Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 242139
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
239 PM PDT Sat Oct 24 2020


.SYNOPSIS...
Strong north to east winds will develop across northern California
on Sunday and Sunday night. These winds combined with very low
relative humidity values and dry fuels will lead to extreme fire
weather conditions.

&&

.DISCUSSION...
The forecast looks to remain on track as, our powerful offshore
wind event moves into the region. Satellite imagery shows the
impending trough sitting over northern Montana this afternoon.
Models depict that northern trough axis to continue to drop
through the Pacific Northwest today and eventually drop into the
Great Basin. At the surface, high pressure will develop along with
a cold airmass in the trough`s wake. Surface pressure gradients
will then strengthen initially in the north- south component
during the day on Sunday, and then in the east- west component
late Sunday and overnight into Monday. This will bring extreme
fire weather conditions to the region as the downslope/katabatic
effects of the winds at the surface will result in an
astonishingly dry airmass. Relative humidity values with this
system are astonishingly low, with most of NorCal sitting below
20 percent with little to no relative humidity recoveries expected
Sunday night. By Monday, we`re expecting to see widespread single
digit humidity values on Monday afternoon with some areas falling
below 5 percent! Dew points will bottom out around 0 deg F,
possibly lower. The already dry fuels will become even drier.

Moving on to winds, the Sac Valley will have the strongest winds along
the west side with gusts of 40 to 50 mph likely during the day on
Sunday. Winds don`t appear to be as strong on Monday as gusts are
expected to be around 15 to 30 mph; however, extreme fire weather
conditions will persist due to the rock- bottom relative humidity
values.

For the southern Cascades, northern Sierra, and foothills, offshore
winds will begin to develop around mid-day Sunday, and accelerate
during the the evening and overnight hours into Monday. These
powerful winds will gust anywhere from 50 to 70 mph -- conceivably
higher in wind-prone canyons, gaps, or in the vicinity of
mountain waves. Single digit relative humidity values will be
widespread in the foothills/mountains on Monday afternoon as winds
slowly subside.

"Product" wise, we now have a Wind Advisory out in addition to the
Red Flag Warning. The Wind Advisory for the Sacramento Valley
starts 8AM Sunday and continues through 11PM, while the Wind
Advisory for the mountains runs from 11PM Sunday and continues
through 11AM Monday Morning. The Red Flag Warning timing remains
in effect beginning Sunday morning for the Sac Valley and southern
Cascades, while the northern Sierra begins Sunday evening. During
Red Flag conditions, any new and/or existing fires have the
potential to spread rapidly and behave erratically. Information
for other locations in northern California can be found in
discussions/fire weather forecasts from NWS San Francisco Bay
Area, NWS Eureka, NWS Reno NV, and NWS Medford OR.

To recap...an extremely critical and dangerous fire weather event
is expected to unfold across northern and central California over
the next 24 to 48 hours. The strongest winds in the local
forecast area impact 1) the southern Cascades and northern Sierra
and adjacent foothills, and 2) the western side of the Sacramento
Valley. The entire region can expect extremely low relative
humidity values. Looking back on past powerful offshore wind
events, there are meteorological similarities to the October
27-28, 2019 event as well as November 8, 2018.

Rowe/KR

&&
.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)...


High pressure will start building into northern California Tuesday
into Wednesday, which will bring warmer temperature back to the
region. Model and ensembles guidance have been bouncing around
with highs for this ridge, with most areas sitting in the mid to
upper 70s mid to late week. The northern Sacramento Valley, will
likely have the hottest highs as they rise to the low 80s Tuesday
and Wednesday. In addition to the warmer weather, extremely dry
conditions remain in the forecast. Poor daytime humidity values,
generally in the teens, are expected each with poor to moderate
recovery expected. The worst humidity recovery is expected to
occur in the foothills. Continuing to practice fire safety
remains essential with the prolonged dry conditions.

By Friday, guidance continues to depict a trough moving over the
PacNW. This should decrease temperatures by a few degrees and
bring some high clouds to the region. Precipitation chances with
this system look to stay north of California.


&&

.AVIATION...


The forecast remains on track as mostly clear skies are being
observed over NorCal this afternoon. Model guidance hints at the
possibility of some MVFR cigs to move in from the Bay Area late
tonight and into tomorrow morning. Hinted at this in the sky grids
of the forecast, but may need to add some lines to the TAFs for
the afternoon/evening package if confidence increase. If MVFR
cigs develop, expect them to be short lived between 9Z and 18Z.
In general, expect most areas to remain VFR for the next 24
hours.

Wind wise, a strong period of north winds begins after 12Z
Sunday. Sustained winds for the Valley are forecast to be between
15 and 30 kt with gusts up to 20 to 40 kt. The strongest winds
are forecast to occur around in the Sacramento Valley. The
mountains and foothills will also experience stronger winds;
however, they`ll have a more north to easterly component. Winds
are forecast to pick up by the afternoon, and then increase
overnight. Sustained speeds should be around 20 to 30kt, with
gusts up to 35 to 55 kt being possible. Favored canyons and ridges
may experience gusts closer to 60kt+.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 8 AM Sunday to 1 PM PDT Tuesday for Burney
Basin and Northeast Plateau in Shasta County Including Northwest
Lassen NF north of Lassen NP-Carquinez Strait and Delta-Central
Sacramento Valley in Glenn, Colusa, Yuba, Northern Sutter, and
Butte County Below 1000 Ft-Eastern Mendocino NF-Eastern Portion
of Shasta/Trinity NF-Northern Motherlode From 1000 to 3000 Ft.
Includes portions of Nevada-Yuba-Placer-Amador and ElDorado
Units-Northern Sacramento Valley to Southern Tehama County Line
Below 1000 Ft-Northern San Joaquin Valley in San Joaquin and
Stanislaus Counties Below 1000 ft-Northern Sierra Foothills from
1000 to 3000 Ft. Includes portions of Shasta-Trinity and Butte
Units-Northern Sierra Including Lassen NP and Plumas and Lassen
NF/S West of the Sierra Crest (West of Evans Peak-Grizzly Peak-
Beckworth Peak)-Northern Sierra Including the Tahoe and ElDorado
NF/S West of the Sierra Crest-Southeast Edge Shasta-Trinity NF
and Western Portions of Tehama-Glenn Unit-Southern Motherlode
From 1000 to 3000 Ft. Includes portions of Calaveras-Tuolumne
Unit-Southern Sacramento Valley in Yolo-Sacramento Far Western
Placer, southern Sutter and Solano County Below 1000 Ft-
Stanislaus NF West of the Sierra Crest.

Wind Advisory from 11 PM Sunday to 11 AM PDT Monday for
Carquinez Strait and Delta-Central Sacramento Valley-Motherlode-
Mountains Southwestern Shasta County to Western Colusa County-
Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-
Southern Sacramento Valley-West Slope Northern Sierra Nevada-
Western Plumas County/Lassen Park.

&&

$$



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