Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 112311 CCA
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
311 PM PST Wed Jan 11 2017

.Synopsis...
Weaker Pacific weather system will continue chance of showers and a
few thunderstorms into Wednesday evening. This weather system will
pass to the south with scattered showers mainly south of I-80 on
Thursday. Flooding concerns and mountain snow will continue on
Thursday. Drier weather expected Friday into the weekend.

&&

.Discussion...
Upper jet has shifted S of our CWA and the TPW plume has eroded.
Much drier air is currently spreading into Norcal in the Wly flow
over our area S of the upper low over the Pac NW. The cross mountain
flow has eroded cloud cover in the Vly and flat-based strato-cu (SC)
clouds have developed due to surface heating. Meanwhile, upslope
flow has continued over the Siernev continuing numerous showers
there.

At the lower elevations, scattered showers along with a few
thunderstorms could develop this afternoon and evening. Shear
parameters much lower than yesterday due to the cold front well to
our south, and lack of a strong mid-tropospheric short wave unlike
last night. So the thunderstorms are not expected to be severe and
should be very brief, unlike last night. This is illustrated with
thin and shallow CAPE up to 10 kt generally, and straight-line
hodographs more suggestive of multicell storms, and not rotating
ones.

The next shortwave nearing 40N/135W will move into the region late
tonight and Thursday. A band of showers should develop over areas
along/south of I-80/Hwy 50 later tonight and Thu as the
aforementioned shortwave energy rotates inland after 12z Thu. IR
imagery shows a narrow band of enhancement along the jet axis
moving across central CA, and short wave energy will roughly
follow a similar track later tonite and Thu. Thus, showers should
be redeveloping over our Srn zones late this afternoon and
evening, and into Thu. QPFs of a third of an inch in the valley
and a half to three-quarters of an inch over the Srn Mother Lode
and Sierra are expected. Snow levels will be lowering to at least
3 kft to 3.5 kft and accumulating snow will continue. We will
continue the WSW for zone 68 thru 18z Thu, and extended the WSW in
time for Srn zone 69 thru Thu afternoon due to additional
accumulations. Another 2 to 5 inches is forecast in zone 68 and
about 5 to 11 inches in zone 69, the heaviest amounts S of the
80/50 corridor at the higher elevations.

The energy will reform the Pac NW upper low along the coast on Thu,
and then eventually just off the LA/SGX coast on Fri. Nly flow
will result in drier weather across the region Friday into the
weekend as ridging from the eastern Pacific extends into NorCal.
Nly flow will preclude widespread fog/stratus, so the most
favorable area would be on the E side of the Vly and lower Sierra
foothills.  JHM

&&
.EXTENDED DISCUSSION (Sunday THROUGH Wednesday)

Upper ridging gradually builds inland through the weekend into
Monday with dry weather and warming temperatures. Patchy morning
valley fog possible from about Sacramento southward and near the
Delta. Models suggest next Pacific frontal system will impact the
area Tuesday as another atmospheric river takes aim on California.
IVT are much stronger on the Oper GFS than GEFS Ensemble means so
forecast confidence limited at this time but appears an extended
period of wet weather could impact Interior NorCal again next
week.

PCH

&&

.AVIATION...

Upr low drops south through NorCal next 24 hrs. In Cstl/Shasta
mtns...mnly VFR conds exc lcl MVFR/IFR conds poss in shwrs til 12z
Thu. Cntrl Vly...mnly VFR exc isold MVFR poss in shwrs ovngt or
isold tstm til 03z Thu. Lcl MVFR conds poss in shwrs mnly I80
Swd. For ern fthls/mtns incldg Siernev...wdsprd MVFR/IFR with lcl
LIFR in shwrs nxt 24 hrs. Sn lvls 020-035. Areas SWly sfc wnd
gsts up to 30 kts over hyr trrn.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 10 AM PST Thursday for Western Plumas
County/Lassen Park.

Winter Storm Warning until 4 PM PST Thursday for West Slope
Northern Sierra Nevada.

&&

$$



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