Area Forecast Discussion
Issued by NWS Sacramento, CA

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000
FXUS66 KSTO 221133
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
433 AM PDT Sun Sep 22 2019

.SYNOPSIS...
Dry and mild weather through the weekend with a few showers
possible over the mountains later today. Breezy northerly winds
early next week will lead to elevated fire weather concerns.
Above normal temperatures are likely Tuesday and Wednesday while a
significant cool down and increasing precipitation chances return
late in the week.

&&

.DISCUSSION...
The transient upper ridge has long exited the region as it tracks
toward the Intermountain West. The southern extension of a modest
upper trough is currently nearing the coast which has spread some
high clouds into western sections of Shasta and Tehama counties.
While scattered showers are possible over the Shasta county
mountains into the far northern Sierra, nearly average moisture
will keep rainfall amounts generally below 0.10 inches. Current
blended TPW imagery supports this idea while keeping the better
moisture support offshore. Overall, most Valley locations will
stay in the mid/upper 80s although a few 90 degree readings are
possible toward Stockton and Modesto.

As this system dips down toward western Nevada, models develop an
upper low forming an "Inside Slider" setup. As is common in these
regimes, dry northerly flow will ensue with gusty winds likely on
Monday. Model surface pressure gradients between KMFR and KSAC
depict 10 mb differences which supports the forecast winds.
Decreasing relative humidities along with poor overnight
recoveries will be the norm early next week with a Fire Weather
Watch in place from Monday afternoon through Wednesday morning.
The affected region includes sections of the Sacramento Valley,
adjacent foothills, and northern Coastal Range.

In addition to the elevated fire weather concerns, temperatures
should be quite warm on Tuesday and Wednesday with Valley highs in
the mid/upper 90s while mountain locations sit in the 70s.
Relative to climatology, this equates to being 5 to 10 degrees
above average. With expected 850-mb temperatures in the 22-25 C
range, a well mixed boundary layer could support an additional
degree or two in the forecast. The overnight warmth will actually
be even more impressive in terms of anomalies. As
north/northeasterly flow events often do, downsloping winds will
keep readings quite mild, particularly in foothill locations with
lows near 70 degrees. Wednesday should mark a close to the brief
bout of heat with a pattern change thereafter. ~BRO

&&
.EXTENDED DISCUSSION (Thursday THROUGH Sunday)...
While the pattern change is slightly delayed compared to
yesterday`s guidance, confidence remains rather high in much
cooler and unsettled conditions arriving by Friday. Anomalously
deep troughing will settle over the western U.S. supporting multi-
day temperature drops into the 20 to 25 degree range. The highly
amplified setup over North America has been rather well advertised
as a powerful ridge across the Gulf of Alaska forces a deep
trough to position itself over the West. During the Friday- Sunday
(Sep 27-29) timeframe, 1000-500 mb thicknesses potentially drop
into the 540-545 dm range which is seasonably low. Dropping
temperatures in the column will lower snow levels considerably,
potentially reaching the 5,500 to 6,500 foot range at times over
the weekend. With the current forecast track favorable for showers
over the region, will need to monitor the potential for snowfall
accumulations. Additionally, it should feel quite Fall like with
Valley locations struggling to escape the upper 60s while 30s and
40s would be more commonplace across the mountains. At this
juncture, ensemble means (ECMWF/GEFS/CMC) are in remarkable
agreement which is not terribly surprising given the level of
amplification in the global pattern. However, details will change,
particularly as individual shortwaves become better resolved. The
system responsible for this pattern shift actually resides near
Siberia at this time. ~BRO

&&
.AVIATION...
VFR conditions are expected over the next 24 hours. There will be
an increase in southerly winds this afternoon, with gusts up to
15 to 20kt being possible in the northern Valley, mountains, and
Delta areas.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Monday afternoon through Wednesday
morning for Central Sacramento Valley in Glenn, Colusa, Yuba,
Northern Sutter, and Butte County Below 1000 Ft-Eastern
Mendocino NF-Eastern Portion of Shasta/Trinity NF-Lake County
Portion of Lake-Napa-Sonoma Unit-Northern Sacramento Valley to
Southern Tehama County Line Below 1000 Ft-Northern Sierra
Foothills from 1000 to 3000 Ft. Includes portions of Shasta-
Trinity and Butte Units-Southeast Edge Shasta-Trinity NF and
Western Portions of Tehama-Glenn Unit-Southern Sacramento Valley
in Yolo-Sacramento Far Western Placer, southern Sutter and
Solano County Below 1000 Ft.

&&

$$


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