

However, a period of moderate to heavy precipitation expected over interior NorCal Tuesday night into Wednesday with strong wind. Main trough with embedded short waves will keep wet weather in the forecast through Friday.

Rainfall amounts Tuesday through Friday look to be around 1 to 3 inches for the Central Valley and 2 to 6 inches for the foothills and mountains. Potential for some urban and small stream flooding given saturated soils and excessive runoff. Several feet of snow expected in the mountains this week with snow levels 4000 to 6500 feet, lowering into the foothills Thursday into Friday.

Extended Discussion (Thursday through Sunday)

Wet weather will continue during the extended period as an atmospheric river exits NorCal. Models are in good agreement on keeping the heaviest rain to our south over central CA and southern CA through mid-week, but we will still see widespread rain and snow across the area beginning to wind down somewhat by Thursday. The good news with this AR is that it is a quick mover unlike the one we saw last week, but we may see some flooding concerns linger.

We will remain in a long-wave trough pattern at the end of the week which will keep shower chances in the forecast, mainly over the mountains. Snow levels will be between 2500 and 3500 feet.