Twain Harte, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

505
FXUS66 KSTO 222120
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
120 PM PST Mon Dec 22 2025

For additional details on weather and expected impacts over the
next 7 days, please visit weather.gov/sto/briefing.

.KEY MESSAGES...

 - Series of atmospheric rivers will bring major holiday travel
   impacts from periods of heavy rain, heavy mountain snow, and
   gusty winds throughout the week.

 - High snow levels begin falling from late Tuesday onward.
   Heaviest snow is expected Wednesday and Thursday, with snowfall
   rates of 1-2" per hour at times.

 - Periods of heavy rainfall are anticipated across the Valley and
   lower foothills with additional flood impacts possible through
   the end of the week.

 - Gusty southerly winds arrive mid to late week, with the
   strongest wind gusts on Wednesday and Thursday.

&&

.DISCUSSION...

...Today-Tuesday...

A longwave trough continues digging offshore of central
California this afternoon introducing a broad fetch of atmospheric
river moisture to interior NorCal. As the latest in a series of
embedded shortwaves travels northward, scattered showers will
follow suit, with most precipitation through this evening expected
from Interstate 80 northward. As this area of precipitation
trends northward, there will be some additional potential for
locally heavier rainfall rates in the northern Sacramento Valley
from this afternoon through the overnight hours.

By Tuesday, the longwave trough looks to deepen over the Pacific
as the shortwave trough departs the region. While showers are
expected to linger into Tuesday morning across the northern
Sacramento Valley and surrounding terrain, a brief lull in
widespread precipitation is anticipated through much of Tuesday
afternoon. As the longwave trough gradually makes a shift toward
the coast later in the day Tuesday, southerly winds will become
breezy through the afternoon with precipitation chances increasing
by late afternoon. Snow levels are expected to remain around
7000-8000 feet through Tuesday.

...Wednesday through Friday...

The aforementioned trough will continue a gradual onshore push
through the mid to late week time frame alongside a deeper
atmospheric river moisture surge. This will introduce additional
periods of widespread heavy rain, heavy mountain snow, and gusty
southerly winds through the end of the week. Additionally,
isolated to scattered thunderstorms will be possible at times from
midday Wednesday through Thursday evening.

Despite initially high snow levels early Wednesday morning, a
rapid drop to snow levels around 5500-6500 feet is anticipated by
mid morning on Wednesday as the offshore system approaches and
pulls colder air into the region. Snow levels then look to
stabilize through early Thursday morning before rapidly dropping
again to around 4500 to 5500 feet along the Sierra and to around
3500 feet for the Coast Range and Shasta County mountains from
midday Thursday through Friday. Heaviest periods of widespread
precipitation are currently expected on Wednesday and Thursday,
with a gradual decrease in areal extent and intensity on Friday.
Similarly, strongest winds associated with this storm are
anticipated on Wednesday and Thursday, remaining breezy on Friday.
As a result, a Flood Watch in effect through Friday and a High
Wind Watch in effect through Thursday remain in place for lower
elevations, with a Winter Storm Warning in effect from Tuesday
evening through Friday evening for the Sierra/southern Cascades
and portions of the Coast Range and Shasta County mountains.

Given the magnitude of expected rainfall, mountain snowfall, and
gusty winds, major travel impacts affecting the holiday travel
period are anticipated throughout the week. The most favorable
travel window at this time is expected through Tuesday afternoon.
A combination of dangerous mountain travel conditions from heavy,
wet snow and gusty winds and periods of difficult travel at lower
elevations from heavy rain, gusty winds, and thunderstorm chances
is likely Wednesday through Friday. If you have travel plans this
week plan ahead, make alternate routes, have backup plans, and
carry safety kits with you in case of emergency. If you see
flooded areas, turn around don`t drown.

...Saturday through Monday...

By Saturday, the longwave trough is forecast to eject out of
NorCal with a trend toward drier weather looking likely through
the weekend and into early next week. As the system is departing
the area on Saturday, there will be some potential for breezy to
gusty north winds in the Valley, but exact details remain
uncertain. Otherwise, with some indications for ridging to build
in by early next week, there will at least be a chance for Valley
fog to return the region.

&&


.AVIATION...

Areas of MVFR/IFR/LIFR in precipitation and low ceilings across
the southern Sacramento Valley, Delta, and Sierra primarily along
and south of I-80 through this afternoon and early evening, then
activity shifts northward across the northern/central Sacramento
Valley and adjacent terrain. Brief lull in activity tomorrow
across most areas, except for the northern Sacramento Valley and
adjacent terrain. Potential for MVFR/IFR/LIFR ceilings and/or
visibilities in dense fog/mist across portions of the southern
Sacramento Valley and northern San Joaquin Valley after 06z-12z
Tuesday. Increased southerly winds after 18z Tuesday, strongest
after 00z Wednesday with sustained winds 20 to 30 kts and gusts
35-50 kts in the Valley and foothills. Southwesterly wind gusts up
to 40-55 kts over the mountains after 18z Tue - 00z Wednesday.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Friday afternoon for Burney Basin / Eastern
Shasta County-Carquinez Strait and Delta-Central Sacramento
Valley-Motherlode-Mountains Southwestern Shasta County to
Western Colusa County-Northeast Foothills/Sacramento Valley-
Northern Sacramento Valley-Northern San Joaquin Valley-Shasta
Lake Area / Northern Shasta County-Southern Sacramento Valley-
West Slope Northern Sierra Nevada-Western Plumas County/Lassen
Park.

Winter Storm Warning from 10 PM Tuesday through 10 PM PST Friday
for West Slope Northern Sierra Nevada-Western Plumas County/Lassen
Park.

Winter Storm Warning from 4 PM Wednesday to 10 PM PST Friday for
Burney Basin / Eastern Shasta County-Mountains Southwestern
Shasta County to Western Colusa County-Shasta Lake Area /
Northern Shasta County.

High Wind Watch from Tuesday afternoon through Friday afternoon
for Carquinez Strait and Delta-Central Sacramento Valley-
Motherlode-Northeast Foothills/Sacramento Valley-Northern
Sacramento Valley-Northern San Joaquin Valley-Southern
Sacramento Valley.

&&

$$