Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 162209

Area Forecast Discussion
National Weather Service Sacramento CA
309 PM PDT Mon Oct 16 2017

Dry weather with less breezy conditions thru mid-week. Rain and high
mountain snow along with cooler temperatures late in the week,
followed by a warming trend this weekend.


Upper air flow has turned SWly over Norcal as the upper ridge axis
shifted Ewd today and a quasi-stationary low pressure area was
located along or just W of 140W. A short wave embedded in the
offshore Sly flow will be moving into our CWA Tue morning preceded
by variable high cloudiness. This wave will phase with the Wly flow
as the ridge flattens due to the passing Pac NW trof. More high
cloudiness is likely over our CWA over the next couple of days as
moisture spreads Nwd from the subtropical low pressure system
vicinity 21N/121W.

Dry wx will continue through mid week with mild days and cool nights.
Radiational cooling under fair skies should drop mins into the 40s
in the Valley the next couple of nights and 20s in the colder mtn
basins. Valley high temperatures will be in the lower to mid 80s with
a big cool change in store later in the week as max temps cool into
the 60s on Fri.

Low pressure system over the Wrn Aleutians is forecast to deepen a
trof over the NErn Pac and into the Wrn States Thu/Fri. This will
drive a frontal band across Norcal mainly Thu nite/Fri morning.
Model QPFS have been trending drier as the Wly flow over Norcal
develops ahead of the deepening NErn Pac trof and shunts the higher
(PW) subtropical air SE of our CWA. NAEFS anomaly table returns
lower PW values (mostly a modest +1 anomaly) to the coast Thu
evening, and spreads it Ewd into the Sierra by 12z Fri, followed
by lowering PW and a drying air mass. Thus, most of the rain in
our CWA should fall Thu nite into Fri morning.

Prefer a blend of the ECMWF/GFS over the Nrn mtns as the ECMWF is
much wetter than the GFS, and prefer the GFS over the Sierra, as
the ECMWF has been trending lower on the QPF than the GFS. The
WSWly flow with the approaching trof/frontal band should provide a
favorable orographic trajectory for the Sierra Thu nite and Fri
morning. Colder air behind the frontal band expected to lower snow
levels below the major passes on I-80/US-50 where an inch or two
of snow could accumulate. About a half foot is expected near
Lassen NP. Wetter mountain areas may see as much as an inch of
liquid precip while the valley will generally see less than a
quarter inch.   JHM



Models are trending a little slower and drier as an upper level
trough moves in late this week. The heaviest precipitation is
expected late Thursday night into early Friday as a cold front
pushes through. Rainfall storm total amounts currently are
projected to reach up to around an inch in the mountains, several
hundredths to a quarter inch in the Valley.

Snow levels will drop behind the front, dropping from around 9000
feet down to as low as 5000 feet for northern Shasta by Friday
afternoon. Snow amounts at this point are expected to be
relatively light, maybe a couple inches at pass levels and for
higher peaks.

Highs on Friday will be cool, upper 60s for the Valley, with the
mountains in the 40s and even 30s for high elevations. The upper
trough pushes east and weakens Friday night into Saturday morning
making way for drier and warmer weather over the weekend. Daytime
highs in the Valley will warm into the upper 70s and low 80s by
Sunday. For the foothills and mountains, Sunday highs will rise to
the 60s to mid 70s. EK



VFR conditions at TAF sites with high clouds. Valley winds will
remain light for the next 24 hours at less than 10 kts. Winds over
higher terrain will be light with a diurnal pattern. EK



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