684
FXUS66 KSTO 221211
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
411 AM PST Fri Nov 22 2024



.SYNOPSIS...
A strong atmospheric river brings periods of moderate to heavy
rain and mountain snow, isolated thunderstorms, and gusty
southerly winds to interior NorCal through the end of the week,
over the weekend and into early next week.


*Winter Storm Warning: From 4 PM Today through 4 PM Tuesday for
 elevations above 6000 feet along the Sierra/ Southern Cascades
 including Interstate 80 and Highway 50.

*Flood Watch:          Through 4 AM Saturday for Valley/foothills
                       locations generally north Interstate 80 and
                       below 3000 feet.

*Wind Advisory: Until 10 AM Friday for the Northern/Central
 Sacramento Valley and Northeast foothills.


.DISCUSSION...
Bands of moderate to heavy precipitation continue to progress
inland early this morning primarily north of I-80 as seen on RADAR.
Hourly precipitation rates are between 0.15" to 0.3" over the
Coastal Range and Sierra foothills. High-resolution convective
models depict precipitation moving over the Coastal Range,
northern/central Sacramento Valley, and northern Sierra early this
morning, before crossing over the I-80 corridor by around 5-7 AM.
Valley locations will primarily see scattered showers, with
isolated areas of moderate to heavy rainfall and gusty winds with
more organized bands of precipitation. Southerly winds are
currently gusting 30 to 40 mph around the northeastern Sierra
foothills and central Sacramento Valley. Winds will be strongest
this morning, with gusts 40 to 50 mph possible across the
aforementioned area. BM probabilities of gusts stronger than 50
MPH are 50-70% across eastern Tehama County. As a result the Wind
Advisory remains in effect through 10 AM today. Winds are
expected to weaken heading into the afternoon, with periodic gusts
25 to 35 mph.

As precipitation pushes further south and east and sliding
eastward across Northern CA today, with lingering showers over
the mountains on Saturday. Active weather reinvigorates on Sunday
evening into early next week as our low pressure system in the
Pacific NW continues troughing over Northern CA bringing continued
rain and mountain snow impacts through Tuesday morning.

Our Winter Storm Warning is in effect for 4 PM today until
4 PM Tuesday when waves of heavy snow are expected to produce
accumulations of 1 to 4 feet, up to 5 feet along the higher
peaks. The first wave of moderate to heavy snow will move through
later this afternoon continuing into Saturday afternoon. A
secondary wave moves through Sunday night into Tuesday afternoon.
Lingering scattered snow showers will be present in between these
two waves, resulting in a brief break or continued, lower-
intensity snowfall for areas in the Sierra. Snow levels will
initially be around 6000 to 7000 feet today, lowering to
4500-5500 feet over the weekend. Monday will see minor rises in
snow levels with 5000-6500 feet expected. Lastly, winds will also
gusts up to 50 MPH.

Also, with this system will see periods of moderate to heavy rain
across the Valley through early next week. Similar to snow, there
will be waves of heavier precipitation over the next several days
with the highest amounts over the mountains, then foothills, and
lastly the Valley floor. The mountains will see anywhere from 5
inches to 1 foot of precipitation through Tuesday, heaviest near
the Feather River Valley. The foothills will see 5 to 9 inches,
increasing as you move northward and approach the Feather R.
Valley. Lastly, the Valley will see 1.50 to 4 inches, heaviest
over the Northern Sacramento Valley. As a result, a Flood Watch
remains in effect for the northern half of the Central Valley for
excessive runoff resulting in sharp rises in rivers, creeks, and
streams, and ponding of water or flooding on roadways and other
low-lying locations. For the mountains and foothills north of Hwy.
50, chances for totals greater than 4 inches is 50-90% between
today and Monday afternoon. For the Valley, there is A 50-80%
chance of totals greater than 3" from today to Monday afternoon.
From this late morning through Saturday, there is a 10-20% chance
of thunderstorm development, primarily from the Northern Sac.
Valley to the eastern portions of the Valley north of Interstate
80.


.EXTENDED DISCUSSION (Tuesday THROUGH Friday)...
Ensemble guidance indicates that the upper level trough continues
to move inland on Tuesday, bringing continued rain and mountain
snow to interior Northern California. The Winter Storm Warning
remains in effect through 4 PM PST Tuesday above 5500 feet as
mountain snow showers continue through much of the day. The
National Blend of Models (NBM) suggests a 45-75% probability of
snow amounts of 6 inches or more from Tuesday through Tuesday
night across the Sierra Nevada. Snow levels are expected to be
around 4500-5500 feet for most of Tuesday, lowering to around
4000-5000 feet Wednesday morning.

Rain showers at the lower elevations are expected over a similar
period, with the NBM suggesting a 35-65% probability of a quarter
of an inch of precipitation or more in the Valley and foothills
from Tuesday through Tuesday night. A few lingering showers and
mountain snow showers will be possible on Wednesday, although
there is still some uncertainty on timing of showers tapering off
vs persisting over the midweek timeframe. With ensembles
indicating some upper level ridging building in offshore of the
Pacific Northwest / Gulf of Alaska behind the trough in the middle
of next week, a relatively drier and quieter Thanksgiving Day is
possible. Be sure to stay up to with the latest forecast at
weather.gov/sto if you have outdoor holiday plans and check
current road conditions at quickmap.dot.ca.gov before traveling!



&&

.AVIATION...
Moderate to locally heavy precipitation continues to move across
the region resulting in periods of VFR/MVFR/IFR conditions across
the Valley and foothills with areas of IFR/LIFR in the mountains.
Gusty southerly winds will continue, with gusts of 20 to 40 kts in
the Valley and foothills, and 30 to 50 kts over the Sierra. Winds
decrease after 18Z but remain breezy through the day with gusts
15-30 kts in the Valley by the evening. Some LLWS possible at RDD
and RBL through about 00Z Saturday. 10-20% probability of
thunderstorms after 17Z.




&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late tonight for Central Sacramento Valley-
Mountains Southwestern Shasta County to Western Colusa County-
Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-
Shasta Lake Area / Northern Shasta County.

Wind Advisory until 10 AM PST this morning for Central
Sacramento Valley-Northeast Foothills/Sacramento Valley-Northern
Sacramento Valley.

Winter Storm Warning from 4 PM this afternoon to 4 PM PST
Tuesday for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

&&

$$