Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 221555
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
855 AM PDT Mon Sep 22 2014

.Synopsis...
Dry and warmer temperatures early this week. Cooler and wetter
weather expected for the middle to end of the week.

&&

.Discussion...
Short-wave ridging developing in the wake of an upper low that
brought cooler temperatures and thunderstorms over the weekend.
The upper low began reintegrating with the mean westerly flow
aloft and quickly ejected out of the area over the last 24 hours.
Fall begins this evening, and a fall-like cool, wet upper trough will
arrive later this weekend to welcome in some seasonal weather.
Increased southwest winds and a period of dry weather just ahead
of the troughs arrival could make containment efforts at the King
fire more difficult.

Near normal temperatures are expected in the benign flow from
today through Wednesday, but temperatures will drop to several
degrees below normal from Thursday into the weekend. This aligns
with the arrival of the main band of precipitation arriving on
Thursday. Main forecast challenge of today is
timing/coverage/intensity of precipitation as models tend to
struggle significantly with seasonal transition systems. No
significant changes are planned to the overnight forecast package
at this time.  DRP
.Previous Discussion...
Mostly clear skies across the interior of NorCal early this
morning as skies have cleared in the wake of the weekend low
pressure system that`s now moving across the Great Salt Lake.
Temperatures are considerably cooler than 24 hours ago and range
from the 40s to lower 50s in the mountain valleys to the upper 50s
to mid 60s elsewhere.

Short-wave ridge will result in dry and warmer weather early this
week. A couple weakening weather systems will ride over the ridge
early this week bringing some clouds, but otherwise will have
little effect for NorCal. High temperatures today are expected to
warm around 5 degrees compared to Sunday`s highs, and little
change is then expected heading into Tuesday. Warmest readings in
the Central Valley will return to around the 90 degree mark both
days.

Ridge weakens and shifts further east by mid-week as upstream
upper low deepens off the PacNW coast and trough approaches
NorCal. While precipitation is forecast to begin across far NW CA
by late Tuesday, the progression of precipitation further inland
will be a slow process as the front becomes parallel to the mid
and upper-level flow slowing its eastward progress. The initial
impacts over interior NorCal will be cooler temperatures along
with breezy to windy conditions, especially over the mountains.

By later Wednesday night and Thursday, there is good agreement in
the models that the upper trough will shift closer to the west
coast and push the frontal system inland across NorCal. Appears
the best chance for precipitation will be mainly from about the
I-80 corridor northward, and while rain amounts are not expected
to be high, this will be the first really significant system of
the fall.

Appears post-frontal shower potential will linger into the weekend
as the EC is favored with its slower eastward progression of the
resulting closed low, then warmer and drier weather forecast to
return by early next week.

&&

.Extended Discussion (Friday through Monday)

Upper level trough along the west coast will move into Northern
California on Thursday bringing cooler temperatures and a chance
of precipitation. Temperatures will be in the 70s in the Valley
and 50s and 60s in the mountains on Thursday and Friday. These
temperatures are around 6 to 12 degrees below normal for this time
of year. Temperatures may warm up over the weekend but how much
depends on how quickly the trough moves to the east.

Precipitation is expected to spread over interior
Northern California on Thursday with the best chance north of
Interstate 80. The models continue to vary with the strength,
track and timing of the trough for low confidence in when the
precipitation will end and coverage of precipitation. The GFS is
the fastest and weaker with the trough and the ECMWF the slowest
and forming a closed low over Northern California. The forecast
leans towards the ECMWF with precipitation possibly lingering into
next weekend especially over the mountains.


&&

.Aviation...

Generally VFR conditions next 24 hrs except local MVFR/IFR conditions due to
smoke around King Fire including nearby foothill locations.

&&

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

$$







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