190
FXUS66 KSTO 212205
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
205 PM PST Thu Nov 21 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 Friday 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:    From 7 PM this evening until 10 AM Friday for the
    Northern/Central Sacramento Valley and
   Northeast foothills.

.DISCUSSION...
Radar this afternoon remains fairly active, continuing rain north
of Interstate 80 with lingering snow showers across the
mountains. With snow levels increasing this morning and the extent
of snow across the mountain decreasing, this morning`s Winter
Storm Warning has been allowed to expire with lingering mountain
travel impacts as slick roads and chain controls persist.

For today, precipitation will slowly begin to lift northward
and weaken through this evening, although the far western sides
of the Valley, Northern Sacramento Valley, and northern portions
of the Sierra won`t see a real break precipitation. Then late
tonight, precipitation makes a return, spreading back further
south and east and sliding eastward across northern CA on Friday.
Saturday will see lingering mountain impacts as showers continue
over the Sierra.

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.

As a result, an assortment of forecast products have been issued
from this evening through early next week.

A Wind Advisory is in effect from 7 PM this evening until 10 AM
Friday for gusts of 40 to 50 MPH expected strongest early to mid-
morning Friday. NBM probabilities of gusts stronger than 40 MPH
are 70-90% across the northeastern foothills and northern portions
of the Central Sacramento Valley. Gusts of 25 to 35 mph linger
through Friday afternoon before tapering off in the evening and
overnight hours.

Our Winter Storm Watch has been upgraded to a Winter Storm Warning
for waves of heavy snow from Friday afternoon until Tuesday
afternoon showing accumulations of 1 to 3 feet, up to 4 feet
along the higher peaks. The first wave of moderate to heavy snow
will be Friday afternoon into Saturday afternoon with a secondary
wave of heavier snowfall Sunday night into Tuesday afternoon.
Lingering snow showers will exist in between these waves so some
areas may not see a break in snowfall but rather just changes in
intensity. Snow levels will initially be around 6000 to 7000 feet
Friday late afternoon and evening, 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 to
1 foot of precipitation from today through Tuesday, heaviest near
the Feather River Valley. The foothills will see 4 to 8 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 65-100% between
today and Sunday afternoon. For the Valley, there is A 50-80%
chance of totals greater than 3" from today to Sunday afternoon.
For Friday 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 (Monday THROUGH Thursday)...

By the start of the work week, rain and mountain showers will
continue to impact interior Northern California through Tuesday as
the aforementioned weather system pushes inland. Snow levels are
forecast to be around 5000-6500 feet Monday through Tuesday, with
additional forecast snow accumulations of 8 to 12 inches and up
to 2 feet along the highest peaks of the Sierra. Probabilities of
exceeding 12 inches are between 50% and 75%. Across the Valley,
there is a 30 to 50% chance of rainfall accumulations of 1.00
inch or more through Wednesday morning. Lower elevation
precipitation is expected to persist as well, with totals of 0.5
to 1.25 inches possible across the Valley and foothills and 1 to
2.5 inches possible across the mountains through Tuesday.

Heading into the midweek (Wed-Thu) timeframe, there is some
uncertainty whether this period of active weather may slowly taper
off or persist. The current forecast solution shows an additional
few inches of mountain snow, tapering off by Wednesday night. The
National Blend of Models projects an a 50 to 70% chance of 2.00
inches of snow or more over the Sierra, Wednesday AM into
Thursday with snow levels around 4000 to 5500 feet, trending lower
compared to yesterday`s forecast. The bulk of the rain ends on
Tuesday, so Wednesday and Thursday rain will be mainly constrained
to the Sierra foothills and eastern Central Valley with amounts of
a trace to 0.10" forecasted.

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 to IFR conditions across
the Sacramento Valley and foothills with isolated areas of
IFR/LIFR in the mountains through 18Z Fri. South winds continue
to trend down this afternoon with lingering gusts of 20 to 30 kts
over the northeastern foothills. South winds gusts increase after
Fri 03z and persists until Fri 18z with gusts of 25 to 35 kts up
to 45 kts expected strongest over the northeastern foothills.
Over the Sierra, southerly winds gusting 30 to 45 kts through 18z
Fri. Valley winds will decrease after Fri 18z, becoming 15-20 kts
by the late evening.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late Friday night 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 from 7 PM this evening to 10 AM PST Friday for
Central Sacramento Valley-Northeast Foothills/Sacramento Valley-
Northern Sacramento Valley.

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

&&

$$