000
FXUS66 KSTO 202231
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
331 PM PDT Thu Sep 20 2018

.SYNOPSIS...
Continued warm with improving humidity levels and the return of
onshore flow on Friday. Cooling trend continues through the
weekend with locally breezy winds. Warming above normal next week.
No precipitation expected over interior Norcal through the period.

&&

.DISCUSSION...
Temps trending up to 7 degrees warmer than yesterday at 20z with
biggest change in the dewpoints/RHs thanks to the drying Nly winds.
Winds will decrease tonight and temps should cool nicely under clear
skies and good radiational cooling. Foothill/mountain locations
above the Valley inversion will remain mild due to developing
thermal belts. Even with return of up valley and modest onshore flow
on Fri, temps will continue above normal as the ridge rebuilds over
Norcal on Fri behind the weak trof moving over central CA overnight.
The marine layer has been literally wiped out so the onshore flow
won`t provide much cooling, although the adiabatic warming effects
from the NLy winds will end.

An upper trof is forecast to move through the Pac NW this weekend
increasing onshore flow and bringing cooler temperatures and higher
RHs to Norcal. Max temps are forecast to return to near or slightly
below normal over the weekend. Some mid to high level cloudiness may
move through the Nrn portion of the CWA, along with the return of
the marine layer.   JHM

&&

.EXTENDED DISCUSSION (Monday THROUGH Thursday)

The weekend trof moving through the Pac NW will carve out out a
broad trof over the mid section of the USA while a Rex Block
pattern develops over the Ern Pac during the EFP. With the models
converging to a similar solution of high pressure building along
the W Coast warm and dry wx is expected during the extended
period. ECMWF model would suggest a period of some gusty north
wind Sun/Mon as the trof moves through the Great Basin, but the
GFS is weaker and more progressive. So this detail will be worked
out in later forecasts. As high pressure settles over the region
Tue/Wed, the Nly flow will weaken. Although not an anonymously
strong pattern aloft the lower level 1000 mbs pattern shows the
anomaly just offshore the CA coast. This will suggest a very
suppressed/mixed out marine layer and lack of onshore flow.

Medium range models are forecasting slightly warmer 850 mbs temps in
the EFP (generally mid 20s Deg C) than in the short term, and this
should push the max temps some 5-12 degrees above normal over much
of our CWA next week. 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.   JHM

&&

.AVIATION...

Mainly VFR conditions the next 24 hours, except local MVFR
conditions due to smoke near the Delta Fire in Shasta County.
Local gusts up to 20 kt near KRDD-KRBL through 00z Friday.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 7 PM PDT this evening 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.

&&

$$