NATIONAL WEATHER SERVICE FORECAST DISCUSSION
Area Forecast Discussion
Issued by The National Weather Service
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000
FXUS66 KSTO 232316
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
316 PM PST Thu Jan 23 2020

.Synopsis...
Valley morning fog will be possible Friday and Saturday. Some
light precipitation possible, mainly over the mountains and
northern portions of the Sacramento Valley, into Friday. Wetter
Pacific storm moves through this weekend. Unsettled weather
continues early next week.

&&

.Discussion...
First some light stratiform precip possible this evening, then
scattered showery, convective activity later this evening and
after midnight.

Frontal band extends from the Pac NW coast southwestward to off the
CA coast will be progressing Ewd this evening. Cloud tops are
generally cooling over Norcal so the moisture is getting thicker
so that the mid level WAA may leak out some light rain/sprinkles
this evening as suggested by the isentropic charts. Orographic
areas most favored for this stratiform type precip this evening,
especially the Sierra and Nrn mtns. This is portrayed in some of
the higher resolution forecast REF models and the precip may
become more widespread late this afternoon and into the evening.

Late this evening and continuing into Fri morning, there is a chance
of convective activity associated with the upper trof and embedded
vort max forecast to brush Norcal during this time. Higher REF
echoes are forecast in the higher resolution REF models (noted
above) suggesting a little heavier shower activity, especially
over Shasta Co. However, this convective activity will likely have
less areal coverage than the stratiform type. Still, we are only
looking at light QPFs. With not much precip expected in the
Sac/Nrn SJV we could see another round of Valley fog, roughly from
CIC Swd into the Nrn SJV Fri morning.

Behind this trof, upper ridging strengthens over Norcal Friday night
into early Saturday, and morning dense valley fog is possible. This
will be a mild air mass with a moderate recovery of temps, if the
fog clears. A stronger Pacific frontal system approaches Saturday
and is forecast to begin spreading precipitation into our Coastal
Range/Nrn mtns later in the afternoon. We have slowed the timing
of precip for areas south of I-80 per the latest guidance until
the Sat nite period. A secondary wave keeps a threat of post-frontal
showers over interior NorCal Sunday into Monday.

The 06z 1/23/20 GFS/ECMWF QPF ensembles have reduced their numbers
by about a third or more for the weekend-Mon precip totals from
previous runs. This of course reduces the snowfall totals along the
80/50 corridor from a swath of 7+ inches to just a couple of inches,
thus limiting the travel impacts once the snow levels fall below
pass levels late Saturday night and Sunday.   JHM

&&

.EXTENDED DISCUSSION (Monday THROUGH Thursday)...

As the weekend system moves toward the Four Corners region early
Monday, any mountain shower activity should begin to wind down.
In its wake, the guidance vary in how developed an offshore
system will become which dictates precipitation chances for Monday
and Tuesday. While a low-amplitude ridge moves in, an upstream
shortwave may bring some light warm advection precipitation to the
northern mountains on Monday. While the 12Z GFS is an outlier in
strength, there are many GEFS members supporting more widespread
precipitation early next week, especially north of I-80. Not
expecting any impacts aside from wet roads and light snow
accumulations around 5,000 feet and above. By mid week this system
will exit the picture while an offshore ridge begins to take
over. This is expected to remain a fixture in the forecast through
the remainder of the work week allowing for drier weather and a
gradual warmup. Expecting late week high temperatures to reach the
low 60s over the Valley and Delta, roughly 3 to 6 degrees above
climatology. Some of the multi-model ensembles could even squeak
out a few mid 60s by next Friday and Saturday (January 31/February
1). ~BRO

&&

.AVIATION...

Mainly VFR conditions with CIGS above 050 this evening with chance
of local MVFR conditions and some -RA this evening. Chance of
-RASH Nrn TAFS after midnight with MVFR conditions. Areas of IFR/LIFR
in BR/F+ developing from KCIC Swd to KMOD developing after 12z Fri.
Improving conditions after 18z Fri.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
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