Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 040607
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1005 PM PST Sat Dec 3 2016

.SYNOPSIS...
A small upper level disturbance will bring chances for small
amounts of rain and snow to the upper Valley and northern
mountains Sunday into Tuesday. Colder air in the Valley could
result in near to below freezing overnight temperatures this
coming week.

&&

.DISCUSSION...
Some big changes from 24 hrs ago with temps some 5-18 degrees cooler
than yesterday due to light winds and lack of the mixing NWly winds.
Forecast cloud cover will mainly spread across the Nrn portion of
the CWA by morning and expect mainly thin cirrus over the Srn
portion. Therefore it looks like radiational cooling will result in
cooler mins in some areas and updated the grids/zones accordingly
this evening. In addition to the cooler temps, dewpoints have
recovered some 4-14 degrees, without the drying winds, with a
recovery of 10-60 percent RH in the Valley. The combination of high
RH. light winds and mainly clear skies overnite brings some morning
fog into play. Also with mins dropping into the mid 30s and possibly
into the 32-34 degree range in the outlying areas brings some frost
into play. The temp is usually a little cooler on the ground than
what it is on the 6m-10m tower where the thermometer is.    JHM

A couple of weak disturbances move through the region late Sunday
and Sunday night and then another one Monday night into Tuesday.
Precipitation amounts look to be very light with both and no
significant problems are expected.

Temperatures will be getting colder at night with low overnight
temperatures Monday night and Tuesday night near or slightly below
freezing for the Valley. The coldest morning day looks to be on
Wednesday. At this time widespread morning lows at or a little
below freezing look likely occur.

.EXTENDED DISCUSSION (Wednesday THROUGH Saturday)


Wednesday morning is expected to start off cool and dry as the
coldest airmass of the season settles over our area. Many of our
valley locations will start off the day with temperatures near to
slightly below freezing. However, this cold air mass is not
forecast to stick around for long as the flow aloft switches
westerly and begins advecting warmer air into our area. For the
most part, this warm air advection (WAA) will bring an increase in
high clouds on Wednesday with valley temperatures still quiet
chilly, around 50 degrees.

Depending on the model, WAA induced precipitation could begin
over our area Wednesday night to Thursday morning. The ECMWF being
the slower solution. What is most interesting is that the warmer
air will be overriding much colder air trapped at the surface.
This could potentially create very low snow levels for places like
Shasta County where cold air is easily trapped this time of year.
The GFS currently has snow levels around 1500 ft Thursday morning
in the northern mountains while the DGEX is even lower. In any
event, it is a pattern that bears watching and it is a bit too
early to start nailing down details.

Precipitation will continue into Friday and snow levels will
gradually rise for those locations with colder air trapped near
the surface. Both the GFS and ECMWF show heights building on
Saturday which should provide mostly dry conditions for our CWA.
However, the ECMWF keeps a few leftover showers over the northern
valley and mountains for Saturday so have kept a slight chance of
PoPs for that area. Wilson


&&

.AVIATION...

Light and variable flow expected to continue at the surface for
the next 24 hours. Medium confidence with the fog forecast
tonight as high clouds moving in may limit fog at TAF sites.
Beyond this, VFR conditions expected tomorrow in the valley with
showers and MVFR conditions in the northern mountains from a
passing system. Wilson


&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$



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