522
FXUS66 KSTO 202129
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
129 PM PST Wed Nov 20 2024

.SYNOPSIS...
A strong atmospheric river storm brings periods of moderate to
heavy rain and mountain snow, isolated thunderstorms, and gusty
southerly winds to interior NorCal across the remainder of the
week, through the weekend, and into early next week.

*Winter Storm Warning: Through 10 PM tonight for elevations above
                       3500 feet along the Sierra/southern Cascades
                       including Interstate 80, the Coastal Range,
                       and above 3000 feet in the Shasta County
                       mountains.

*Flood Watch:          Through 4 AM Saturday for Valley/foothills
                       locations generally north Interstate 80 and
                       below 3000 feet.

*Wind Advisory:        Through 10 PM tonight for the Northern/Central
                       Sacramento Valley and Northeast foothills.

.DISCUSSION...
As of early this afternoon, latest GOES-West satellite imagery
continues to depict a robust trough spinning off the coast of the
Pacific Northwest, with attendant moisture plumes advecting inland
across interior NorCal. Resultant composite radar shows steady
precipitation falling along a line from about Interstate 80
northward. Heaviest precipitation (0.5 to 1.5 inches) through
midday today has fallen across the northern Sacramento Valley and
surrounding terrain, with lighter accumulations (0.1 to 0.5
inches) toward Interstate 80. Despite the higher precipitation
totals, precipitation rates so far have generally remained steady
around 0.25 inches per hour. Even still, there remains some 20% to
40% probabilities of hourly rainfall rates up to 0.5 inches as
precipitation persists. Sub-3000 foot snow levels early this
morning have since risen closer to 4000 to 5000 feet as of this
afternoon as well. Despite this, mountain travel impacts have been
reported from Interstate 80 northward.

Even with rising snow levels expected to continue through the
remainder of the day, mountain travel impacts are expected to
persist, with the Winter Storm Warning above 3000 feet in Shasta
County and above 3500 feet elsewhere remaining in effect through
10 PM PST tonight. Additional snowfall accumulations of 6 to 12
inches will be possible, with up to 2 feet still possible over
peaks as well. As snow levels jump closer to 7000 to 8000 feet by
Thursday morning, rainfall is expected to become the primary
precipitation impact through midday Friday as high elevation snow
likely continues. Given the persistent fetch of moisture
advection into interior NorCal as the aforementioned trough
generally stalls offshore, precipitation totals north of
Interstate 80 through Friday evening are expected to range from 3
to 6 inches, with totals up to 10 inches possible along the
surrounding terrain. From Interstate 80 southward, precipitation
totals of 1 to 3 inches are expected, with totals up to 6 inches
possible along the Sierra.

Moving toward the weekend, the trough is expected to begin an
onshore push toward NorCal, although exact details on the timing
and eventual progression remain somewhat divided between current
ensemble guidance. Current consensus indicates the "final" push
of offshore influenced precipitation occurring later Friday into
midday Saturday, followed by a brief lull or at least
comparatively lighter period of precipitation through the
remainder of Saturday and possibly into Sunday. While the
uncertain timing will inherently impact thunderstorm potential
Friday evening through Saturday, nonzero chances (10% to 20%) for
isolated thunderstorm development persists primarily for Valley
and foothills locations from Interstate 5 eastward. Additionally,
snow levels are expected to fall to around 4000 to 5000 feet once
again by Saturday, generally stabilizing there through the
weekend. As a result, additional snowfall is expected as the final
offshore influenced push moves through.

.EXTENDED DISCUSSION (Sunday THROUGH Wednesday)...
Ensemble guidance notably begins to diverge through the end of
the weekend and into next week with respect to the eventual
progression of the aforementioned trough. The two favored
solutions indicate the trough ejecting out of the region as early
as Tuesday or as late as Wednesday evening at the moment, with the
latter solution resulting in more prolonged precipitation
impacts. Regardless of timing differences, overall precipitation
totals are expected to be comparatively lighter with the weekend
into early next week waves.

With persistent snow levels around 4000 to 5000 feet and a
broader area of expected precipitation (expected to span the
majority of interior NorCal, rather than a precipitation cutoff
around Interstate 80), additional snowfall totals of 1 to 3 feet
will be possible along the Sierra generally above 6000 feet, with
totals of 6 to 12 inches possible between 5000 and 6000 feet
through Tuesday morning. Resultant probabilities of exceeding 12
inches of snowfall are around 60% to 90% above 5500 feet at this
time. Lower elevation precipitation is expected to persist as
well, with totals of 0.5 to 1.5 inches possible across the Valley
and foothills and 1 to 3 inches possible across the mountains
through Tuesday morning. This is further evidenced by 40% to 60%
probabilities of precipitation exceeding 1 inch across the Valley
and foothills, jumping to 70% to 90% probabilities along the
Sierra. There are some indications that active weather may linger
or persist toward the upcoming holiday period, so be sure to stay
up to date with the latest forecast if you have holiday travel
plans.

With the active weather forecast to persist, be sure to stay up
to date with the latest forecast at weather.gov/sto and check
current road conditions at quickmap.dot.ca.gov before traveling!

&&

.AVIATION...
General MVFR/VFR conditions expected for most interior NorCal
locations through 18Z Thu, with IFR/LIFR in the mountains as a
storm system continues moderate to heavy precipitation across the
region, spreading inland from the northwest to the southeast
through this afternoon and evening. Gusty southerly winds
continue, with areas of gusts 15 to 30 kts, up to 35-40 kts for
the Delta, Valley, and foothills, strongest across the
northeastern foothills. Gusts of 30 to 55 kts along the Sierra
forecasted today into tomorrow morning.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 10 PM PST this evening for Mountains
Southwestern Shasta County to Western Colusa County-Shasta Lake
Area / Northern Shasta County-West Slope Northern Sierra Nevada-
Western Plumas County/Lassen Park.

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 until 10 PM PST this evening for Central
Sacramento Valley-Northeast Foothills/Sacramento Valley-Northern
Sacramento Valley.

&&

$$