Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 172206

Area Forecast Discussion
National Weather Service Sacramento CA
306 PM PDT Tue Oct 17 2017

Dry weather thru mid-week. Rain chances along with high mountain
snow and cooler temperatures late in the week followed by a
warming trend this weekend into early next week.


Seeing more high cloudiness over Norcal today as one offshore
convective complex moves onto the central CA coast to be followed by
a second one overnite. These two systems are embedded in the WSWly
flow aloft and ahead of a short wave trof. Some of this moisture
appears to be the remnants from the decaying low pressure system
that was off the Baja Coast and is now moving inland. These
complexes are quite moist and unstable with numerous C-G strikes.
This moisture gets shunted SE of our CWA today and tonight by the
WNW flow developing over Norcal as a progressive short wave moves
across the Pac NW today and the Nrn Rockies tonite.

WNWly flow will transition to WSWly on Wed as low pressure deepens
over the NErn Pac/GOA on Thu and the Wrn States on Fri and a lot of
high cloudiness over Norcal will exit the area. The deepening trof
will drive a frontal band across Norcal mainly Thu nite/Fri morning.
NAEFS anomaly table returns 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. Increasing clouds and onshore
flow expected to lower Max Temps some 5-10 degrees below normal on
Thu, and 10-15 degrees below normal behind the front on Fri, Thus,
60s to low 70s in the Valley on Thu and 60s on Fri.

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. The system looks rather dynamic over the GOA at this time
with cold air cumuliform cloudiness preceded by a baroclinic leaf
and associated with a decent moisture plume. Although some eroding
of the plume is forecast as it encounters lower (drier) PW air and
can`t tap into the subtropical moisture, orographic and dynamic
effects should provide for at least a modest soaking over portions
of the Sierra. The wetter mountain areas are expected to see as much
as an inch of liquid precip while the valley will generally see less
than a quarter inch and most locales probably just a few hundredths.
Colder air behind the frontal band is expected to lower snow levels
below the major passes on I-80/US-50 where an inch or two of snow
could accumulate.

Gusty ridge winds can be expected late Thu as the upper trof moves
inland with an 8 mbs Sly RDD-SAC gradient forecast by 00z Fri.
Chance of seeing wind advrys in the Nrn half of the Sac Vly and
surrounding NE foothills in future forecasts as onshore gradients
increase along with topographic forcing of the wind. Also, chance of
a stratus intrusion into the Sac Vly Thu morning ahead of the
deepening trof, unless the trof mixes it out beforehand.

Warm frontal clouds could return Fri nite with WAA precip mainly
staying to our N as strong ridging begins to build for the weekend.
Expect good radiational cooling in the mtn valleys/basins tonite
until there is some moderation due to clouds and precip Thu/Fri
mornings.   JHM



Drier and warmer weather returns for the weekend as cool upper
trough exits eastward. Some lingering light showers are possible
over northern and eastern Shasta County, with dry weather
elsewhere across the area. 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.

Dry weather with additional warming continues into early next
week as high pressure ridging builds along the coast. NAEFS
temperature anomalies show some unusually warm air near the
surface, along the central California coast by the ridge axis.
Currently, Valley highs in the low to mid 60s are projected, and
this could potentially trend forecast temperatures even warmer.
For reference, Valley highs for that time of the year are normally
in the mid 70s, with record highs in the low to mid 90s. EK



General VFR conditions with light winds are expected over the next
24 hours, though local HZ/FU MVFR visibility is possible in the
Delta into southern Sacramento Valley overnight into early
Wednesday. Valley winds at TAF sites will be less than 10 kts.
Winds over higher terrain will be light with a diurnal pattern.



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