944
FXUS66 KSTO 121024
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
324 AM PDT Wed Mar 12 2025

.SYNOPSIS...
A pattern shift toward active weather begins today as the first
in a series of weather systems impacts the region. Periods of
moderate to heavy rain and isolated thunderstorms, heavy mountain
snow, and gusty winds are expected to bring major travel impacts
at times through the end of the week. A brief lull in impacts is
expected over the weekend before another system arrives into early
next week.

&&

.DISCUSSION...

*Storm #1 (Today-Thursday):

While radar representation of precipitation remains scattered and
mostly confined to the northern Sacramento Valley early this
morning, a rapid transition to widespread precipitation is
expected through the morning hours. This initial wave of
precipitation will be aided by a plume of atmospheric river
moisture advancing into interior NorCal by the afternoon and
evening hours. As the approaching trough nears the coast at the
same time, enhanced precipitation rates are expected across the
region, particularly along the Sierra/southern Cascades this
afternoon and evening.

Some 5 to 15 percent probabilities of thunderstorms exist today,
primarily centered along the northern/central Sacramento Valley
and adjacent foothills. With the trough becoming centered over the
region by midday Thursday, probabilities of thunderstorm
development jump up to 20 to 35 percent for Thursday afternoon and
evening. Instability on the order of 200 to 400 J/kg will be
possible on Thursday as potential for ample surface heating is
expected amidst remnant available moisture. Magnitude of shear
profiles will be limited underneath the trough, but proximity to
the trough itself may offer some localized enhancement.
Regardless, any thunderstorms that develop will be capable of
brief heavy rain, gusty winds, hail, lightning, and possibly a
brief funnel cloud.

As a tightening surface pressure gradient evolves underneath an
advancing mid level jet streak, gusty south-southwest winds are
also expected throughout much of the day today. Widespread wind
gusts 40 to 55 mph are expected through the Delta, Valley, and
foothills today, with strongest gusts to 65 mph possible along the
higher terrain. Outside of thunderstorms, heaviest precipitation
is expected to occur this afternoon and evening, with more
moderate to locally heavy rates into Thursday. At this time,
probabilities of exceeding 1 inch of precipitation sit around 15
to 30 percent for most locations throughout the Delta and Valley,
increasing to 50 to 80 percent probabilities across the northern
Sacramento Valley and throughout the foothills. Along the higher
terrain, probabilities of exceeding 2 inches of precipitation
persist around 60 to 90 percent.

Snow levels are still expected to vary through the event,
starting around 4500 to 5500 feet at precipitation onset this
morning, then rapidly falling to 3000 to 4000 feet by the
afternoon and evening, before settling around 2500 to 3500 feet by
Thursday. With heaviest precipitation expected this afternoon and
evening along the Sierra/southern Cascades, the timing of
dropping snow levels will have a significant bearing on what lower
elevation snowfall accumulation potential looks like. At this
time, most accumulations are expected above 3000 feet across the
Coast Range/Shasta County mountains and above 4000 feet along the
Sierra/southern Cascades, although some nonzero probabilities do
exist for light accumulating snowfall as low as 3500 feet.
Forecast snowfall totals from today through Thursday remain
largely unchanged at this time, with 4 to 12 inches possible above
3000 feet across the Coast Range and Shasta County mountains and
up to 2 feet over peaks, with 1 to 3 feet possible above 4000 feet
along the Sierra/southern Cascades and up to 4 feet over peaks.
Probabilities of snowfall rates exceeding 3 inches per hour peak
around 40 to 70% along the Sierra/southern Cascades this afternoon
and evening.

