000
FXUS66 KSTO 201120
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
420 AM PDT Wed Oct 20 2021

.SYNOPSIS...
A period of wet weather is expected across the region into next
week as several Pacific weather systems affect NorCal. Moderate to
heavy rain will be possible at times along with gusty winds,
particularly Friday and Sunday. Heavy snow will be possible in the
northern Sierra early next week.

&&

.DISCUSSION...
Satellite imagery shows the upper trough lifting to the north into
the PacNW early this morning. Rainfall amounts since late Tuesday
have been generally light, around 1/4 to 1/2 inch across the
northern Sacramento Valley and surrounding terrain with only
sprinkles or a few hundredths of an inch reported so far further
south. Radar shows scattered light to very light precipitation
over much of the region early this morning.

Current temperatures are considerably milder compared to 24 hours
ago with many areas reporting readings that are 10-20 degrees
warmer.

Southerly breezes are picking up across the northern half of the
Sacramento Valley and over higher terrain as the surface gradient
tightens. The SAC-MFR gradient is around 8 mbs while the SAC-RNO
gradient is approaching 6 mbs. Gusts in the 20s to mid 30s mph are
common across that area.

Minor disturbances embedded in the west-southwest flow, along with
warm-advection, will keep mostly light precipitation going across
the region today and Thursday. Some light snowfall accumulations
(around an inch) will be possible this morning near the pass
levels in the northern Sierra before snow levels rise further.
Precipitation will likely pick up later tonight and Thursday
across the orographically favored areas of the northern mountains
and northern Sierra as flow backs ahead of Friday`s trough.

Moisture transport increases across NorCal on Friday as the next
incoming trough draws 1 to 1.5 inch TPW northward. A relatively
quick shot of moderate to heavy precipitation will move across
the region Friday with 2-4 inches of rain possible in the northern
mountains with 1-2 inches in the northern Sierra (mainly north of
Highway 50). Lighter amounts are likely in the valley. Snow
levels will be very high (above 9k ft) limiting accumulations to
just the highest peaks.

A relative short break in precipitation is likely Friday night
into Saturday before increasing again later Saturday ahead of
Sunday`s stronger system.

&&

.EXTENDED DISCUSSION (Sunday THROUGH Wednesday)...
High confidence continues in a moderately strong AR forecast to
affect the region on Sunday as a wide TPW plume of 1.5 to 1.75
inches is drawn northward into NorCal. Strong upward vertical
motion from the strong polar jet nosing into the region will
support an extended period of moderate to heavy rain with 1-3
inches possible in the valley with 3-5 inches in the foothills and
mountains. Significant impacts, including ash and debris flows,
will be possible particularly in and near recent burn scars,
though the details on where the heaviest precipitation will setup
remain somewhat uncertain.

Light to moderate precipitation continues Monday as the trough
swings through, then gradually tapers into the middle of next week
as ridging from the eastern Pacific edges into NorCal. Focus of
more significant impacts will shift to the higher elevations of
the northern Sierra early next week as snow levels lower below the
pass levels bringing several feet of accumulation above about 6k
ft, mainly Monday and Monday night.

&&

.AVIATION...
Areas of MVFR conditions, locally IFR, next 24 hours in showers.
Local SE wind gusts to 20 to 25 kt over the northern Sacramento
Valley through 18z. Gusts to 30 to 40 kts are possible over higher
terrain through this afternoon.

&&

.STO WATCHES/WARNINGS/ADVISORIES...None.

&&

$$