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FXUS66 KSTO 022113
AFDSTO
Area Forecast Discussion
National Weather Service Sacramento CA
113 PM PST Sun Feb 2 2025
.SYNOPSIS...
Despite a relative lull today, impactful weather is expected to
return and persist for much of the week ahead as an elongated
atmospheric river plume and associated systems bring periods of
moderate to heavy rain and mountain snow and gusty southerly winds
to interior NorCal. Major mountain travel impacts are anticipated
throughout the week as a result.
&&
.DISCUSSION...
As of early this afternoon, radar imagery shows early morning
precipitation drifting eastward off of the Sierra crest, with
additional lingering rainfall across the northern Sacramento
Valley and surrounding terrain. Elsewhere, a few isolated, light
showers are evident as well, with overall minimal impacts. Current
probabilities of additional precipitation exceeding 1 inch today
around 60 to 90 percent continue to favor the northern Sacramento
Valley and surrounding terrain and portions of the Sierra/southern
Cascades north of Interstate 80. Otherwise, gusty winds continue
through the early afternoon across portions of the northern and
central Sacramento Valley, but will be gradually decreasing
through the remainder of the day.
The primary longwave trough driving weather impacts remains
firmly positioned over the Pacific Northwest via fluctuating, but
amplified, ridging aloft in place across the central and eastern
CONUS. As this ridging amplifies over the Plains into the early
week period, the longwave trough is expected to retrograde and
deepen westward as an amplified shortwave trough begins to travel
through the flow pattern toward the central CA coast. As these
systems become more in sync, late Monday into Tuesday, heavier
precipitation potential is expected to rapidly increase across the
region. Additionally, southerly wind gusts are expected to be
their strongest as this system builds in and progresses. Prior to
this shortwave introduction, light to moderate precipitation will
persist alongside still breezy southerly winds, with highest
precipitation totals remaining confined to areas generally from
Interstate 80 northward through midday Monday. Shasta County and
the surrounding terrain will see the highest potential for
impactful rainfall, with continued 70 to 95 percent probabilities
of precipitation exceeding 1 inch through Monday evening.
Rainfall will gradually spread southward on Monday as the system
progresses through the region, with heaviest precipitation rates
anticipated late Monday through Tuesday as the shortwave becomes
more centered over interior NorCal. Current probabilities of
precipitation exceeding 2 inches through Wednesday AM continue to
favor (90%+) the northern Sacramento Valley and surrounding
terrain and the length of the Sierra/southern Cascades, with
generally 40 to 60 percent probabilities across the central and
southern Sacramento Valley, dropping to 15 to 30 percent
probabilities for the northern San Joaquin Valley. Snow levels
will vary with this early week wave of precipitation, beginning
around 6000 to 7000 feet on Monday then falling to near 4000 to
5000 feet by midday Tuesday and further to 3000 to 4000 feet by
Wednesday morning. With heaviest precipitation expected to occur
late Monday through Tuesday, most appreciable snowfall
accumulation through midweek is expected to occur above 5500 feet
at this time, although some lower elevation snowfall down to 4000
feet will be possible across the Shasta County mountains and Coast
Range. Resultant probabilities of snowfall exceeding 1 foot sit
around 40 to 60 percent for most locations along the
Sierra/southern Cascades above 5500 feet, with up to 80 to 90
percent probabilities over peaks. Across Shasta County and
surrounding terrain, probabilities of exceeding 4 inches above
4000 feet sit around 30 to 50 percent as well.
With a break between the departing early week shortwave and the
expected late week shortwave, a brief reprieve in the form of a
return to scattered light to moderate precipitation will be
possible on Wednesday. Winds will remain breezy, but are also
expected to decrease as the aforementioned trough ejects eastward.
Resultant probabilities of exceeding a half inch of additional
precipitation on Wednesday sit around 20 to 40 percent across the
northern Sacramento Valley and surrounding terrain and along the
Sierra/southern Cascades, with 10 to 20 percent probabilities
elsewhere.
&&
.EXTENDED DISCUSSION (Thursday THROUGH Sunday)...
As the next shortwave begins to approach the west coast on
Thursday, more widespread precipitation potential will once again
begin to gradually increase. Ensemble guidance has begun to
indicate a slight lag in timing of this wave progression and
attendant precipitation impacts, with heaviest precipitation
trending toward mid Thursday through Friday at this time.
Regardless, the ejecting shortwave is expected to bring the
longwave trough eastward with it as it progresses, which will
result in another period of moderate to heavy precipitation
through the end of the week, alongside likely lower snow levels
than the previous systems as colder air filters in. Given the
uncertainty in timing and exact trajectory to an extent, snowfall
accumulations, particularly at lower elevations, remain somewhat
variable. Still, probabilities of precipitation exceeding 1 inch
through Saturday morning sit around 70 to 90 percent for the
northern Sacramento Valley and surrounding terrain and along the
Sierra/southern Cascades, with 20 to 40 percent probabilities
elsewhere at lower elevations. Snowfall totals of 1 to 5 feet are
expected along the Sierra/southern Cascades, with resultant
probabilities of exceeding 1 foot of snowfall above 5000 feet
around 70 to 95 percent at this time. Given uncertainty in snow
levels, snowfall into lower elevations remains a bit tougher to
gauge at this time, but there is certainly potential to see
accumulating snowfall into the foothills with this late week
system.
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 following weekend, ensembles continue to
suggest a trend toward a drier pattern.
&&
.AVIATION...
Widespread MVFR conditions with areas of IFR/LIFR conditions in
the northern Sacramento Valley are expected to continue for the
next 24 hours across interior NorCal as an atmospheric river
continues to move through the area. Widespread precipitation is
forecast to continue today, with the heaviest precipitation in the
northern Sacramento Valley and foothills/mountains of the Sierra.
Southerly surface winds in the Valley are forecasted around 15
knots with gusts up to 30-40 knots. Southerly gusts in the Sierra
are forecasted up to 65 knots.
&&
.STO WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 10 PM PST 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-Shasta Lake Area / Northern Shasta County-
Southern Sacramento Valley-West Slope Northern Sierra Nevada-
Western Plumas County/Lassen Park.
Wind Advisory until 4 PM PST this afternoon for Central
Sacramento Valley-Northeast Foothills/Sacramento Valley-Northern
Sacramento Valley.
Winter Storm Warning from 10 AM Monday to 10 AM PST Wednesday
for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.
&&
$$
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