Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 030525
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1025 PM PDT MON MAY 2 2016

.Synopsis...

Showers and thunderstorms will increase in coverage across
Northern California on Wednesday and Thursday. The storm system
will linger through the weekend, which will cause more chances for
showers and storms.

.Discussion...
Not much in the way of shower and thunderstorm activity today...as
a more stable atmosphere moved in. High clouds are increasing
ahead disturbance approaching from the Pacific. Dry weather with
generally light winds overnight...current forecast is on track and
no updates are needed.

.Previous Discussion...

Water vapor analysis and upper level model guidance indicates an
upper level high pressure system positioned over the northern third
of the CONUS, while a trough of low pressure is positioned in the
eastern Pacific this afternoon. An influx of moisture and cloud
cover is beginning to move into Northern California.

High resolution short term model output shows chances for
thunderstorms (mainly over the foothills and the Sierras) this
afternoon and Tuesday afternoon.  Storms that may develop could
bring brief heavy downpours and small hail.

As the upper level trough pushes eastward Tuesday night, showers and
storms should increase in coverage across much of Northern
California for Wednesday and Thursday.  The greatest instability in
the atmospheric profile should be positioned over the mountains,
thus resulting in the greater threat for thunderstorms. The valley
also has chances to see thunderstorms, but the risk is lower than
the upper elevations. The GFS and ECMWF are in good agreement to
the amplitude and progression of the upper level trough feature,
and both models indicate a slow moving, lingering system. Because
the storm system does not move much from Wednesday to Thursday,
unsettled weather should continue across the region through
Thursday night.

Snow levels should remain at and above 7000 feet, so some mountain
passes could be affected for this weather pattern. Travel may
become difficult for some mountain passes Wednesday night through
Thursday.

.EXTENDED DISCUSSION (Friday THROUGH Monday)

Negative anomaly (minus 3) 5H closed low over Socal on Fri forecast
to track slowly ENEwd to near the 4-Corners through the upcoming
weekend. The deep low finally relaxes its dominance on Norcal wx by
Mon as high pressure builds inland. Cyclonic flow and wrap around
moisture will keep a cool and unsettled weather pattern over most of
our CWA through the weekend...followed by a drying and warming trend
early next week. The GFS/GEM show strong ridging over the area
then...while the ECMWF shows a weak trof moving trough the Pac NW
and Nrn Rockys. Will keep some "silent PoPs" over the mtns for now.

Can`t rule out the potential for thunderstorms given the pattern
especially over our interior mtns as long as the cyclonic flow/wrap
around moisture pattern continues through Sun. 02/12z GFS
instability progs favor the Trinity/Shasta Co mtns for now...but
this can vary in future model runs. Winter may not be over yet
either as snow levels could easily lower to pass levels in
convective showers.

Temps will gradually warm through the period. Maxes will start out
just a little below normal over the Nrn portion of the CWA to 5-10
degrees or more cooler than normal over the Srn portion on Fri...to
a few degrees above normal on Mon. Forecast highs in the low to mid
70s in the Central Vly on Friday are forecast to warm into the
low to mid 80s Monday. JHM


&&

.AVIATION...

VFR conditions for TAF sites next 24 hours with increasing
high clouds. Light winds generally under 10 kts. Scattered
afternoon/evening thunderstorms over the mountains.




&&

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

$$



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