Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 171002

Area Forecast Discussion
National Weather Service Sacramento CA
302 AM PDT Sun Oct 17 2021

Cooler weather along with a chance of precipitation returns
this afternoon and overnight. Mountain snow will bring travel
impacts this evening into tomorrow morning. More unsettled
weather mid to late week into next weekend.


A pattern change begins today for interior Northern California as
multiple weather systems are forecast to impact the area through
the week. A quick-moving upper trough is approaching West Coast
early this morning, increasing cloud cover across the area. The
trough and associated surface front will begin to move inland
later today, bringing cooler temperatures and precipitation
chances to Northern California. The surface front and
precipitation isn`t expected to reach the Coastal Range until
late afternoon, with precipitation spreading east through the
evening and continuing through the overnight hours, mainly over
the Sierra and Southern Cascades. Snow levels will begin around
6000 to 7000 feet Sunday afternoon, falling to around 5000 to
6000 feet after sunset. CAMs continue to suggest a period of weak
instability tomorrow evening as the trough axis passes through,
mainly along the Sierra and foothills. This could produce some
locally enhanced rain and snow showers and possibly even brief,
isolated thunder. The highest QPF amounts will be over the
mountains and foothills with 0.25 to 0.75 inches, locally up to
an inch, with lighter amounts of a trace up to 0.20 inches inn
the Valley. 3 to 8 inches of snow is expected above 6000 feet,
locally up to a foot near Lassen Park, with lighter amounts down
to around 5000 feet mainly in the early morning hours on Monday.
Mountain travel and recreational impacts are likely with this
snow. A Winter Weather Advisory remains in effect for the Sierra
and Southern Cascades for elevations above 5500 feet Sunday
evening through early Monday morning.

Breezy winds are also forecast today as the trough passes through.
Winds will begin picking up this morning with the strongest winds this
afternoon through early evening, slowly decreasing overnight. In
the Valley, expect winds generally 10 to 20 mph with gusts up to
25 to 30 mph. Strongest winds will be in the mountains and
foothills with west winds 20 to 30 mph, gusts up to 35 to 45 mph,
locally higher over ridgetops and favorably oriented canyons.
Temperatures will also be much cooler with 70s in the Valley and
40s to 60s in the mountains and foothills.

Shortwave ridging builds over California Monday into Tuesday, with
dry weather returning. However, temperatures will cool even more
behind the trough passage with Valley temperatures in the 60s and
30s to 60s in the mountains and foothills. The next weather system
is forecast to impact Northern California late Tuesday into
Wednesday. Current ensemble guidance shows this shortwave will
likely clip the interior with the majority of precipitation
remaining northwest of the forecast area. However, lighter showers
are forecast late Tuesday into Wednesday as a frontal boundary
moves through the area, mainly for locations north of Highway 50.
The majority of QPF is expected over the Coastal Range and
Southern Cascades, with snow levels generally above 6000 to 7000
feet. Uncertainties still remain in specific precipitation timing
and amounts, though accumulation looks limited at this time. -HEC


Long wave trough off the West Coast Thursday as embedded short
wave frontal system approaches NW CA coast. Band of moisture ahead
of main baroclinic zone progged to bring a threat of some light
precip Thursday. Frontal precip band with associated 1-1.5 PWS
progged across the CWA Friday. Significant model differences exist
by this time period leading to forecast uncertainty with QPF.
Forecast leans towards wetter solutions. Precip threat ramps up
next weekend as models indicate a 160+ kt zonal polar jet
stretched across the Pacific with LFQ extending over NorCal.



Pac fntl sys movs acrs intr NorCal nxt 24 hrs with incrg MVFR/IFR
conds sprdg fm NW to SE acrs CWA this aftn thru tngt. Lcl LIFR
omtns aft 00z Mon obscg trrn. Isold tstms poss ovr ern ptns of
Cntrl Vly into fthls btwn 00z-06z Mon. Sly sfc wnd gusts to 35 kts
poss ahd of fnt in Cntrl Vly and up to 50+ kts ovr hyr mtn trrn.
Sn lvls arnd 060 AMSL this eve lwrg to arnd 040 AMSL Mon mrng.


Winter Weather Advisory from 5 PM this afternoon to 5 AM PDT
Monday for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.


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