Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 282220
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
320 PM PDT WED SEP 28 2016

.SYNOPSIS...
Cooling to below normal Friday and 15 to 25 degrees below normal
by Sunday. Chance of showers and isolated thunderstorms Sunday
into Monday with snow showers over higher terrain. Drier with
some warming Tuesday into Wednesday.

&&

.DISCUSSION...
Cooling trend in store for Interior NorCal through the remainder
of the week and into the weekend as deep low pressure currently
in the GOA tracks into the area. Cooling had begun today with
afternoon temps running upwards of 5 to 7 degrees lower than 24
hours previous. Upper low over SoCal has drawn some moisture and
instability farther north than previous expected and potential
exists for an isolated shower or thunderstorm over the higher
Sierra Nevada through early evening. KSFO-KSAC surface pressure
gradient is trending up this afternoon and Delta breeze forecast to
increase this evening and overnight.

Upper low progged to dig near 130 W Thursday into Friday with
increasing onshore flow. Weak dry front pushes inland later
Thursday into Friday bringing some gusty wind to Interior NorCal.
Synoptic cooling increases with high temperatures near normal
Thursday and 5 to 10 degrees below normal Friday.

Upper low weakens to trough and moves through Interior NorCal
Saturday bringing some breezy wind and additional cooling. Colder
more dynamic secondary low is forecast to follow inland Sunday.
Models showing some QPF over the entire CWA Sunday into Sunday
night with GFS painting some afternoon CAPE up over 400 J/Kg over
portions of the Central Valley to support development of isolated
thunderstorms. Highly varied precip amounts expected with
unstable pattern. Some snow possible over the higher terrain
Sunday into Monday as snow levels lower to pass levels.

PCH

&&

.EXTENDED DISCUSSION (Sunday THROUGH Wednesday)
An active weather pattern is expected for the long term forecast
period for Northern California. Cooler temperatures will be felt
on Sunday as a trough of low pressure begins to come onshore from
the Pacific with increased chances for showers and slight chances
for thunderstorms through Sunday evening. Both GFS and ECMWF
models are in good agreement with placing surface based and upper
level instability across the coastal mountains and the central
valley south of Redding, and increased moisture within forecast
profiles would indicate good chances for rain. The cool system
will bring snow levels down to around 5000 feet, so higher
elevations could see some snow accumulations affecting mountain
travel Sunday afternoon through early Monday morning.

The main trough base will swing through the forecast area by early
Monday morning, and thunderstorm chances will diminish. However,
chances for showers will continue to linger behind the passage of
the system as upper level spin continues to give a chance for rain
showers through Tuesday evening. High temperatures will warm up a
bit to allow snow levels to rise on Monday and Tuesday, so
chances for higher elevations to see additional snow on Tuesday is
minimal at this time.

&&

.AVIATION...

VFR conditions expected at the terminals for the next 24 hours.
Expecting mainly southerly flow below 10 kts, except for gusts up
to 30 kts for the delta and Sierra ridgetops. Local gusts up to 20
kts in the northern Sacramento valley also possible Thursday
afternoon.

&&

.STO Watches/Warnings/Advisories...
None.
&&

$$


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