Area Forecast Discussion
Issued by NWS Sacramento, CA

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123
FXUS66 KSTO 131130
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
330 AM PST Fri Dec 13 2019


.SYNOPSIS...
Light to moderate valley rain and moderate to heavy mountain snow
Friday through Saturday night. Mountain travel delays are likely.
Drier weather Sunday and early next week.

&&

.DISCUSSION...
Upper level flow remains mainly zonal flow over NorCal today.
Multiple impulses along the flow will bring another weak Pacific
moisture plume into NorCal today. Periods of light to moderate
precipitation is expected to return to NorCal today through
Saturday creating travel concerns over the Sierra. The first wave
of precipitation today is expected to begin in the northern
mountains & Valley early this morning slowly spreading south
through the day. Hires models show heavier band of precipitation
reaching the Central Valley by late afternoon, impacting the
evening commute. Snow amounts of around 10 to 15 inches are
forecast at pass level with the heaviest falling after 4pm this
afternoon through tonight. Snow levels will be high today
generally above 6500-7000 feet, beginning to fall overnight to
around 4500 to 5500 feet by early Saturday. Liquid precipitation
totals of a few hundredths up to half an inch are expected in the
Valley with a half an inch to 2 inches forecast in the mountains.
Winds will become breezy Friday over the mountains with gusts up
to 30 to 40 mph possible at higher elevations. This combined with
falling snow could create low visibilities over mountain passes.

Models continue to indicate a brief break in precipitation
Saturday late morning and afternoon before a trough originating
from the Gulf of Alaska brings another round of precipitation
Saturday evening through the night, mainly limited to the
mountains and foothills. As this is a colder system, snow levels
will be around 4000 to 4500 feet. However, this will also be not
as wet with additional liquid precipitation amounts of a few
hundredths in the Valley to up to half an inch in the mountains.
Additional snow amounts of 4 to 8 inches are possible above 5000
feet with a few inches possible down to 4000 feet. Confidence is
still fairly low as higher resolution models are not bringing in
as much precipitation with this wave as GFS & Euro.

Upper level ridging returns Sunday, bringing dry conditions by
Sunday morning through Monday. Overnight temperatures will drop
Saturday night around 5 to 10 degrees after passage of surface
front, then will drop another few degrees Sunday night as lack of
cloud cover will allow radiational cooling effects. Valley
temperatures expected to fall into the 30s with widespread teens
and 20s in the mountains through Monday night. Daytime
temperatures will remain fairly steady with a slight cooling
Sunday. HEC

&&

.EXTENDED DISCUSSION (Tuesday THROUGH Friday)...
The brief dry trend from the weekend will continue into early
next week. On Tuesday, the transitory ridge to likely shift over
toward the Central Great Basin ahead of a compact upper low
nearing the California coast. Given the complex of nature of this
system separating from the westerlies and eventually closing off,
uncertainty still plagues the forecast. While the GFS/ECMWF
ensembles agree on the existence of this system, they continue to
exhibit spatio-temporal differences. The former model suite
remains the wetter solution with widespread precipitation entering
the picture by mid-week. On the contrary, The ECMWF ensemble
system takes a more southern track favoring impacts down in
southern California. There is enough ensemble spread to keep
either scenario in play so will continue maintaining precipitation
in the forecast. Expect increasing cloud cover given the influx
of Pacific moisture with near average temperatures for mid-
December.

Looking toward the following weekend, there are still fairly
strong signals of wet weather in response to a full-latitude
trough approaching the Pacific coast. While some of the 24-hour
probabilities for 0.50 and 1.00 inches of precipitation have come
down a bit, this will continue to be watched. The Climate
Prediction Center experimental Day 8-14 hazards supports this
notion with Dec 20-22 being highlighted for heavy precipitation
which includes snow in the mountains. ~BRO

&&

.AVIATION...
Areas of MVFR/IFR conditions, local LIFR, next 24 hours, becoming
more widespread by early morning. Pacific system will bring
widespread precipitation after 14z-16z Friday. Snow levels
lowering to 6500 feet Friday. Light winds generally less than 12
kts.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from noon today to 4 AM PST Sunday for
West Slope Northern Sierra Nevada-Western Plumas County/Lassen
Park.

&&

$$



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