Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 232340

Area Forecast Discussion
National Weather Service Sacramento CA
340 PM PST MON NOV 23 2015

Dry and mild weather will continue through this afternoon with
above normal temperatures. A cold Pacific storm will bring heavy
snow in the mountains with snow extending down into the upper
foothills late Monday into Wednesday. Dry and cold weather
expected Thanksgiving.


The last of a series of pleasant, warm days is winding down with
temperatures well above normal levels, reaching the mid to upper
60s in the Valley. A weak disturbance touched off some light
sprinkles and flurries in the Sierra around mid day before
shifting out of the area. A cold storm from the Gulf of Alaska
now located along the coast of British Columbia is approaching.
This storm will spread inland early Tuesday. bringing heavy snow
to the mountains extending down into the upper foothills (2500
feet), impacting communities such as Grass Valley with several
inches of wet snow. Some light snow is possible down to 1500 feet
by Wednesday morning in heavier showers, although accumulation is
not expected. Very heavy snow is possible Tuesday afternoon and
early evening in the mountains with the potential for whiteout
conditions in intense snow. Storm total amounts of around 10 to
14 inches are projected above 5000 feet, with up to 18 inches over
higher mountains. Motorists are strongly urged to avoid travel
during this time period. The Winter Storm Watch has been upgraded
to a Winter Storm Warning, with a Winter Weather Advisory for the
northern mountains/Burney Basin, from Tuesday morning through
Wednesday morning.

This storm will come in two parts, with the main front on Tuesday
and then showers in cold are behind the front late Tuesday into
Wednesday. The main frontal band will spread into the area
Tuesday morning, from north to south, with highest intensity snow
in the mountains in the afternoon and early evening. Snow levels
will start around 4000 feet for the northern Mountains, 5000 feet
for the Sierra Tuesday morning, and are then expected to drop
into the foothills (2500 feet) late Tuesday night. Snow levels
could lower to 2000 feet or locally lower in showers Wednesday
morning. Showers through the day Wednesday will add some
additional mountain accumulation. The best potential for foothills
to see accumulating snow is late Tuesday into Wednesday morning. The
showers should be mainly over by around sunset Wednesday, except
for a few lingering ones over the Sierra crest.

The Valley will see rain, starting early in the morning over the
northern Sacramento Valley, and reaching the I80 corridor by late
morning. The heaviest rain will be in the afternoon. Rain amounts
generally in the 0.15 to 0.40 in range forecast. More is possible
locally in Tuesday afternoon thunderstorms, though. Clearing
behind the cold front along with cold air aloft could bring
enough instability for thunderstorms in the Sacramento Valley,
mainly north of I80. There is enough shear for some isolated
stronger thunderstorms to develop ,though confidence remains low.
There is also the possibility for some convection into Wednesday
but will leave thunderstorms out of the forecast for now.

Thursday looks dry and cold, with just a few light snow showers
over the Sierra Crest. Temperatures in the mountains and mountain valleys
could be quite cold dropping to teens and single digits. A
limiting factor for cold morning temperatures will be northerly
winds which could keep the atmosphere more mixed and a little
warmer. EK


Development of an Omega-Rex Block along the W Coast late in the week
will slow the Ewd progression of the Thanksgiving Holiday Storm. In
fact...anticyclonic flow around the high pressure cell over Wrn
Canada may tend to retrograde the blocking low into CA from NV Fri
and Sat. This forecast package includes higher PoPs given this
retrograding scenario. There will be the continued potential for
freezing/near freezing temperatures in the Valley and very cold
temperatures in the mountains and mountain valleys dropping into the
teens and single digits. A limiting factor for the cold morning
temperatures will be mixing winds and cloud cover.

Model differences Sun and Mon add more uncertainty to the forecast
with the GFS showing ridging and the ECMWF showing lingering effects
of cyclonic flow over the CWA from the upper low to the east. Worse
case scenario would be lingering snow showers mainly over the
Siernev and colder temps. This feature (upper low to the east) also
slows down the progression of the next upstream trof. The slower
ECMWF maintains the closed low offshore while the GFS is more
progressive with the upstream trof as it nears the coast by 12z Tue.
In any event...Sun and Mon should be mainly dry days with moderating
temps.   JHM


Incrsg SWly flow alf as cold Pac stm apchs. In Cntrl Vly, wdsprd
MVFR/IFR dvlpg aft 10-12z Tue with areas Sly sfc wnd gsts to 25
kts. Omtns, areas of S-SW sfc wnd gsts to 30 kts with wdsprd
MVFR/IFR with lcl LIFR in Nrn mtns aft 10z, sprdg to Siernev by 18z
Tue. Sfc wnd gusts to up to 45 kts ovr hyr mtn trrn Tue. Sn lvls
040 to 060 Tues mrng lwrg to 020-025 Tue aftn/eve.


.STO Watches/Warnings/Advisories...
Winter Storm Warning from 7 AM Tuesday to noon PST Wednesday for
Motherlode-Northeast Foothills/Sacramento Valley-West Slope
Northern Sierra Nevada-Western Plumas County/Lassen Park.

Winter Weather Advisory from 7 AM to 6 PM PST Tuesday for Burney
Basin / Eastern Shasta County-Mountains Southwestern Shasta
County to Northern Lake County-Shasta Lake Area / Northern
Shasta County.


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