Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS66 KSTO 180650

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

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.


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 

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


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. 




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. 



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-

National Weather Service Glossary of Abbreviations