Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 161109
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
409 AM PDT Wed Apr 16 2014

.Synopsis...
Breezy northerly winds developing today in the valley. A slight
chance of showers over the mountains Thursday afternoon into Friday
and possibly over the Northern Sacramento Valley Thursday evening.
Temperatures above normal through Sunday then may cool to around
normal early next week due to an unsettled weather pattern moving
over the region.

&&

.Short Term Discussion...
The forecast continues to be driven by a progressive upper air
pattern over the Ern Pacific and West Coast as a series of
trofs/ridges affects Norcal wx for the rest of the week. The minor
impact today will be the Nly winds that are expected to develop in
the late morning and lasting into the afternoon...and decreasing
late afternoon. Otherwise...mostly dry wx and above normal max temps
continuing for the rest of the work week...albeit for a slight
chance of showers Thu into Fri mainly mtns and in Nrn Sac vly Thu
nite.

While the main short wave energy over the Nrn Rockys moves SEwd
today...secondary energy embedded in the NWly flow aloft from the
rebuilding Ern Pac ridge in the wake of the main short wave is
forecast to dig into the Great Basin today. This short wave is
tracking farther E and is weaker so it will have less of an affect
on our pressure gradients. The forecast Nly pressure gradients are
just a little shy of our rule-of-thumb for wind advisory criteria...
especially the RDD-SAC gradient which is forecast to be 2+ mbs
when we would like to see at least 4 mbs. The NAM 925 mbs winds
(~2 kft) peak out in the 25+ kt range from 12z-18z Wed...a little
premature for the time of max heating to optimize downward
momentum transfer which is also problematic for wind advisory
criteria this time around. Gradients and winds are forecast to
drop off this afternoon. High temps are forecast to be some 10-15
degrees above normal today...as the weak CAA will be offset by
downslope warming from the northerly winds. Low-mid 80s are expected
in valley...with mostly 60s and 70s mtns.

The progressive upper air pattern will bring a cooler day on Thu
than what we expected a couple of days ago as the next short wave
trof is now forecast to near the coast by 00z Fri. Onshore flow
should increase as the trof approaches bringing synoptic cooling to
the CWA as well as a chance of showers over the Nrn mtns and
possibly the Nrn Sac vly by evening. The chance of showers will
spread SEwd over the Siernev into Fri as well. This positively-
tilted and splitting system won`t bring much precip...just a
reminder that we are still in the spring season.

Instability associated with the stronger piece of energy rotating
across Socal Thu nite and Fri (in the bottom of the trof) and
lingering over the 4 corners area on Sat may trigger some
thunderstorms during the afternoons/evenings over the Siernev mainly
S of our CWA...but close enough to warrant mentioning some low PoPs
S of Hwy 50 near the Sierra Crest. Otherwise...model differences in
the timing of the next short wave on Sat leads to low confidence
with precip chances for the weekend. Our forecaster consensus
leans to the GFS with the GEM/ECMWF similar and the faster of the
three. The dprog/dt of the ECMWF shows the last two runs becoming
more consistent...so this forecast is problematic and subject to
change.   JHM

.Extended Discussion (Sunday through Wednesday)

All models showing a large upper level low dropping southward
from the Gulf of Alaska towards the West Coast in the extended
forecast. Details of this system are not consistent across the
models, but a pattern change is expected. Sunday should remain
fairly dry and warm, but wetter and cooler conditions are expected
to arrive by Monday. The low could start bringing light
precipitation to Coastal & Northern mountains late Sunday. Showers
will then spread southward across our CWA Monday into Tuesday.
Valley highs will go from the low to mid 80s on Sunday cooling to
mid-upper 70s on Monday and upper 60s-low 70s by Tuesday.  JBB

&&

.Aviation...

Currently VFR conditions with light winds for valley TAF sites.
Northerly winds will increase this morning into afternoon for the
valley. In KRDD-KRBL vicinity, expect winds to be enhanced between
15-22z with sustained winds 10-20 kts and gusts up to 30 kts. For
areas southward, winds will be enhanced between 17-22z with
sustained winds 5-15 kts and local gusts up to 25 kts. Strongest
winds will be along the western side of the northern/central
Sacramento Valley. Winds become light again after 0z as VFR
conditions continue.  JBB


&&

.STO Watches/Warnings/Advisories...
None.
&&

$$







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