Area Forecast Discussion
Issued by NWS Sacramento, CA

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472
FXUS66 KSTO 181538
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
838 AM PDT Fri May 18 2018

.SYNOPSIS...

Afternoon and evening thunderstorms possible over higher terrain
into next week. Temperatures will gradually warm through Saturday,
and then cool slightly on Sunday.

&&

.Update...
Made a few small updates to the forecast. The marine layer is
thinker than what models were indicating and we are seeing some
scattered marine stratus make it into the Sacramento area. These
clouds should diminish quickly as things heat up. We are also
seeing some valley fog in Plumas and parts of Lake County. This
fog should burn off within the next hour or two. Afternoon showers
and isolated t-storms are still expected across the Sierra mainly
south of I-80 this afternoon. A subsidence inversion around 500
mb will limit this activity though. Highs today will be similar to
yesterday to a few degrees warmer.

.Previous Discussion...

Weak ridging will build over the CWA today with WNWly flow limiting
the convection to the Sierra Crest. Then more areal coverage of
storms expected as the weekend wears on as a weak trof is progged to
move through on Sat, followed by a digging trof over the Wrn States
on Sun leading to the development of a closed low over the Srn half
of CA on Mon.

There are still some clouds moving Swd along the Sierra Crest in the
lingering cyclonic flow from the Great Basin upper low. This flow
will back to Wly as the weak ridge builds and this cloudiness
should erode. Some cumuliform cloudiness is likely to develop over
the mtns as the flow turns upslope. As the flow turns Wly, the
convection will move off/over the Sierra Crest.

A stronger onshore pressure gradient is leading to a stronger Delta
Breeze this morning. Ft Ord profiler shows the marine layer is
slightly elevated and has spread into the Wrn portion of Solano Co.
It looks doubtful the stratus will advect inland as the T/Td spreads
are too wide and RHs lower than yesterday.

Generally, the SREF MUCAPE plumes show nil CAPE over the Coastal
Mtns and RDD area this afternoon, modest values Sat, and higher on
Sun. However, the values are similar each day over the Sierra
today and through the weekend. The synoptic pattern would suggest
stronger forcing/ascent with the approaching trof on Sat and then
with the digging trof on Sun. A marked increase in instability is
likely over the Shasta County and Coastal Range mtns with the
digging trof on Sun, with an increasing chance of convection over
the N end of the Sac Vly.

Modest warming is expected over Norcal through Sat, before a
cooling trend on Sun. Onshore flow should yield a Delta Breeze Sat
and Sun with the two trofs, then modest warming again on Mon as
the low moves into Socal.   JHM

&&

.EXTENDED DISCUSSION (Tuesday THROUGH Friday)

While model discrepancies exist in the extended period, there is
broad agreement that a closed upper low will remain over the
Pacific Southwest Tuesday, possibly into Wednesday. This will
continue to bring the chance of showers and thunderstorms mainly
over the higher elevations through at least the middle of next
week.

Model solutions diverge toward the end of next week. The ECMWF
remains more progressive, with another closed low approaching the
West Coast. The GFS amplifies an upper ridge over the West Coast,
bringing warmer and drier weather. Without a clear or preferred
solution, our forecast leans heavily on persistence, with
continued chances of showers and thunderstorms over the mountains.

Dang

&&

.AVIATION...

Mainly VFR conditions the next 24 hours. Showers and a few
thunderstorms are likely to develop over the high Sierra this
afternoon. South winds will strengthen to 10-15 kt this afternoon
& evening with local gusts up to 20 kt. Near the Delta...gusts up
to 30 kt.

Dang

&&

.STO WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$



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