Area Forecast Discussion
Issued by NWS Sacramento, CA

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000
FXUS66 KSTO 180650
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1050 PM PST Sat Feb 17 2018

.Synopsis...
A cold weather system brings breezy winds and snow over the
mountains and foothills Sunday into Monday, impacting travel.
This will be followed by near record cold temperatures with a
possible widespread freeze.

&&

.Discussion...
Some clouds from the incoming GOA trof are finally making their way
into the Nrn portion of our CWA (Shasta Co) with the surface front
nearing the CA/OR border. As the front sags Swd, cloud cover expected
to increase over the CWA. This system has a narrow PW Plume around
.80" but is also getting eroded with the drier air associated with
the trof. Deeper moisture is associated with the vort max near
VCRISL which is forecast to move into WA/OR and never makes it into
our CWA.

Coldest air of the winter season expected to impact Norcal beginning
Sun. Coldest morning forecast to be Tue (20th) seemingly "likely" to
set Min Temp records at some of the main climo sites. Current GOA
short wave will be driven Swd by the Nly (meridional) flow between
the ERn Pac ridge and the Polar Vortex (PV) over the Canadian arctic
archipelagos. This system will give Norcal some showers mainly Sun
afternoon/Mon, but moisture will be limited due to a lack of a tap
of subtropical moisture. This will keep amounts limited to a
few/several tenths of an inch. However, the colder temps will create
high snow ratios with the plumes showing a mean of 3" for BLU
through Mon nite, down from 5" yesterday which shows the lack of
moisture with this system.

Temp plumes for SAC on Mon (19th) morning indicate a mean of 29 and
25 on the 20th, with some colder members, and 26 and 22 for RDD.
Thus, Min Temp records look vulnerable at the main climo sites early
next week.  JHM


    Feb 18       Feb 19      Feb 20      Feb 21      Feb 22
MOD 29(1956)     29(1955)    28(1955)    29(1955)    32(1975)
RBL 29(1956)     28(2006)    28(2006)    27(2006)    30(1969)
RDD 29(2013)     25(2006)    25(2006)    29(2006)    29(2011)
DTS 31(1990)     31(1882)    31(1953)    31(1955)    32(2013)
SAC 29(1956)     33(1961)    30(2013)    29(2006)    30(1975)
SCK 29(1975)     28(1955)    28(1955)    27(1953)    28(1951)

.Previous Discussion...
A change in the weather pattern is in the works, as upper level
ridging shifts westward. This will allow a cold system from
western Canada to drop into the region tonight through Monday.
This system does not have a strong moisture tap, but with cold air
aloft will be efficient as squeezing out what moisture is
available and producing high ratio snow. Snow levels are
expected to drop Sunday through Monday, starting around 5500 and
dropping to around 1000 feet by Monday morning. Periods of snow
will bring slippery conditions at times to mountain roads. Total
snow amounts of 4 to 6 inches are expected by late Monday
afternoon over the northern Sierra. The foothills could see light
snow, with a dusting possible down to 1000 feet. A Winter Weather
Advisory is in effect for the northern Sierra, with impacts to
busy holiday travel expected. If you plan on traveling into the
northern Sierra Sunday, travel in the morning if possible, the
earlier the better. The southern Cascades, Coastal Range, and
mountains of western Plumas County could see an inch or so of
snow.

Temperatures on Monday morning will be quite cold across the
region as cold air advects into the area with the cold trough. A
Freeze Watch remains in effect for late Sunday night through
Wednesday morning. The coldest temperatures are expected on
Tuesday morning, as skies clear further. Forecast lows have
trended a little cooler, with widespread lows in the mid to upper
20s, and several hours of subfreezing temperatures. These could
break daily records, which are in the upper 20s to lower 30s for
Valley locations. Northerly winds may keep temperatures slightly
less cold over the western side of the Sacramento Valley.
Widespread frost is expected. Agricultural impacts could be
significant, due to an early bloom from unusually mild weather in
recent weeks. People should begin planning now on ways to protect
sensitive plants, outdoor pets, and outdoor pipes. Wednesday looks
slightly less cold, as the airmass warms a little. EK

&&
.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)


Models show a series of short wave troughs embedded within the
long wave trough to pass through the region during the extended
period. There is quite a bit of disagreement with timing and
placement of these short waves though. The GFS has them set up to
our east and is much drier while the EC is further west and is
able to pull in some moisture and is also deeper with them.
Unfortunately confidence in the extended is quite low because of
this. Right now the greatest rain/snow chances look to be Thursday
and again Saturday. With the systems originating from the NW they
will be cold systems with low snow levels. Snow levels with both
systems look to be around or below 3000. High temperatures for the
period will be slightly below normal to near normal and will be
slightly warmer each day. We will have to watch out for patchy
frost Thursday morning in the Valley but otherwise overnight lows
should remain warm enough to avoid frost.

-CJM

&&

.AVIATION...


VFR conditions expected at the TAF sites the next 24 hours. Low
clouds around 5000 feet will start to build in from north to south
this evening and overnight. Scattered snow showers can be expected
in the higher elevations and MVFR will be possible in any shower
activity. Cig will become more BKN after 18z Sunday between 5000
and 10000 feet. Winds increase this evening becoming 5-15 knots
and continue into Sunday.

-CJM


&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 10 AM Sunday to 4 PM PST Monday for
West Slope Northern Sierra Nevada.

Freeze Watch from late Sunday night through Wednesday morning
for Carquinez Strait and Delta-Central Sacramento Valley-Clear
Lake/Southern Lake County-Motherlode-Mountains Southwestern
Shasta County to Northern Lake County-Northeast
Foothills/Sacramento Valley-Northern Sacramento Valley-Northern
San Joaquin Valley-Shasta Lake Area / Northern Shasta County-
Southern Sacramento Valley.

&&

$$



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