Area Forecast Discussion
Issued by NWS Sacramento, CA

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000
FXUS66 KSTO 201735
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
935 AM PST Sat Jan 20 2018


.Synopsis...
Dry today then a series of Pacific storms move through Sunday
through Thursday bringing rain, mountain snow, and gusty wind at
times.

&&

.Discussion...
Upper level ridging builds into the forecast area today bringing
drier weather with some light northerly wind. High temperatures
this afternoon expected to be near normal temperatures. Ridge
axis shifts through overnight allowing upstream baroclinic cirrus
to advect into the CWA. Guidance showing overnight low
temperatures in the Central Valley expected to be similar to this
morning. However amount of higher clouds moving in could impact
morning fog or frost development.

Pacific frontal system begins to spread precipitation into
western portions of the CWA tomorrow afternoon, and then across
the entire CWA Sunday night into Monday. Models agree on bulk of
precip expected over the northern and western portions of our
forecast area but have significant QPF differences over Interior
NorCal. GFS20 is the driest with 00Z EC wettest, and NAM-12
between the two. This adds some uncertainty to the Winter Storm
watch currently in effect. With colder air already in place, snow
levels will be low enough to impact travel along I-5 through the
Shasta-Siskiyou mountains. Rain totals in the Central Valley range
from around a tenth of an inch to over three-quarters of an inch,
highest in the Northern Sacramento Valley. For the foothills and
mountains, values range from around half an inch to 2 inches.

Precip diminishes by Monday afternoon as subsidence increases over
Interior NorCal. Upper ridging brings drier weather Monday night
and Tuesday. Night and morning valley fog may be more prevalent
during this period. Temperatures trend up Monday into Tuesday to
near or slightly above normal.

PCH

&&

.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)
Mid range models in fairly good agreement in bringing a Pacific
frontal system into northwest California Wednesday morning then
through the north state during the day on Wednesday. 00z runs
slowed this system down about 6 hours but 06z run sped it back up
again. This system looks to be of moderate strength with moderate
snow levels between about 4500 and 5500 feet. Main frontal band
moves into the Sierra Thursday night with several inches of
snowfall expected for likely travel impacts. Main upper trough
pushes through the region on Thursday with cold air behind the
front bringing snow levels down to 2500 to 3500 feet. Instability
will keep showers going through the day with daytime highs
remaining a little below normal. Upper ridge begins to build over
the eastern Pacific and west coast on Friday pushing the shower
threat northward to north of about Red Bluff. Upper ridge
continues to amplify on Saturday bringing daytime temperatures up
to a few degrees above normal. Most of the CWA should remain dry
next Saturday if current models hold but a shortwave slipping over
the ridge could bring a few showers over the mountains of
northern Shasta county.


&&

.AVIATION...
VFR conds ovr Intr NorCal nxt 24 hrs exc isold MVFR/IFR conds poss
in Cntrl Vly in BR/FG btwn 10z-18z Sun. Dtrtg conds Sun aftn into
ngt as nxt Pac fntl sys movs inld. LCL N to NW sfc wnds up to 12
kts poss in Cntrl Vly til arnd 00z Sun.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Sunday morning through Monday morning
for Mountains Southwestern Shasta County to Northern Lake County-
Shasta Lake Area / Northern Shasta County-Western Plumas
County/Lassen Park.

&&

$$


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