Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 160502
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
902 PM PST Mon Jan 15 2018

.Synopsis...
A weak and warm weather system passes through the region tonight
into Tuesday, followed by stronger and colder storm systems
Thursday-Friday, and next Sunday-Monday.

&&

.Discussion...
Updated the forecast to add dense fog and drizzle in addition to
the rain chances already in the forecast overnight. Patches of
dense fog already starting to form and with the very weak front
there is no wind to mix out the fog but should be enough to give
us some drizzle. Conditions will continue to deteriorate this
evening so have issued a dense fog advisory with the main areas of
concern from around Marysville southward. Conditions may improve
for a few hours as the rain moves through late tonight but should
deteriorate after the front moves through in the morning.

.Previous Discussion...
High pressure ridge has begun to shift eastward as a high-
amplitude frontal system approaches the West Coast. Considerable
high cloudiness has spread over the region, on top of an extensive
low stratus deck across the Valley floor, a holdover from last
night`s fog. Temperatures have been slow to climb today as a
result of the cloudiness, as readings across the Valley are in the
lower 50s as of this writing.

The aforementioned storm system will begin to spread into the
interior late tonight, and across the region Tuesday. This system
is already showing signs of weakening as it runs into the upstream
ridge, and should continue to do so. The bulk of precipitation
should remain along Interstate 80 and northward, and precipitation
totals should be on the light end of the spectrum. Perhaps a
couple of inches of snow will be possible over the higher mountain
peaks.

A brief period of dry weather is expected Wednesday, with the
potential for morning Valley fog. The next storm then arrives
beginning late Wednesday night across the far north, and spreading
southward through the day Thursday and Thursday night. This 2nd
system looks to be stronger than the 1st, but the bulk of the
precipitation looks to favor the mountains with this system. This
system will be considerably colder than the first, with snow
levels rapidly lowering below Sierra pass levels, and down to
3000-4000 Thursday night into Friday. Showery weather should
continue on Friday as a baggy upper trough passes overhead. A
Winter Storm Watch has been issued to address these snow concerns.

Dang

&&


.EXTENDED DISCUSSION (Friday THROUGH Monday)


A cold front will be pushing to our south to start the extended
forecast. Cold air will continue to build into NorCal behind the
front and we will see snow levels fall below 3000 feet by the end
of the day Friday. The trough axis will swing through the region
on Friday and that will keep shower activity in the forecast
mainly through the mid evening. Poor Mountain travel conditions
can be expected to continue throughout the day. We do see some
instability build into the Valley during the afternoon and can`t
rule out some thunder with any of the Valley showers along with
some small hail in the most intense ones. High temperatures on
Friday will also be about 5-15 degrees colder compared to
Thursdays forecast highs.

A weak ridge will pass overhead during the day Saturday and that
will bring mainly dry conditions with similar highs to Friday. An
upper level low is expected to drop out of the Gulf of Alaska
Sunday and that will push rain back into the forecast from north
to south. Models are more aligned with each other today but still
differ a bit with the timing. This system looks to be a wetter
one and snow levels look to remain relatively low and Mountain
travel will likely be affected.

-CJM

&&

.AVIATION...


Areas of MVFR/IFR will continue in the central Valley this
afternoon and evening due to fog and low cigs. VFR expected at the
northern terminals through 6z Tuesday. Light rain pushes in
overnight continuing into tomorrow morning causing MVFR/IFR.
Conditions improve by 21z Tuesday but areas of MVFR/IFR are
expected to continue. Winds remain under 10 knots.

-CJM
&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Thursday afternoon through Friday
afternoon for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

Dense Fog Advisory until 11 AM PST Tuesday for Carquinez Strait
and Delta-Central Sacramento Valley-Northern San Joaquin Valley-
Southern Sacramento Valley.

&&

$$


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