Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 281120

Area Forecast Discussion
National Weather Service Sacramento CA
320 AM PST SAT NOV 28 2015

Mainly dry conditions this weekend with cold morning
temperatures. The next chance for precipitation will arrive late
Wednesday or Thursday.


.SHORT TERM DISCUSSION (Today through Tuesday)...
Dry northerly wind the last two days has lowered dewpoints into
the teens to mid 20s over most of interior Norcal and this will
preclude fog or frost in the valley once again.

Wonder twin powers activate; one in the shape of an Upper high
centered over Vancouver Island and the other as the Eastern Great
Basin low. Together they form the broad Rex block ridge over the
western US. At the center of these two features sits the very
cold surface high pressure area across the Western Great Basin.
This cold air is separated from the Central Valley by the Sierra
Cascades, and physics dictates that wind blows from higher to
lower pressure areas. Voila, the winds across the region. Anyway,
the main short wave energy offshore will be rotating underneath
the Rex Block over the Great Basin today and finally ejecting the
block eastward enough to weaken its influence over Norcal on Sun.
Much lighter winds and clear skies are expected over the region on
Sun making it conducive for precipitous drops in temps below
freezing Sun morning, especially since the drying winds have
graciously lowered dewpoints. We upgraded the freeze watch to a
warning, because we are more serious about temperatures dropping
in the Valley. Temperature are already very cold up the hill.

Next system of uninterest will come aboard Monday, but this weak
system is trying to beat down the Rex Block, so not much is
expected other than some cloudiness and spitting over the northern
mountains. Tuesday may have increasing cloudiness as the next
system creeps eastward in the Pacific.   JClapp


Unsettled weather is expected for the extended with improving
confidence for a moderately wet system Thursday into Friday.

A weak wave will move in Wednesday with some light precipitation,
mainly over the northern mountains, with a slight chance for rain
into the northern Sacramento Valley.

Both the ECMWF and GFS models show a stronger system moving
through Thursday into early Friday. This could bring a quick shot
of moderate rain and mountain snow. Due to relatively mild air
ahead of this system, snow levels will likely start a little above
pass levels, then fall during the day behind a cold front. As a
result, accumulating snow will not reach as large of an area as
the last system, and are currently expected to be focused above
6000 feet. This could cause travel problems over the Sierra
passes, especially for the Thursday afternoon and Friday morning
commute times. Breezy southerly winds in the Valley and over the
mountains could be an issue as well.

The latest ECMWF now has a more progressive trough like the GFS,
so it is expected to exit fairly quickly on Friday, with a
building ridge bringing drier and warmer conditions by late in the
day. Dry weather is expected on Saturday, though there are some
hints another wet system could move in later in the weekend. EK


VFR conditions expected today with mostly clear skies. Northerly
winds around 5-12 kt in the Valley. EK


.STO Watches/Warnings/Advisories...
Freeze Warning from 1 AM to 9 AM PST Sunday for Carquinez Strait
and Delta-Central Sacramento Valley-Northern Sacramento Valley-
Northern San Joaquin Valley-Southern Sacramento Valley.


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