Heaviest precipitation today, heavy mountain snow Monday through Wednesday

January 8, 2017 – Heavy rain, gusty winds, and flooding concerns today. Flooding concerns continue Monday into Wednesday with very heavy mountain snow expected. Another weather system possible Thursday.

Flood Watch through Wednesday afternoon, Wind Advisory until 6 PM this evening.

Discussion

Moderate to heavy precipitation has spread across interior Northern California early this morning as the relatively narrow TPW plume is shifting northward across the region. Most Valley locations have already seen 0.75 to 1.50 inches of rainfall since the event began Saturday morning. The orographically-favored west slopes of the Sierra have received 2 to 4 inches of rain, with no sign of letting up. Winds have also steadily been ramping up this morning, with wind gusts of 20 to 30 mph being reported across the Valley.

For today, expect the narrow band of heavy precipitation to continue shifting northward toward the far north end of the state by midday. That band begins to shift southward again this afternoon and evening as a deep upper low off the Pacific Northwest coast pushes the TPW plume south.

Precipitation forecasts remain on track: with 2 to 6 inches of rain expected across the Valley, and 7 to 15 inches possible over the Sierra. Wind gusts should also increase to 40 to 50 mph during the day today, with moderate-sized power outages possible.

Showery weather will continue across the region into Monday as a weaker shortwave moves through. Snow levels should lower to the 3000-4500 ft range. There won’t be much of a break in the mountain showers between that shortwave and a more potent storm Tuesday into Wednesday. Given the heavy precipitation expected with the Tues-Wed storm and lower snow levels, we’ve issued a Winter Storm Watch to cover the entire Mon-Wed timeframe. Several feet of snow are expected along the mid and upper slopes of the Sierra in that timeframe.

Extended discussion (Thursday through Sunday)

Large level in uncertainty in the extended forecast as models continue to struggle. Looks like wet pattern extends at least into Thursday with GFS and ECMWF showing some level of troughing over the west while GEM shows overrunning precip ahead of a more offshore trough solution. High chance pops seem appropriate at this time with moderate snow levels. Forecasting wind direction nearly impossible with all models showing different scenarios but at least strong winds are not indicated. GFS and ECMWF show drying on Friday under upper ridge but GEM keeps things quite moist bringing a moderately moist system across the coast. Have left in chance pops for now but could see a change to a drier solution if model solutions become more consistent. Models actually become a little more consistent with each other going in to next weekend showing a fairly flat ridge over the west and overrunning precipitation moving into the region at least by the end of the day Saturday. Again, have kept chance pops in for now but details still fuzzy. GFS and ECMWF agree on continued break down of any ridge by next Sunday so at least chance pops will remain in the forecast for second half of next weekend.

Not much change in the extended forecast. Wednesday continues to look wet as that is part of the second wet storm expected to hit NorCal on Tuesday-Wednesday with snow levels below passes at approximately 4000-5000 feet. The models continue to struggle on how the wet pattern will look between Thursday through Saturday. At this point, it looks like Thursday will at least see some scattered showers (GFS), but it could be more widespread (ECMWF). Both models are showing a break Friday with an additional storm arriving either early Saturday (GFS) or late Saturday (ECMWF). Chance pops continue to be best bet at this time. Snow levels remain moderate through the extended period.

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