000
FXUS66 KSTO 251117 AAA
AFDSTO

Area Forecast Discussion...UPDATED
National Weather Service Sacramento CA
417 AM PDT Sun Oct 25 2020


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

&&

.DISCUSSION...
A powerful and dangerous offshore wind event will materialize
across northern and central California over the next 12 to 24
hours. This event is forecast to be the strongest event of the
year so far for the region. Consequently, the potential fire
weather impacts will be extreme. The Storm Prediction Center has a
large portion of the region under an "extremely critical" risk,
the highest possible categorical assignment for fire weather.
Fuels are dry and will only further dry, winds and gusts will be
very strong, and relative humidity values will be desert-dry. The
meteorological synoptic setup is uncomfortably similar to recent
past events in northern California such as October 27-28, 2019
(Kincade Fire rapid growth), November 8, 2018 (Camp Fire rapid
growth), and October 8-9, 2017 (2017 Wine Country Fires rapid
growth).

For today, positively-tilted upper level trough axis will dive
south from the Pacific Northwest towards the Great Basin. Behind
this trough, surface high pressure will develop to our north/east
along with plenty of cold, dense air. This sets the stage for the
north-to-south and east-to-west offshore pressure gradients that
will impact the region.

North winds will first increase in the northern Sacramento Valley
around mid-morning Sunday. Elsewhere at that time, winds will
still be rather light. In fact at sunrise, there may be some
elevated stratus with lingering onshore flow in/around the Delta.
Northerly winds will increase from north to south along the
western half of the Sacramento Valley through the day, peaking in
the late afternoon and evening hours between 20 to 30 mph
sustained and gust of 40 to 50 mph. Relative humidity values will
crash to the single digits to middle teens for most locations in
the Valley, foothills, and mountains.

The east-west pressure gradients will significantly strengthen
late day and overnight into Monday. This is when the winds in the
southern Cascades, northern Sierra, and foothills along the
western slopes will dramatically increase and become dangerously
gusty. Northeast to east winds of 20 to 35 mph with gusts of 40 to
60 mph will be likely during this period of time. Isolated gusts
in favored canyons, gaps, and ridgetops can conceivably exceed 70
mph. Additionally, can`t rule out the potential for some
developing mountain waves along the lee west slopes of the Sierra
as the cold, dense air descends and accelerates from the crest.
Various cross section analyses from the North Bay to the northern
Sierra in the latest 3-km NAM support this possibility, which
could push localized wind gusts to even more extreme levels.
Relative humidity recoveries overnight Sunday and into Monday
morning will effectively be non-existent with many locations in
northern California holding between 10 to 25 percent.

While winds are forecast to gradually subside through the day on
Monday, relative humidity values will drop to remarkably low
values -- less than 5 percent for some locations -- with dew
points at or below 0 deg F. Therefore, extremely critical fire
weather conditions will continue through Monday and at least into
Tuesday morning.

As far as legacy products -- Red Flag Warning goes into effect at
8 AM PDT Sunday for the west side of the Sacramento Valley,
northern San Joaquin valley, southern Cascades, and the northern
Sierra. Red Flag Warning then expands through Tuolumne County at 8
PM PDT Sunday. Additionally, a Wind Advisory goes into effect
locations that will experience the strongest winds: at 8 AM PDT
Sunday for the west side of the Sacramento Valley and at 8 PM PDT
Sunday for the northern Sierra to Yosemite.

To recap...all necessary ingredients will be in place to produce
extremely critical fire weather conditions: 1) strong winds, 2)
low relative humidity values, and 3) dry wildland fuels. Any new
ignitions will be present in an environment that will promote
rapid, explosive, and dangerous spread of fire. Have a plan in
place if you need to evacuate will little/no notice. While Fire
Season 2020 has already been historic from the countless lightning-
based ignitions in August, remember that peak offshore wind
season is right now for much of the Golden State. Be safe and
vigilant during this extremely high-impact and particularly
dangerous fire weather event.

Rowe

&&
.EXTENDED DISCUSSION (Thursday THROUGH Sunday)...


High pressure continues to build over the west coast mid-week
allowing for mostly clear skies and lighter winds on Wednesday.
Highs look to be a few degrees above normal with this ridge, with a
few Vally locations possibly reaching into the low 80s on Thursday.
Deterministic models have diverged at this point for next weekend
with the GFS continuing high pressure across NorCal and the ECMWF
bringing a system across the PacNW with some precipitation chances
in far northern California. Ensemble guidance suggests the former is
more likely and have continued a dry forecast through the extended
period. With this said, very dry conditions continue from mid week
into the weekend with low daytime humidity expected for most of the
area and practicing fire safety remains critical.


&&

.AVIATION...


Mainly VFR conditions for the next 24 hours, with possible brief
MVFR cigs at Sac terminals from 13-17Z. Main concern of the TAF
period will be strong sfc winds and potential LLWS. Gusty N winds
from 15-30 knots with gusts between 20-40 knots expected in the
Valley with the strongest winds between 15Z to 06Z. Stronger N to NE
winds in the foothills/mountains picking up after 03Z. Gusts up to
55 knots possible through Monday morning. LLWS possible at RDD with
onset of stronger winds this morning mainly between 14-18Z.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from 8 AM this morning 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 8 PM this evening 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-
Shasta Lake Area / Northern Shasta County-Southern Sacramento
Valley-West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

&&

$$