Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 171040

Area Forecast Discussion
National Weather Service Sacramento CA
340 AM PDT Sat Mar 17 2018

Unsettled weather today with a chance of showers then drier and
warmer Sunday into Monday. Wet weather returns Tuesday into


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



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.



Pacific disturbance will bring showers with mountain snow over
interior NorCal through Saturday evening. VFR occasional MVFR/IFR
conditions TAF sites with scattered thunderstorms this afternoon
and evening over the valley and foothills. Generally IFR over the
mountains. Snow levels 2500 to 3500 feet. Conditions improving
after 06z Sunday. Southerly winds generally below 10 knots at TAF


Winter Storm Warning until 5 AM PDT early this morning for
Western Plumas County/Lassen Park.

Winter Storm Warning until 5 AM PDT early this morning for West
Slope Northern Sierra Nevada.


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