*Storm #2 (Friday-early Saturday)

A brief lull in precipitation impacts late Thursday evening into
the overnight hours is expected, but will be conditional upon the
end timing of convection moving up the terrain Thursday evening. A
quick moving shortwave is then on track to bring renewed
precipitation chances by early Friday morning. Some uncertainty
still remains regarding the timing of warm air advection as the
shortwave approaches, with a slight lag in this feature resulting
in some potential for lower elevation snowfall across Shasta
County and into the upper foothills of the Sierra/southern
Cascades. Even the most generous estimates indicate snow levels
remaining around 2000 to 3000 feet at best for much of the event,
lending credence to at least some potential for accumulating
snowfall in these locations. Current probabilities of up to 2
inches of snowfall between 2500 and 3500 feet along the
Sierra/southern Cascades sit around 30 to 60 percent at this time,
with some nonzero probabilities of a dusting across the lower
elevations of Shasta County as well.

The expected trajectory of this shortwave will likely limit
overall precipitation potential through the remainder of the
Valley and Delta, with probabilities of exceeding 0.5 inches of
precipitation around 20 to 40 percent. Attendant probabilities of
precipitation exceeding 1.0 inches for Friday continue to sit
around 30 to 60 percent across the mountains and foothills. Aside
from lower elevation snowfall potential, more appreciable snowfall
accumulations are expected above 3500 feet at this time, with
probabilities of exceeding 6 inches of snowfall around 60 to 90
percent, and additional probabilities of 50 to 70 percent for
exceeding 1 foot of snowfall above 6000 feet.

Ensemble guidance remains in good agreement that the Friday
system will be fairly progressive, as most precipitation is
expected to come to an end by Saturday morning. Additionally, a
transient period of ridging aloft building in between systems over
the weekend looks to keep Saturday dry. Despite this relative
lull, with a prolonged period of active weather expected to
encapsulate much of the next 7 days ahead, be sure to take time
now to prepare for adverse weather impacts, monitor the forecast
for additional updates, and be sure to have a reliable way to
receive weather and travel alerts.

&&

.EXTENDED DISCUSSION (Sunday THROUGH Wednesday)...

*Storm #3 (Sunday-Tuesday):

Notable uncertainty exists pertaining to the evolution of the
next expected trough late weekend into early next week, but
additional periods of at least moderate precipitation are
anticipated as it moves through. Ensemble guidance generally
agrees that the overall trajectory of the system would favor
potential for slightly higher precipitation totals relative to the
Friday system, but some uncertainty regarding the magnitude of
the attendant atmospheric river plume keeps forecast confidence in
exact details low at this time. Current probabilities moving into
next week are as follows:

Probability of Rain > 1" Sunday-Tuesday AM: 20-50%
Valley/foothills

Probability of Snow > 12" Sunday-Tuesday AM: 60-95% above 5000`

Probability of Wind Gusts > 40 mph Sunday: 55-90%

Probability of Thunderstorms Sunday-Monday: 10-20%

At this time, most precipitation is expected to be coming to an
end by Tuesday morning, with primarily lingering mountain showers
anticipated at this time. Ensemble guidance differs on the upper
level pattern moving past the extended forecast period, but
consensus indicates dry weather continuing much of Tuesday through
Wednesday. Regardless, be sure to monitor the latest forecast at
weather.gov/sto and plan ahead for continued cool, wet and windy
weather into early next week!

&&

.AVIATION...
VFR conditions early this morning are expected to rapidly
deteriorate to nearly widespread MVFR by 15z. Additional
reductions to IFR/LIFR expected in periods of low elevation rain
and mountain snow. Heaviest precipitation rates resulting in
IFR/LIFR during the forecast period expected 18z-00z through the
Valley and 15z-06z along the Sierra. Increasing south to west
winds gusting 30 to 45 kts through the Valley today, expected to
peak 16z-22z. Wind gusts 55 to 65 kts anticipated along the Sierra
crest as well.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 11 PM PDT Thursday for Burney Basin /
Eastern Shasta County-Mountains Southwestern Shasta County to
Western Colusa County-Shasta Lake Area / Northern Shasta County-
West Slope Northern Sierra Nevada-Western Plumas County/Lassen
Park.

Wind Advisory until 11 PM PDT this evening for 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-Southern Sacramento Valley.

&&

$$