Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 152220
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
320 PM PDT Wed Apr 15 2015

.Synopsis...
High pressure builds over NorCal behind a low pressure system
moving eastward across the Great Basin. Breezy to windy conditions
continue tonight before slackening on Thursday. Gradual warming
trend through the end of the work week.

&&

.Discussion...
Upper flow becoming more Nly over Norcal as upper low settles-in
over the Great Basin/4-corners area through Thu. 925 mbs winds were
forecast to slacken to 15-25 kts this afternoon when deeper mixing
occurred which made the wind advisory a hit or a miss at the METAR
sites. Valley gusts did not quite reach criteria and arguably the
sustained winds lacked duration as they bumped up/then came down. In
addition to the decreasing 925 mbs winds...the surface pressure
gradients (MFR- SAC) were forecast to weaken from 10+ mbs at 12z
Wed to 8 mbs by 00z Thu. KDAX VWP continued to show the 2 kft
winds decreasing...from 40 kts at 1530z to 20-25 kts at 2016z
which seemed to confirm the 925 mbs wind forecast. In any
event...the current valley wind advsry will not be extended and
allowed to expire. We will have to monitor a wind advsry for some
areas as Ely pressure gradients increase tonite.

Pressure rises over NV tonite are forecast to increase the RNO-SAC
gradient to 7 mbs by 12z Thu...almost doubling it from this morning.
Instead of a Nly barrier jet developing along the W side of the Sac
Vly (ala this morning)...the downslope/katabatic wind is forecast to
develop from the NE Sac Vly foothills into Srn Lake/Napa and
portions of Solano Co centered around 12z Thu. Sites such as Jarbo
Gap...Knoxville Creek...High Glade LO will be susceptible to this
pressure pattern and gusty winds. Pressure gradients and upper
support weaken drastically Thu afternoon and winds are expected to
diminish.

Another chilly/cold nite is expected in the NE mtn valleys/basins
due to NEly flow from the cold NE plateaus. Otherwise...adiabatic
warming and mixing will bring a rapid warmup to the region as high
pressure strengthens over Norcal and the air mass modifies. As 850
mbs temps warm into the lower teens Thu-Fri and mid teens degrees C
Sat...max temps will warm well into the 80s...assisted by
adiabatic warming/drying effects from Nly winds at times. Warming
aloft will result in a strong subsidence inversion...conducive for
mild overnite temps and low daytime RH and poor to moderate RH
recovery in the foothills and thermal belt areas at night.

A weak upper trof is forecast to develop over Nrn CA on Sat with a
weak closed low developing over NW CA. Enough moisture and
instability may be present for a few late day showers and
thunderstorms over the mountains.    JHM


.Extended Discussion (Sunday through Wednesday)

Warm, above normal temperatures will continue on Sunday and Monday with
highs generally in the mid to upper 80s in the Valley and mid 50s
to mid 70s in the mountains. Upper level disturbance will be off
the Nor Cal Coast bringing some instability for a slight chance of
showers and possibly thunderstorms over the mountains Sunday and
Monday. A cold and deep trough is expected to dig down along the
West Coast Tuesday into Wednesday bringing cooler temperatures
and potentially a more widespread threat of precipitation.
Although there is less confidence in the details (such as
precipitation amounts, timing) due to the models handing the system
differently. Temperatures are expected to drop a few degrees below
normal on Wednesday with high temperatures in the upper 60s to
around 70 in the Valley and upper 30s to mid 50s in the mountains.
The current forecast spreads precipitation threat over all of interior
northern California on Wednesday.


&&

.Aviation...

VFR conditions expected to continue through Thursday. Northerly
winds 10-25 kts with local gusts up to 32 kts expected in the
Valley until 03z then decrease and not as gusty then they will
pick up again around sunrise 10-20 kts with gusts up to 30 kts.
NE winds will strengthen across the Sierra, with local gusts
above 40 kt possible overnight into Thursday morning.


&&

.STO Watches/Warnings/Advisories...
wind advisory until 6 pm pdt this evening carquinez strait and
delta...central sacramento valley...northern sacramento valley...
southern sacramento valley.

&&

$$






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