000
FXUS66 KSTO 230927
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
227 AM PDT Sat Oct 23 2021


.SYNOPSIS...
A significant weather system will bring heavy precipitation
leading to flooding concerns Saturday evening into Monday. Strong
southerly winds are also expected to develop. Heavy snow will be
possible in the Sierra by Sunday night into early Tuesday.

&&

.DISCUSSION...
Radar imagery shows some light returns scattered throughout
interior northern California this early morning. During the late
evening hours, some embedded heavier rain showers impacted parts
of the Shasta County area. These have since dissipated and not
expecting any weather-related impacts in the immediate near-term.
Beyond the next 12 to 18 hours, things will begin changing in a
hurry.

The much-anticipated atmospheric river will materialize along a
deepening trough in the North Pacific Ocean over the next 24
hours. With Integrated Vapor Transport values forecast to peak in
the 1000 to 1250+ kg/m/s range, combined with strong upper level
jet of 100 to 125+ kt, a powerful system will bring widespread
heavy rain to much of northern California. Onset of the
precipitation will begin overnight Saturday and into Sunday
morning with periods of the heaviest precipitation occurring
during the day on Sunday and into Sunday night. The warm air
associated with this system will result in high snow levels at the
onset of the rain, starting in the 8,500 to 10,000 ft range early
Sunday. This means that nearly all pass levels will start off
with heavy rain rather than snow. The colder air will gradually
filter in from north to south late Sunday and Sunday night,
resulting in a drop in snow levels to around 5,550 to 6,500 ft by
mid-day Monday. Strong south/southwest winds will be accompanied
by the heavy precipitation; a Wind Advisory is in effect for the
Delta, Sacramento Valley, and lower foothills early Sunday morning
through Sunday evening.

Recent burn areas will be extraordinarily vulnerable to debris
flows, mudslides, and rock slides, which has prompted the issuance
of a Flash Flood Watch for 13 various burn scars from 2018 to
2021 that starts early Sunday and continues into early Monday.
This includes the 2018 Camp, Carr, Delta, Hirz Fires; the 2020
August Complex, North Complex, LNU Complex, Zogg Fires; and the
2021 Caldor, Dixie, McFarland, River, and Salt Fires. In our
county warning area alone, this encompasses over 2.2 million acres
of burn area under the Flash Flood Watch (with additional areas
outside our CWA are also under a Flash Flood Watch). With storm
total precipitation forecast in the 2 to 5 inch range in the
Valley, and 4 to 12+ inches in the foothills and mountains, the
Weather Prediction Center has issued a moderate and high risk of
excessive rainfall over much of the region. The highest risk is
focused between the Dixie and Caldor burn areas where the
heaviest rainfall is forecast. Nearly all of these burn areas have
yet to be tested with such a magnitude of a storm, and the
forecast rainfall is deeply concerning. Even if there were no burn
areas to be concerned about, this is still far too much rainfall
forecast in such little amount of time. For those living near and
downstream of burn areas, have a plan in place to evacuate if
necessary. Please stay safe and weather-aware during this
impactful storm.

Precipitation will begin to taper off in intensity and coverage
late Sunday night and into Monday morning. On/off precipitation
will continue through most of the day on Monday, becoming more
scattered in nature Monday night and into Tuesday. Winter weather-
related impacts will begin to materialize then in the mountains
with the lowering of snow levels. Presently a Winter Storm Watch
remains in effect for the mountains above 6000 feet from Sunday
evening through late Monday night. // Rowe

&&

.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)...
EPAC upper level ridge builds inland over interior NorCal midweek
resulting in drier and warmer weather. High temperatures in the
Central Valley climb upwards of 10-12 degrees on Wednesday and
inch up into the upper 60s to lower 70s Thu/Fri. Ridge axis shifts
into the Intermountain West Friday as next upstream Pacific
frontal system approaches. Forecast uncertainties with speed of
progression of this system as significant model differences exist.
Forecast remains dry at this time into Saturday.

&&

.AVIATION...
Lcl MVFR poss omtns this mrng in lgt shwrs otrw incrsg areas MVFR
with lcl IFR poss sprdg fm NW to SE acrs intr NorCal tda as nxt
Pac fntl sys movs inld. Lcl Sly sfc wnd to 15 kts with gusts to 25
kts poss in Cntrl Vly tda ahd/alg fnt. Wly sfc wnd gsts up to 35
kts poss ovr hyr mtn trrn this aftn and tngt. Sn lvl arnd 6000 ft
AMSL this mrng incrsg to abv 7000 ft AMSL tngt.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from this evening through late tonight 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.

Flash Flood Watch from late tonight through late Sunday night
for Burney Basin / Eastern Shasta County-Central Sacramento
Valley-Motherlode-Mountains Southwestern Shasta County to
Western Colusa County-Northeast Foothills/Sacramento Valley-
Northern Sacramento Valley-Shasta Lake Area / Northern Shasta
County-Southern Sacramento Valley-West Slope Northern Sierra
Nevada-Western Plumas County/Lassen Park.

Winter Storm Watch from Sunday evening through late Monday night
for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

Wind Advisory from 5 AM to 11 PM PDT Sunday for Carquinez Strait
and Delta-Central Sacramento Valley-Northeast
Foothills/Sacramento Valley-Northern Sacramento Valley-Northern
San Joaquin Valley-Southern Sacramento Valley.

&&

$$