Area Forecast Discussion
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FXUS66 KSTO 242220

Area Forecast Discussion
National Weather Service Sacramento CA
320 PM PDT Sun Oct 24 2021

A high-impact weather system continues to bring widespread heavy
precipitation to NorCal. This has lead to flooding and some debris
flows. Strong southerly winds will diminish during the evening
but will remain breezy overnight. Rain will switch to heavy snow
over the Sierra tonight into early Tuesday. Quieter and drier
weather expected by the middle of next week.


We are in the middle of our strong Atmospheric River and we have
been seeing flooding impacts across the area especially in and
around the Dixie Fire. You can see the trough spinning off of the
coast of the PacNW pushing a cold front into NorCal. Ahead of this
front we are seeing deep moisture with PWATs 0.80 to 1.75", the
12z OAK sounding had 1.62". Rain rates over the foothills and mountains
have generally ranged from 0.30 to 0.80" per hour and 0.15 to
0.60" in much of the Valley with lesser amounts in the northern
Sac Valley. Locally higher rain rates will be possible right along
the front. Heavy rain will continue with these rain rates into
the evening slowly working their way to the south with the front.
The majority of the heavy rain will clear the area by 10 pm. We
are going to see heavy precip linger over the mountains and some
of the foothills into Monday morning mainly from I-80 south as the
trough axis moves through.

Snow levels are currently above pass level but we are going to
see them fall from north to south reaching 5500 to 6500 feet by 4
am. Mountain travel will be very difficult above 6000 feet by
daybreak tomorrow morning and heavy snow will continue throughout
the morning. It will become more showery in the afternoon but
mountain travel will likely remain poor. We will also see some
instability build in tomorrow mid morning into the afternoon in
the valley and foothills. With some forcing from the trough axis
we can expect a few scattered showers and thunderstorms with the
best chances along I-5 east into the foothills. PWATs will remain
elevated tomorrow (0.40-0.75") and brief heavy rain will be
possible with any thunderstorms along with some small hail.
Right now the threat for thunderstorms over the burn scars look
to remain low with the better chances staying in the lower

We are seeing gusty winds across the area and they will gradually
diminish this evening but will remain on the breezy side
overnight. Less wind is expected by tomorrow afternoon.

A few short wave troughs will pass to our north on Tuesday and
that will keep light showers in the forecast from I-80 north with
the best chances over the higher elevations. Snow levels Tuesday
will be between 5000 and 6000 feet, snow accumulation will
remain light. Upper level ridge begins to build in on Wednesday
brining quiet and dry weather. Cold below normal daytime highs
will continue with some warming Wednesday.



Ensembles in good agreement with upper level ridge over the CWA
Thursday, shifts into the Great Basin Friday as upstream Pacific
storm approaches. There is considerable uncertainty with the
strength and progression of this next system, though. Some light
precipitation may be possible during the Friday night to Saturday
morning time frame over Shasta County as it moves through. The NBM
is now picking up on this and so light rain has been added to the
forecast for that period. Most of the forecast area will remain
dry. Dry weather across the area Sunday as upper ridging looks to
rebuild over the forecast area.


Wdsprd MVFR/IFR with areas of LIFR conds ovr intr NorCal nxt 24
hrs as stg Pac stm movs thru. In Cntrl Vly, wdsprd Sly sfc wnd
sust 20-30kts with lcl gsts up 25 to 35 kts poss til 12z. In mtns,
stg S-SW sfc wnd with gusts up to 60 kts poss nxt 24 hrs. Sn lvls
abv 8000 ft AMSL til 08z, lwrg to 5500-6500 ft AMSL Mon mrng.


Flood Watch from late tonight through Monday afternoon 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 through late tonight 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 Warning from 11 PM this evening to 11 PM PDT Monday
for 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-Northeast
Foothills/Sacramento Valley-Northern Sacramento Valley-Northern
San Joaquin Valley-Southern Sacramento Valley.


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