Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 171103

403 AM PDT Thu Apr 17 2014

Weak low pressure system will bring a slight chance of showers over
the mountains this afternoon and into Friday and over the Northern
Sacramento Valley Thursday evening. Temperatures will continue above
normal through Sunday...then are forecast to cool to near normal to
below normal early next week. An unsettled weather pattern moving
over the region will bring a chance of showers over interior
Northern California Monday night into Tuesday night along with a
slight chance of thunderstorms in the Valley Tuesday.


.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 through the forecast period. There are
indications of a deeper closed low impacting Norcal early next week
with more substantial wx than the progressive short-term trofs.
Above normal max temps will continue through the weekend before
substantial cooling early next week.

Today...two wx systems will be approaching from the NW
and the other from the SW. However...the northern most one will
prevail and will shunt the Srn system well to our S. Some cloud
cover from the Srn system may spread into the Srn portion of our
CWA...otherwise the Nrn trof will prevail...and will move inland
this evening and across our CWA tonite...exiting on 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 this 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. Models forecast
some instability near or within our CWA Fri afternoon over the
Siernev...but the synoptic forcing weakens during this time. The GFS
shows greater areal coverage of some convective precip over our
Siernev Fri afternoon while the NAM keeps the instability farther S.
Given the contrasting forecasts...we continued with our forecast
continuity for now.

Increasing onshore flow with the approaching trof should strengthen
the Delta Breeze late this afternoon and evening. Given the synoptic
cooling and local wind conditions...max temps will be mostly 5-10
degrees above normal on Fri...instead of the 10-15 degrees above
normal today. Cooler yes...but still above normal.

A modest recovery in 5H heights is expected Fri nite and early Sat
before another progressive trof moves into the Pac NW and brushes
Norcal. The GEM/ECMWF/UKMET indicated a deep enough short wave trof
to bring some showers to our Wrn Shasta Co mtns late Sat...with
the latest 17/06z GFS trending drier/weaker for our CWA. Otherwise
5H heights recover further over Norcal on Sun ahead of the deepening
large closed low over the NErn Pac which is destined to bring a
drastic change to our wx early next week. Until then...Eastern Sun
looks dry and warm with max temps some 5-10 degrees above normal. JHM

.Extended Discussion (Monday through Thursday)

Pattern change starts Monday with cooler and wetter weather on the
way. The ECMWF, GFS, and GEM models are finally trending toward a
consensus for the main arrival of a large upper low from the Gulf
of Alaska. Although light mountain showers could start Monday
morning, the brunt of this system will start impacting NorCal
Monday evening. Showers will spread southward across our CWA
Monday into Tuesday. Have kept the mention of slight thunderstorm
chances Tuesday as forecast instability appears favorable for the
valley. Cooler temperatures will be noticeable as valley highs go
from the low 80s on Sunday cooling to mid-upper 70s on Monday and
mid 60s-low 70s by Tuesday. Those cooler temps on Tuesday will
continue into Wednesday with lingering showers.  JBB



VFR conditions will continue today for TAF sites. Winds will be
light this morning into early afternoon, but onshore flow will
develop later in the afternoon into early evening. This will bring
some moderate winds to the delta region while also bringing local
gusts of 15-18 kts to KSMF, KSAC, and KSCK between 22-03z today.


.STO Watches/Warnings/Advisories...

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