Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 180539

Area Forecast Discussion
National Weather Service Sacramento CA
1040 PM PDT Sat Mar 17 2018

Drier and warmer Sunday into Monday. Wet weather returns Tuesday
into Friday.


Update: Upper level trof axis has moved east of our CWA into NV and
drier Nly flow aloft has eroded much of the cloudiness over our CWA
except for the NE portion of Shasta Co and near Tuolumne and Alpine
counties. With clear skies and light winds, strong radiational
cooling will cool temps into the 30s in the Valley where some areas
of frost will be possible. HRRR suggests a narrow area of fog/ST
will form Sun morning in the Sac River/San Joaquin River Vly,
otherwise a mostly sunny day before increasing clouds late Sun. JHM

.Previous Discussion...

The closed upper low that has brought the last several days` worth
of weather is still hovering over eastern Oregon, with cyclonic
flow remaining over Northern California. A vort max is moving
through the region this afternoon, and has aided in kicking off
convection. The Sierra Nevada, Coastal Range, and portions of the
Valley are all seeing showers with a few thunderstorms this
afternoon. Mostly light snow accumulations are expected over the
mountains today, but even light snowfall can cause big impacts
driving over the Sierra.

The upper trough will shift eastward into the Great Basin tonight
as an upstream ridge moves overhead. This will bring drier weather
tonight into Sunday. A couple of mountain showers will be
possible Sunday afternoon with some residual moisture lingering
across the region.

Mostly dry weather with seasonal temperatures will continue into
Monday. The next moisture plume then arrives on Tuesday. This has
the look of an atmospheric river, with a warmer subtropical
moisture tap. Much of the precipitation is expected to continue
through the end of the week.



Confidence is high that we will see a wet system to start the
extended period. Models show an upper level low to our west over
the Pacific pulling moisture into the state of CA. This will bring
Valley rain and Mountain snow to NorCal on Wednesday. The upper
level low will then be phased into a trough digging south out of
the Gulf of Alaska later Wednesday and we will see the trough
axis move into NorCal Wednesday night into Thursday. This will
bring the heaviest rain and snow to the area with at times heavy
rain expected in the Valley and heavy snow in the higher
elevations. Snow levels on Wednesday will start out between 6000
and 7000 feet but will start to fall Thursday as the colder air
builds into NorCal. Snow levels will be falling to 3000 to 4500
feet by thursday afternoon. Hazardous mountain travel can be
expected Thursday through Friday morning.

Light showers continue into Friday and then we see
models diverge on solutions into next weekend. The GFS pushes a
few short wave troughs into the region while the EC is further to
the east with this feature and builds in a ridge for us. Kept
rain chances in the forecast for the weekend for the higher
elevations but confidence in low. With wet weather expected we
will see high temperatures run 5-10 below average throughout the




VFR conditions are expected at the TAF sites the next 24 hours.
SCT to BKN clouds at 3500 to 5000 feet. Scattered showers
possible in the southern Valley and Mountain snow showers through
2z Sunday. MVFR in any showers. Winds remain under 10 knots.




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