Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 222326

Area Forecast Discussion
National Weather Service Sacramento CA
326 PM PST Wed Nov 22 2017

Mild temperatures this week, with weak weather systems possible on
Thanksgiving and Saturday. More widespread rain Sunday, followed
by a colder system early next week.


Upper level ridging has brought dry weather and above normal
temperatures. Fog developed over the Valley and over mountain
valleys this morning, persisting into the mountain valleys into
the early afternoon. High and mid level clouds have spread across
the area, limiting sunshine and temperature rises to some degree.
Some fog and mist may again develop overnight, but the greater
cloud cover will keep dense fog from becoming as extensive for
Thanksgiving morning as it was early today.

A Pacific moisture plume will bring some precipitation into Norcal
for Thanksgiving day. Rainfall amounts will be light, and focused
over the northern mountains, extending into the Valley north of
I80. Snow levels with this mild system will be above 10000 feet.
After dry weather returns for Friday, another weak, mild system
arrives on Saturday.

Temperatures will remain above normal through Saturday, with
Valley and Delta highs in the upper 60s to around 70, 60s for the
foothills. EK


Models indicate a couple of precip frontal bands will move into
Norcal Sun into Mon morning. WAA pre-frontal rain should begin over
much of the region on Sat and become more widespread Sun morning as
it moves inland as the initial short wave moves through the
deepening Ern Pac trof. Model timing is similar, but amounts vary
with the GFS wetter than the ECMWF. Precip may taper on the west
side of the CWA Sun afternoon with orographics aiding the frontal
band over the Sierra.

The second, more dynamic wave, will increase precip Sun evening
into Mon morning. The GFS continues to be a little faster/wetter
with this system, while the slower ECMWF lingers precip over the
Sierra Mon morning. In the end, the QPFs could be similar over the
Sierra for both models. Slightly positive TPW anomaly expected to
yield a modest precip event with amounts ranging from .25" south
to around .75" north in the Valley and 1 to 2 1/2 inches in the
mountains. This is double in the mtns from yesterday and about a
third increase in the North Valley from yesterday.

Little snow impact is expected in the Sierra until Sun nite and
Mon as snow levels should remain generally above pass levels until
Sun nite, around midnight or so. Then snow levels are forecast to
drop below pass levels late Sunday night into Mon morning. This
is where the wetter ECMWF could mean more snow for the Sierra
highways than the GFS with snow levels dropping below 4000-5000 ft
by the end of the precip event.

Seasonably mild temperatures on Sat cool to near/below normal
Sun/Mon. Then drier conditions Tue/Wed as high pressure rebuilds
over Norcal leading to a warming trend. Nly winds should keep fog
at bay Tue morning with some valley fog Wed as winds subside. JHM



Mid cloud CIGS bases AOA 10 kft along the I-80 corridor are forecast
to erode after 06z-08z Thu yielding mostly SKC conditions over
interior Norcal by Thu sunrise. Areas F+ expected to develop by Thu
morning and lower conditions to IFR/LIFR in the Valley in the
morning with conditions improving after 18z-20z Thu. Light winds.



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