Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 212326

Area Forecast Discussion
National Weather Service Sacramento CA
326 PM PST Tue Nov 21 2017

Mainly dry and mild weather this week with rain chances limited
to far northern CA. Cooler, wetter system possible early next


Dry weather across the area today. Lingering low clouds have
broken up, with some high clouds. Have kept just a slight chance
of light rain showers over northwest Shasta County into the early
evening. So far, radar is just showing a few weak returns in that
area. Low cloud deck and northerly winds limited the development
of fog this morning, but expect this will not be the case for late
tonight through Wednesday morning, with dense fog developing in
the Valley and in mountain valleys.

While mild overall today, low clouds have kept down temperatures
where they have persisted longer. A few spots may yet reach 70,
such as Redding and Vacaville, but many Valley locations will
remain in the 60s. Temperatures are tricky again tomorrow, with
fog likely developing in the Valley. If this fog lingers for a
while, it could keep some spots from reaching the low 70s that are
forecast. Temperatures this time of year in the Valley are
typically in the low 60s. For the mountains, 50s and 60s are
expected, which is relatively mild.

A weak weather system will make its way into NorCal for
Thanksgiving, bringing the potential for some light rain to the
area from I80 northward. Snow levels are highs, so even mountain
travel will not be impacted by snow. Highs look mild again,
similar to those of Wednesday. Friday will also be mild, and has
trended dry as the weak system exits Thursday night/early Friday
morning. EK


After a dry day on Fri, initial Holiday Weekend trof moves
into/across Norcal shunted to the N by the seasonably strong Baja
High Center. The frontal wave is forecast to remain offshore, so
this will be a relatively warm, weak/glancing-blow system for our
CWA. Shower chances will be mainly N of I-80 and the heavier
amounts in the mountains (up to around 1/2 inch or so) with high
snow levels.

A deeper low and associated trough will approach the area Sunday
and into Mon with the GFS 6-12 hrs faster than the slower and
slightly colder ECMWF. There remains uncertainty with precipitation
amounts, primarily due to the timing/speed of this system, with
the slower/deeper ECMWF the wetter model. For now, amounts range
from .10" south to around .50" north in the Valley with a little
over an inch in the Nrn mtns/Sierra. Snow levels are forecast to
drop below pass levels Sunday night, but precip could taper off as
colder air moves in Mon morning as snow levels drop below 4000-5000
ft. Seasonably mild temperatures expected Sat, then cooling
Sun/Mon to below normal mainly N of I-80 and near or slightly
above normal south of I-80. Drier conditions Mon/Tue as high
pressure rebuilds over Norcal leading to a warming trend on Tue.
Nly winds should keep fog at bay Tue morning and local adiabatic
warming effects from the katabatic winds.   JHM



Areas of MVFR/IFR will be possible late tonite and again Wed morning
due to patchy fog and low cigs. Conditions improve to VFR during the
day. Winds remain under 10 knots.



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