Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 281030

Area Forecast Discussion
National Weather Service Sacramento CA
330 AM PDT WED SEP 28 2016

Cool down begins today with much cooler temperatures by the end
of the week as a weather system moves toward NorCal. Chance of
precipitation early next week.


Clear skies and mild temperatures across interior NorCal early
this morning as strong high pressure is gradually giving way to a
trough approaching from the Gulf of Alaska. Offshore flow is
slackening allowing shallow stratus to return to the coast, and
consequently a weak Delta Breeze is developing, but yet to
provide much cooling inland.

The approaching trough will gradually deepen off the PacNW coast
over the next several days bringing a cooling trend to NorCal
along with increasingly breezy southwest winds. In addition to
the synoptic cooling associated with the weakening high pressure,
the increasing onshore flow will provide additional cooling. It
looks like the Delta Breeze will gradually pick up today and
become robust tonight and Thursday. By Friday and Saturday, highs
in the Central Valley will be down into the mid 70s to lower 80s.

A few showers will be possible across the far northern mountains
of California by Saturday as an initial short-wave lifts out ahead
of the main offshore low.


Confidence is increasing in idea of precipitation across portions
of NorCal on Sunday with models agreeing on solution of upper
trough dropping into Norcal by afternoon. Small differences in
timing still exist but GFS and ECMWF in pretty good agreement.
Best chances of precipitation will be across the northern most CWA
with decreasing chances farther south. All areas will see cooling
for the second half of the weekend as cooler air continues to push
into the area. Combination of cooler airmass and cloud cover will
bring daytime highs on Sunday down into the 60s and 70s throughout
the valley or about 15 to 20 degrees below normal.

Models diverge significantly on Monday and as result, forecast
confidence plummets as well. ECMWF model moves the upper low into
the Great Basin on Monday bringing upper north flow and drying to
Norcal. GEM model moves the low along much slower keeping
precipitation over much of the forecast area. GFS moves the low
out so quickly the next upstream system is on the coast by
afternoon. With such disparity, did an average of models and
surrounding office blending for now. If more ridging solutions
verify, then there might be a slight warm up Monday but daytime
highs will remain well below normal.

Model differences continue into Tuesday but all show slight
ridging and northwest flow aloft so kept precip primarily over the
mountains and north valley. Both GFS and ECMWF show ridging over
the west coast by Wednesday with GEM the outlier showing a trough
over Normal. Have gone with model majority for middle of next week
with a forecast of a drying and warming trend.


VFR conditions over interior NorCal next 24 hours. Sustained
winds southerly generally below 12 knots except west 15 to 20
gusts to 30 knots through the delta this evening and tonight.


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


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