NATIONAL WEATHER SERVICE FORECAST DISCUSSION
Area Forecast Discussion
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000
FXUS66 KSTO 061115
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
315 AM PST Mon Dec 6 2021

.SYNOPSIS...
Dry weather with areas of dense Valley fog this morning. A few
showers will be possible this afternoon and overnight as a weak
system moves through. A colder weather system could bring
mountain snow, light Valley showers, and colder temperatures late
week.

&&

.DISCUSSION...
IR satellite imagery shows high clouds streaming into Northern
California early this morning ahead of the weather system that is
slated to move through the area later today. Fog developed late
last evening for portions of the Valley and Motherlode and will
continue through the early morning hours. A weather system will
drop north from the Gulf of Alaska into Northern California today.
This will bring a quick round of light precipitation hitting
Shasta County by late morning and moving south through interior
Northern California through the afternoon and evening hours. QPF
will be limited with this system with only a few hundredths for
most locations and up to 0.25 inch for the Sierra south of Highway
50. Light snow showers are possible at the highest elevations with
a couple of inches forecast south of Highway 50. Temperatures will
also be much cooler yesterday for most locations (those that were
not impacted by stratus). 50s are forecast across the Valley and
foothills, a 10-20 degree drop from yesterday for many locations,
with 40s and 50s in the mountains.

Dry weather returns Tuesday with clearing skies and slightly
warmer temperatures. Then ensemble guidance continues to indicate
another weather system pushing south from the Gulf of Alaska
Wednesday into Thursday, bringing another round of precipitation.
At this point there looks to be more moisture associated with
this system than the Monday system, though the track of the
weather system means the majority of precipitation will fall over
the Sierra. QPF did increase with the latest model guidance,
mainly for the Sierra and foothills, with around 0.5-1.5 inch of
liquid precipitation over the Sierra and only 0.3-0.25 inch in the
Valley. This is a colder system so snow levels will quickly fall
from around 6500 feet Wednesday evening to 4000-4500 feet by
Thursday morning, which corresponds with the period of heaviest
precipitation. With the higher QPF, snow amounts have increased
slightly as well. Anywhere from 6-14 inches is possible above 5000
feet, with lighter amounts down to 4000 feet. Uncertainty still
remains in forecast timing and amounts with this system, so keep
an eye out for forecast updates. -HEC

&&

.EXTENDED DISCUSSION (Friday THROUGH Monday)...
Long wave trough that should bring Norcal`s first significant
precip in about a month`s time (i.e. Nov 8-9) during the middle of
this week is forecast to progress Ewd across NOAM Fri and through
the weekend. A drier but cool NW flow pattern is likely to
develop over Norcal on Fri before transitioning to cyclonic Wly
flow Sat/Sun with the positioning of the GOA/Yukon upper low. The
associated frontal precip band should move onshore into the Pac NW
early in the weekend trailing to the SW and across the NW portion
of CA and potentially into our NW mountain zones. Many of
ensemble members show an aggressive digging trough over the Ern
Pac into early next week which may temporarily lift the frontal
band Nwd and slow its Ewd progression on Sun before it moves
onshore early next week. This looks as if it will be a multi-day
precip event for Norcal with at least a weak AR, if not moderate
AR intensity especially for coastal locations next week. With the
origin of this low from the Bering Sea and Arctic influences as
well, this will be a cold system with the potential for heavy
Sierra snowfall as well. Timing may vary given the amplitude of
this digging system, but as of now the 13th-15th looks to be main
impact period. Until this system impacts Norcal, the clearer skies
Fri/Sat mornings could produce frosty/freezing temps which may
warrant the initial frost/freeze headlines. Max temps will be
slightly below normal as well. JHM

&&

.AVIATION...
Areas of LIFR conditions Nrn Sac Vly until 15-17z before
improving to VFR with chance of showers and MVFR conditions after
20z. Mostly MVFR conditions in the Srn Sac/Nrn San Joaquin Vly
improving to VFR conditions with chance of showers after 20z.
Widespread MVFR to LIFR conditions developing over the mountains
including Sierra Nevada this afternoon as precip develops. Then
clearing from N to S beginning this evening in the Nrn Sac Vly and
working Swd overnight. Winds mostly below 10 knots.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
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