River Flood Warning 2, NWS Sacramento, CA

Area Forecast Discussion
National Weather Service Sacramento CA
500 AM PST Mon Feb 3 2025

.SYNOPSIS...
Stormy weather is expected to return today and persist for much
of the week ahead as an atmospheric river plume brings periods of
moderate to heavy rain and mountain snow and gusty southerly
winds to interior NorCal. Major mountain travel impacts are
anticipated as a result.

&&

.DISCUSSION...
The first of three storms systems is winding down, with the next
system arriving today. As a result there is a relative lull in the
precipitation this morning, except for radar imagery showing
moderate showers over the northern mountains and moderate to
locally heavy showers over Shasta County. A Flood Warning for
Shasta County was issued yesterday afternnoon due to heavy rain
from a convergence area which developed. This produced a band of
heavier showers that have since diminished and so the Warning was
cancelled. Redding received a total of 4.22 inches of rainfall
over the past 24 hours. Cow Creek near Whitmore recorded 5.28
inches of rainfall. Some roadway flooding was reported in the area
by the CHP through the evening. There is the potential for this
convergence area to possibly develop again this evening/tonight.
Precipitation will remain mainly north of I-80 in the morning,
then drift southward through the rest of the area by afternoon.
Latest probabilities of additional precipitation exceeding 1 inch
for the next 24 hours are around 100 percent for the northern
Sacramento Valley, trending to around 40 percent for the
Sacramento metro area, and 10 to 30 percent for the northern San
Joaquin Valley. So heavier precipitation amounts will tend to be
over the northern Sacramento Valley and surrounding foothills and
mountains north of Interstate 80. Heavier precipitation spreads
through the entire area tonight through Tuesday night. Rainfall
totals through the event around 1 to 3 are expected in the Valley,
A Flood Watch has been issued from 4 pm this evening to 4 am
Wednesday, for below 3500 feet for the northern Coastal Range and
Shasta County, for below 5500 feet for the rest of the area.
Expect 3 to 7 inches of rainfall for Shasta County and the Sierra
foothills. For the mountains 4 to 10 inches of liquid equivalent,
though for elevations above 5500 feet much of this will largely be
in the form of snow. Excessive runoff may result in flooding of rivers,
creeks, streams, and other low-lying and flood-prone locations.
Creeks and streams may rise out of their banks. Flooding may occur
in poor drainage and urban areas. There is the potential for mud
and rockslides in the foothills and mountains.

Cool air from the Gulf of Alaska will gradually drop southward
today and tomorrow, with lower snow levels than we saw with the
first storm. Snow levels early this morning start around 7500
feet, then lower some to 6000 to 7000 feet by this evening. More
significant lower is expected Tuesday afternoon, when snow levels
for the northern mountains drop below 4000 feet, then below 3000
feet by Tuesday evening. A Winter Weather Advisory has been issued
for the northern Coastal Range and the mountains of Shasta County
below 3500 feet from 10 pm tonight through 10 pm Tuesday night,
with 4 to 8 inches of snow, and up to 16 inches over peaks.
Probabilities of exceeding 4 inches above 4000 feet sit around 40
to 60 percent. A Winter Storm Warning continues for the northern
Sierra and southern Cascades, mountains of western Plumas County
and Lassen National Park from 1 pm this afternoon to 10 am
Wednesday for total snow accumulations of 1 to 3 feet above 5500
feet, up to 4 feet over peaks. Winds could gust to 60 to 70 mph,
which along with heavy snowfall rates of 1 to 2 inches an hour at
times could bring periods of whiteout conditions.

Gusty southerly winds develop over the northern and Sacramento
Valley today, with gusts at Red Bluff already reaching 65 mph
early this morning. A Wind Advisory is in effect for the
northern and central Sacramento Valley and the northeast
foothills for south winds 15 to 25 mph with gusts up to 45 mph.
This continues through 4 pm Tuesday. That Wind Advisory has been
extended into the southern Sacramento Valley, the Delta, and the
northern San Joaquin Valley for 10 am to 4 pm Tuesday.

With a break between the early week trough(storm #2) and then a
late week shortwave (Storm #3), there will be a brief lull in the
precipitation with some scattered light to moderate precipitation
will be possible on Wednesday. Winds will remain breezy, but are
also expected to decrease as the trough exits eastward.

Storm #3 arrives on Thursday as a cold trough off the Pacific
Northwest approaches and begins to move inland Thursday night.
Snow levels are expected to be around 3000-4000 feet over the
northern mountains and 4000 to 5000 feet over the northern Sierra.
Moisture amounts have trended a little lower than previously seen,
but expect to see heavy snow again, with 1 to 3 feet possible
through Thursday night, and another 1 to 2 feet of additional
snow possible through Friday.
&&

.EXTENDED DISCUSSION (Friday THROUGH Monday)...
Ensemble guidance has cold longwave trough (Storm #3) moving
eastward bringing moderate to heavy precipitation through the end
of the week, with lower snow levels, thought snowfall
accumulations at lower elevations remain somewhat uncertain. The
upper foothills (3000 feet) have some potential to see some snow,
currently forecast by the NBM to see light accumulations around an
inch. Rainfall amounts look less than the first two storms.
Probabilities of precipitation exceeding 1 inch for 48 hours
through early Saturday are around 80 percent for the northern
Sacramento Valley and surrounding terrain and along the
Sierra/southern Cascades, with 20 to 40 percent probabilities
further south in the Valley. Snowfall totals of 1 to 4 feet are
expected along the Sierra/southern Cascades, with resultant
probabilities of exceeding 1 foot of snowfall above 5000 feet
around 75 to 95 percent. This heavy snowfall combined with that of
earlier storms will make mountain travel even more difficult.

Be sure to prepare now for the continued snowy and difficult to
impossible wintry travel in the mountains! In addition to
precipitation, periods of gusty southerly winds are also expected
to continue. For the weekend, ensembles continue to show a trend
toward a drier pattern.

&&

.AVIATION...

MVFR conditions with areas of IFR/LIFR conditions are expected
this morning, mainly in the northern Sacramento Valley and
adjacent foothills as a result of scattered showers. Isolated
areas of MVFR/IFR will be possible elsewhere in the Valley and in
the mountains through 21Z today. MVFR conditions will become more
widespread with areas of IFR/LIFR as precipitation coverage
increases throughout interior Northern California after 23Z today.
Heaviest precipitation will be in the northern Sacramento Valley
and Sierra foothills/mountains. Snow levels will be between
6000-7000 feet. Southerly surface winds 15 to 25 kts with gusts
20 to 40 kts until 21Z Tuesday in the northern Sacramento
Valley/adjacent foothills and from 00Z Tuesday to 03Z Wednesday
elsewhere in the Valley. northern Sacramento Valley. Over the
mountains, southerly wind gusts up to 65 kts.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 4 PM PST this afternoon through late Tuesday
night 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-Shasta Lake Area /
Northern Shasta County-Southern Sacramento Valley-West Slope
Northern Sierra Nevada-Western Plumas County/Lassen Park.

Winter Weather Advisory from 10 PM this evening to 10 PM PST
Tuesday for Mountains Southwestern Shasta County to Western
Colusa County-Shasta Lake Area / Northern Shasta County.

Wind Advisory until 4 PM PST Tuesday for Carquinez Strait and
Delta-Central Sacramento Valley-Motherlode-Northeast
Foothills/Sacramento Valley-Northern Sacramento Valley-Northern
San Joaquin Valley-Southern Sacramento Valley.

Winter Storm Warning from 10 AM this morning to 10 AM PST
Wednesday for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

&&