Area Forecast Discussion
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FXUS66 KSTO 192211

Area Forecast Discussion
National Weather Service Sacramento CA
311 PM PDT Wed Sep 19 2018

Warmer weather continues into next week. Critical fire weather
concerns tonight and Thursday. Patchy smoke and haze will persist,
mainly locally over northern portions of Shasta County.


It is a warm, sunny day across interior NorCal. Temperatures this
afternoon are warmer than this time yesterday, especially in the
Valley where temperatures are ranging from 5 to 10 degrees warmer.
Afternoon highs will peak in the upper 80s to low 90s in the
Valley and 60s to 70s in the mountains.

A weak trough moved over the area this morning and will slowly
move east across the Great Basin through Thursday. Northerly winds
have returned to the forecast area as we remain on the western
side of the trough. Winds are forecast to increase overnight
through Thursday, with gusts of up to 20-30 mph possible Thursday
afternoon over the northern portion of the forecast area.

Ridging will continue to build over California through the rest of
the week. Temperatures continue to increase with most locations
hitting about 4 to 8 degrees above normal Thursday and Friday.
Widespread Valley temperatures in the 90s are forecast through

The combination of northerly winds and warmer temperatures have
caused relative humidities to drop significantly today in portions
of the northern Valley and Coastal Range. RH values are expected
to remain low tomorrow with poor overnight recoveries. As such,
the dry conditions and gusty winds will produce critical fire
weather conditions, and the Fire Weather Watch has been upgraded
to a Red Flag Warning through Thursday evening. HEC



Upper trof exits the Pac NW and moves over the Nrn Rockys on Sun.
NWly flow in its wake will bring seasonably warm temps to our CWA to
end the weekend. But this is only the beginning of a warm period as
the aforementioned trof in part, begins to dig out a broad trof over
the mid section of the USA and a Rex Block pattern develops over the
Ern Pac for the remainder of the EFP. With the models converging to
a similar solution of high pressure building along the W Coast
during this period, some warm temps are expected in our CWA.

The WR ModTrend tool continues to show higher 5H heights along the W
Coast over the last several runs, and thus we have continued to
trend the EFP warmer with a reasonable amount of confidence. Short
term NWP indicates max temps in the mid 90s over the N end of the
Sac Vly with 850 mbs temps in the low to mid 20s Deg C. With the
medium range models forecasting slightly warmer 850 mbs temps in the
EFP (mid to upper 20s Deg C) we have pushed the max temps even
warmer there and elsewhere in our CWA, eventually to some 5-10
degrees above normal over much of our CWA Tue/Wed. In the middle of
summer, these 850 mbs temps would support temps in the low to mid
100s, but have tempered the warming due to the waning sun angle as
the Autumnal Equinox begins this Sat at 654 pm PDT.

There will be the possibility of some moderate to locally breezy
Nly winds at times Sun/Mon, but nothing anonymously strong. As
high pressure settles over the region Tue/Wed, the Nly flow will
weaken. JHM



Dry/stable air mass over interior Norcal through Thu with VFR/SKC
conditions except for local MVFR/IFR VSBYS due to smoke near the
Delta wild fire in Shasta Co. Nly flow gradually increasing
tonight as high pressure builds over OR. Nly winds expected to
gust to 25-30 kts over KRDD/KRBL TAF sites after 20/1600 UTC.
Band of 25-30 kts 925 mbs winds (roughly 2 kft MSL) forecast to
increase after 20/03 UTC to 20/18 UTC mainly on W side of Sac Vly
and into Lake/Napa Counties/N Bay Mtns causing light to moderate
turbulence for light aircraft. Nly flow will negate marine layer
and its influence inland.


Red Flag Warning from 7 PM this evening to 7 PM PDT Thursday 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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