Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 182152
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
252 PM PDT Sat Apr 18 2015

.Synopsis...
Very warm weather continues across the region into early next
week. Isolated late day showers and thunderstorms will be possible
over the mountains. Another low pressure system will move into the
region by the middle of next week bringing cooler temperatures and
continuing the threat of mainly mountain showers and thunderstorms.

&&

.Discussion...
Very weak closed mid/upper level low drifting southward into far
northern California this afternoon from Oregon. Just enough
moisture (TPW around 1/2 an inch) and instability with day time
heating to kick off a few showers and thunderstorms over the
northern mountains and northern Sierra Nevada. Expect these to
dissipate shortly after sunset with loss of surface heating.

Temperatures remain very warm across the region once again this
afternoon with most areas running a few degrees ahead of yesterday
at this time. Mid-afternoon readings range from the 60s and 70s in
the mountains to mainly the 80s in the Central Valley. Warmest
readings are at the north end of the Sacramento Valley where
residual northerly wind is providing local enhanced downslope
warming, and readings at Redding may eclipse the 90 degree mark
later today.

The closed low over far northern California is forecast to drift
back to the southwest on Sunday as strong ridging over the eastern
Pacific moves closer, evolving into a weak Rex Block. This may cap
off deeper convection for a day across the northern mountains
while the focus of late day thunderstorms shifts to the Sierra to
the south of Lake Tahoe. Temperatures will likely be the warmest
of the past week on Sunday as northerly/northeasterly flow briefly
reinvigorates. Readings through much of the Central Valley will
likely climb into the upper 80s to lower 90s. The exception may be
in the Delta where a trickle may keep Fairfield in the lower 80s.

Diurnal mountain showers and thunderstorms are forecast to be on
the upswing again across NorCal on Monday as the ridge weakens
and a little more mid-level moisture is forecast to entrain up
from the southeast ahead of the weak low off the Central
California coast.

More widespread showers and thunderstorms along with cooler
weather will be possible beginning Tuesday as a stronger trough
moves into the region.

&&

.Extended Discussion (Wednesday through Saturday)...
The longwave trough will dominate through the early portions of
the extended period, keeping temperatures moderated. Showers and
thunderstorms still look like a pretty good for the mountains
Tuesday through Thursday, and possibly into Friday. A few of these
showers may make it down to the Valley as well, though timing and
location of these showers could be tricky.

Significant model differences arise by the end of next week. The
ECMWF and GEM both show a trough moving through Northern
California next Friday, whereas the GFS keeps a drier, broad
westerly flow solution. With no inkling toward any solution, our
forecast Friday and beyond leans heavily on climatology. Expect
temperatures to remain around or slightly warmer than normal. Dang

&&

.Aviation...
VFR conditions next 24 hours. Isolated thunderstorms will continue
over the mountains through 03z. Light winds will continue except
in vicinity of thunderstorms. Dang

&&

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

&&

$$



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