804
FXUS66 KSTO 191200
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
400 AM PST Tue Nov 19 2024

.SYNOPSIS...
Cold temperatures this morning with low elevation frost.
Storm system arrives midday, with periods of moderate to heavy
rain, mountain snow, and gusty winds then expected through the
end of the week and into the weekend.

*Freeze Warning:        10 PM tonight through 10 AM Tuesday morning
                        for elevations below 2000 feet.

*Cold Weather Advisory: 10 PM tonight through 10 AM Tuesday
                        morning for elevations above 6000 feet along
                        the Sierra.

*Winter Storm Warning: 4 PM Tuesday through 10 PM Wednesday for
    elevations above 3500 feet along the
   Sierra/southern Cascades, Coastal Range,
   and above 3000 feet in the Shasta County
   mountains.

*Flood Watch:           10 PM Tuesday through 4 AM Saturday for
                        Valley/foothills locations generally north
                        Interstate 80 and below 3000 feet.

*Wind Advisory: 10 PM Tuesday through 4 PM Wednesday for the
 Northern/Central Sacramento Valley and
Northeast foothills.

.DISCUSSION...
Current GOES-West satellite imagery shows mid to high level
clouds spreading inland across interior northern California early
on this Tuesday morning. Current temperatures observations are
trending approximately 5 to 15 degrees cooler than this time
yesterday morning, with temperatures in the 30s to around 40 in
the Valley and foothills, and in the teens and 20s in the
mountains, valid at 345 AM PST. As temperatures continue to cool
as the morning progresses, frost development will be possible
throughout much of the Valley and foothills. The National Blend of
Models (NBM) suggests a 40-80% probability of minimum
temperatures less than 36 degrees this morning across elevations
less than 2000 feet. For the mountains, lows in the single digits
to teens are forecast, generally above 6000 feet. As a result, the
Freeze Warning and Cold Weather Advisory both remain in effect
through 10 AM PST this morning.

Ensembles continue to indicate the trough digging southward from
the Gulf of Alaska mid to late week, allowing for widespread
moderate to heavy rain and mountain snow beginning Tuesday
afternoon and continuing through the end of the week.
Precipitation is expected to arrive to northwestern California
this afternoon and spread inland overnight, continuing over the
next several days as the trough stalls and provides for additional
rounds of precipitation. Storm total precipitation amounts from
this afternoon through Friday morning are around 3 to 6 inches in
the Valley and lower foothills north of Interstate 80, locally up
to 8 inches in some of the upper foothill locations. South of
Interstate 80, storm total precipitation amounts are generally in
the 0.50 to 2.5 inch range. Latest guidance shows the trough
moving inland sometime over the Friday into Saturday timeframe,
which will bring additional precipitation chances to the area that
will continue into the extended forecast period.The Flood Watch
remains in effect from 10 PM PST this evening through 4 AM PST
Saturday.

Moderate to heavy mountain snow will also be produced by this
storm system over the next few days. Snowfall is expected to first
arrive this afternoon to the Coast Range and northern mountains
before spreading inland moreso on Wednesday. The majority of
snowfall with this wave will be over the Coast Range, Shasta
County Mountains, Southern Cascades, and Sierra north of
Interstate 80. The NBM offers up a 65-95% probability of snow
amounts of 12 inches or greater, with best chances above 4500 feet
in those aforementioned locations. Snow levels initially start
out around 3000-4000 feet, before rising on Wednesday to around
4000-5500 feet, and with continued rise to around 7000-8000 feet
on Thursday. Motorists should plan on difficult to impossible
mountain travel in those areas due to snow covered roads, chain
controls, travel delays and gusty winds. The Winter Storm Warning
remains in effect from 4 PM PST this afternoon through 10 PM PST
Wednesday.

Periods of gusty southerly winds are also anticipated with this
storm system. Strongest winds are currently expected to be with
the initial wave, late Tuesday into Wednesday. The NBM offers a
60-90% probability of wind gusts of 30 mph or greater, with best
chances in the northern/central Sacramento Valley, adjacent
foothills and over the mountains where a Wind Advisory has been
issued from 10 PM this evening through 4 PM Wednesday. Gusty
southerly winds continue at times through the end of the week,
although tonight through Wednesday still looks like the strongest
period. Isolated thunderstorms will also be possible beginning
late Friday, as the NBM offers a 10-20% probability.


.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)...
An active weather pattern is expected to continue into the
weekend, with renewed chances for precipitation across interior
NorCal. Ensemble guidance and cluster analysis depict the
deepening trough offshore slowly progressing eastward toward the
California coast. This will result in prolonging the window for
additional precipitation impacts through the weekend and
potentially into early next week.

As the trough becomes more centered over NorCal, snow levels look
to fall to around 4000 to 5500 feet over the weekend as a colder
airmass moves in. While exact details regarding precipitation
totals from Friday into early next week remain somewhat uncertain,
current probabilities of precipitation exceeding 2 inches linger
around 40% to 70% for the Delta, central/southern Sacramento
Valley, and northern San Joaquin Valley, jumping to 70% to 90%
probabilities across the northern Sacramento Valley as well as
along the Sierra. Given lowering snow levels through the weekend,
additional mountain travel impacts will be possible along the
Sierra, with current probabilities of exceeding 1 foot of snow
around 60% to 80% along the Sierra above 5000` through Monday.
Additionally, chances for isolated thunderstorms appear on
Saturday (15 to 20%) and Sunday (10-15%) during the afternoon and
evening hours, with Saturday being more favorable for convection.
As the trough progresses further towards the coast on Saturday, southerly
winds over the Sierra are forecast to periodically gust 40 to 50
mph Saturday morning with gusts 20 to 25 mph in the Valley.

With the active weather in the forecast this week, 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...
Primarily VFR conditions across Interior Northern California
through at least 18z. Around 19z Tuesday, MVFR conditions possible
due to showers in the Valley. Winds generally 12 kts or less
through 17Z. By 19Z, southerly winds are expected to gust 15 to 22
kts in the northern and central Sacramento Valley. These winds
become more widespread across the entire Valley by around 06z
Wednesday lasting into Thursday, with the strongest gusts 25 to 37
kts in the northern and central Sacramento Valley from 06Z Wed to
00Z Thurs. Over the mountains, southerly gusts 35 to 50 kts are
expected between 04z Wednesday to 00z Thursday.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Freeze Warning until 10 AM PST this morning for 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.

Winter Storm Warning from 4 PM this afternoon to 10 PM PST
Wednesday 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 from this evening 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 from 10 PM this evening to 4 PM PST Wednesday for
Central Sacramento Valley-Northeast Foothills/Sacramento Valley-
Northern Sacramento Valley.

Cold Weather Advisory until 10 AM PST this morning for the West
Slope Northern Sierra Nevada and Western Plumas County/Lassen Park
above 6000 feet.

&&

$$