Winter Storm and Flooding in California, October 24 to 25, 2021

Area Forecast Discussion
National Weather Service Sacramento CA
151 AM PDT Sun Oct 24 2021

.SYNOPSIS...
A dangerous, high-impact weather system will bring widespread
heavy precipitation to NorCal. This will lead to flooding on
Sunday and into Monday with debris flows expected over the 2021
burn scars. Strong southerly winds are also expected to develop
today. Heavy snow will be possible in the Sierra by Sunday night
into early Tuesday. Quieter and drier weather expected by the
middle of next week.

&&

.DISCUSSION...
Night shift concurs with the discussion analysis from the day
shift. A powerful atmospheric river is on our doorsteps, and will
bring widespread heavy precipitation to a vulnerable northern
California. Flash flood watches are in effect for the recent burn
scars, with high-resolution forecast models indicating that many
of these scars from this year will exceed their established
guidance thresholds for debris flows. The public is urged to pay
attention over the next 24-36 hours as there is a very real
potential for life- threatening impacts around and downstream of
recent burn areas. Stay safe and weather-ready.

See previous discussion section for further details. // Rowe

&&

.PREV DISCUSSION...as of 4:42 PM Saturday...
It will be a wild 24-36 hours across northern CA as we will see
an extreme and possible historic atmospheric river push through
the region. The IVT (Integrated Vapor Transport) pushes into the
1000 to 1250+ kg/m/s for a time on Sunday. This system will have
tons of moisture with PWATs 0.80 to 1.75" and flooding impacts can
be expected across the entire region. The biggest concern will be
over recent burn areas especially the Dixie, River, and Caldor.
Those are the burn scars where debris flows are expected, debris
flows will be possible over 2020 burn scars also. Flash Flood
Watches are in effect for most of the 2018 to 2021 burn scars. If
you live near any of the recent burn scars you need to be prepared
for debris flows and follow local officials for evacuation
information.

We are seeing light showers across the area and that trend will
continue with them becoming more widespread overnight as warm air
advection increases ahead of the approaching trough. The cold
front will push into the area during the morning and will slowly
work south into the evening. We will see heavy rain just ahead and
along this front. The period of heaviest rain over the Coastal
Range and northern Sac Valley will be between 4 am and noon and 7
am to 8 pm over the southern Cascade including the Dixie Fire, and
11 am to 11 pm for the southern Sac Valley, northern San Joaquin,
Sierra and Sierra Foothills. Widespread precip will continue over
the Sierra into Monday morning as activity else where becomes
more showery. The trough axis moves through on Monday and this
will keep shower activity in the forecast. We do see some
instability build into the Valley and Foothills Monday late
morning into the afternoon. Some scattered thunderstorms will be
possible especially along and east of I-5 into the Foothills.

Snow levels will remain high above pass level for much of
tomorrow but will begin to fall from north to south Sunday evening
falling to 5500 to 6500 feet shortly after daybreak. This will
bring mountain travel issues to the higher elevations Sunday night
into Monday. Snow showers will continue throughout the day on
Monday. Snow totals are looking to be 6 to 14 inches with amounts
up to 3 feet above 7000 feet.

Wind is going to be another major part of the this storm system.
We will see winds increase overnight tonight and will be peaking
ahead of the cold front mid morning into the early evening.
Breezy winds will linger into Sunday night with them becoming
lighter Monday. Downed trees and power outages can be expected
with these types of winds.

Showers will linger on Tuesday as troughing continues over the
west and we see some waves pass to our north. Additional rain/snow
on Tuesday will be limited.

-CJM

&&

.EXTENDED DISCUSSION (Thursday THROUGH Sunday)...
Upper level ridge over the CWA Thursday, shifts into the Great
Basin Friday as upstream Pacific storm approaches. Models differ
with strength and progression of this wave leading to forecast
uncertainty with POPs. Some light precipitation may be possible
during the Friday night to Saturday morning time frame over Shasta
county as it moves through, but NBM has not picked up on this
yet. Drier Sunday as upper ridging looks to rebuild over the
forecast area.

&&

.AVIATION...
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 to 45 kts poss til 00z. In mtns,
stg S-SW sfc wnd with gusts up to 60 kts poss nxt 24 hrs. Sn lvls
abv 8000 ft AMSL tda, lwrg to 5500-6500 ft AMSL Mon mrng.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 5 AM PDT early this morning 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 5 AM PDT early this morning 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 from 5 AM early this morning to 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.

&&