March 17, 2018 – Unsettled weather today with a chance of showers then drier and warmer Sunday into Monday. Wet weather returns Tuesday into Friday.
Upper trough and cyclonic flow will keep a threat of showers over Interior NorCal today. Best chances will be over the foothills and mountains of Western Plumas county and the Northern Sierra Nevada. Additional light snow accumulations are possible today. Models showing some afternoon CAPE over southern half of the Central Valley and eastern foothills to support a slight chance of an afternoon thunderstorm. High temperatures this afternoon forecast to be around 10 degrees below normal.
Upper trough shifts east into the Great Basin tonight as upstream upper ridging begins to move onshore. Shower threat diminishes overnight as subsidence increases over Interior NorCal. Stabilizing AMS, decreasing cloudiness and moist boundary layer point to the potential for some late season patchy valley fog Sunday morning.
Dry weather expected Sunday as short wave upper ridging moves inland. AMS begins to warm tomorrow but temperatures remain below normal. Models show weak upper trough moving through Sunday night into Monday followed by another Monday night. These waves will bring some clouds to the CWA, and a slight chance of precipitation to the Coastal and Shasta mountains.
GFS/GEFS Integrated Water Vapor Transport showing moisture plume in EPAC taking aim on California Tuesday. While much of this higher level moisture remains south of the forecast area, approaching mid latitude wave will tap into it, spreading precipitation over much of Interior NorCal Tuesday into Tuesday night.
Extended discussion (Wednesday through Saturday)
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Models now fairly consistent in shifting upper ridge eastward and bringing first shortwave disturbance through Norcal late Tuesday and early Wednesday. Snow levels with this system will be fairly high at between 5500 and 6500 feet. Precipitation amounts with this first disturbance are not expected to be especially heavy but the highest pass levels could have some minor impacts from a few inches of snowfall Wednesday.
By Wednesday evening an upper level trough dropping out of the Gulf of Alaska will tap into subtropical moisture bringing an increase in precipitation across the north state.
Snow levels will start out relatively high but lower during the day on Thursday as cooler air from the Gulf system filters into the region. Moderate precipitation rates and lowering snow levels will bring significant snowfall to the higher elevations so mountain travel impacts are likely during the Thursday timeframe.
Lighter showers continue on Friday with fairly low snow levels for this time of year. Model forecasts become much more uncertain for next weekend with ECMWF predicting a strong ridge of high pressure along the west coast while GFS predicts weak troughing and light precipitation over at least the northern portions of the forecast area. Have kept chance pops in the forecast for now for next Saturday but forecast changes are likely during this time period as models come more in line.