Area Forecast Discussion
Issued by NWS Sacramento, CA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

000
FXUS66 KSTO 220616
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1015 PM PST Sun Jan 21 2018

.Synopsis...
Pacific storm brings light to moderate precipitation to Norcal into
Monday. Brief period of dry weather as high pressure rebuilds over
the area later Monday and Tuesday. Stronger system impacts the area
midweek into Friday bringing moderate to heavy rain and mountain
snow, and gusty wind.

&&

.Discussion...
Update: Shasta Dam profiler shows snow level around 2 kft with
reports of snow on I-5 north of RDD down to around 1500 ft. Farther
SE, OVE profiler shows the snow level around 4 kft. Radar shows
precip just beginning to move into the I-80 corridor where the
precip should be in the form of snow at BLU. Also Freezing/Melt
level around 3500-4000 ft showing up as higher reflectivities in the
Nrn Sac Vly indicating some moderate precip moving through that
area.

Short wave trof forecast to move through the Pac NW early Mon and
ridging will return to the region. This will generally end the
precip, but moisture will remain trapped below the subsidence
inversion so clouds will be stubborn, persistent and slow to clear.
There should be some clearing on the west side of the Valley in the
afternoon due to the NWly downslope flow off the coastal range,
while upslope flow over the Sierra will likely keep skies cloudy
there.

Precip timing from the HRRR and NAM3km suggest precip will be
winding down over our advisory area after 3 am or so. So we will
look at the status of the advisories overnight.   JHM

.Previous Discussion...

Pacific frontal system continues to make a slow progression inland
this afternoon as 5H flow nearly parallels surface boundary. Run
to run of the HRRR has trended slower with front advancement.
Latest showing precip spreading over the Coastal and Shasta
mountains through the afternoon and across the Sacramento Valley
into Western Plumas county this evening. Bulk of precip moves
through overnight. Snow levels expected to slowly rise to 3500 to
4000 feet by Monday morning. Snow accumulations up to 5 inches
possible in the Shasta and Coastal mountains, with local amounts
up to 10 inches over highest terrain. Slightly less accumulations
expected in the mountains of Western Plumas overnight, while only
up to 3 inches forecast for the Northern Sierra Nevada.

Showers Monday morning decrease in the afternoon as subsidence
increases over Interior NorCal. Upper ridging brings drier weather
Monday night into Tuesday. Some patchy morning fog may develop in
southern portions of the Sacramento Valley and Northern San
Joaquin Valley Tuesday morning. Temperatures trend up early next
week to near or slightly above normal Tuesday.

Stronger Pacific frontal system spreads inland over far NW CA
Tuesday night then across all of Interior NorCal Wednesday.
Breezy to windy conditions develop ahead of the front Wednesday.
Models currently showing significant QPF with this storm. Snow
levels attm initially look to be around 5000 to 6000 feet
Wednesday morning, but lower to 2500 to 3500 feet Wednesday night
into Thursday in post-frontal colder AMS. Showery weather follows
Thursday.

PCH

&&

.EXTENDED DISCUSSION (Thursday THROUGH Sunday)
A pacific front will be tracking to our south and east to start
the extended period with the tough axis working through NorCal.
This will bring showers to the area throughout Thursday. Cold air
quickly builds in behind the front and we will see low snow
levels, 2000 to 3000 feet. Mountain travel will likely be
difficult on Thursday and this includes both I-80 and I-5.
Mountain showers will continue into Friday due to onshore flow but
should diminish by Friday afternoon. A warm front pushes into the
PacNW late Friday into Saturday and that may bring a few isolated
showers to the Shasta Mountains Friday night. Otherwise quiet
weather settles in for the weekend as a ridge builds in overhead.
We will see a cool start to the period with highs generally
running 5-10 degrees below normal but as the ridge builds in
towards the weekend we will see them return to near normal.

-CJM

&&

.AVIATION...
VFR conditions expected through 02z Monday. Cigs lower to MVFR,
1500 to 3000 feet after 02z from north to south. Light showers are
also expected through the overnight. Showers diminish between
12-15z with improving conditions into Monday afternoon but areas
of MVFR will still be possible. 5-10 knot winds are expected
throughout the period.

-CJM

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 4 PM PST Monday for Mountains
Southwestern Shasta County to Northern Lake County-Shasta Lake
Area / Northern Shasta County-Western Plumas County/Lassen Park.

&&

$$



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.