Area Forecast Discussion
Issued by NWS Sacramento, CA

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000
FXUS66 KSTO 220555
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1055 PM PDT Thu Sep 21 2017

.SYNOPSIS...
Unseasonably cool weather system will bring mountain rain and snow
showers through Friday night. Several inches of snow are possible at
high elevations. Turning warmer and drier this weekend.

&&

.DISCUSSION...
Moist cyclonic flow on the backside of the upper trof moving into
the Great Basin will keep spreading mainly mid clouds over Norcal
this evening. After lull in the shower activity overnight, the next
wave of energy working on the backside of the trof is progged to
move over the Sierra Fri afternoon. This should result in another
round of scattered rain/snow showers. Another cool day on Fri,
before warming trend on Sat with breezy Nly winds Sat afternoon/
evening with gusts to 25 mph.  JHM

.Previous Discussion...

A winter like trough will continue to move across interior northern
California and exit early tonight. Snow levels have been generally
above 6000 feet but current weather observations support that they
could lower to 5000-5500 feet including Blue Canyon. Higher
elevations above 8000 feet in Tuolumne County have received at least
3 inches of snow as of this afternoon. In addition to the Sierra
snow, instability has lead to the development of thunderstorms along
the crest this afternoon. Convection will continue generally through
00z Friday. Have introduced a slight chance for showers this evening
for Coastal Range as instability progs and high resolution guidance
suggest possible showers.

A Winter Weather Advisory remains in effect for locations above
7000 feet with main concern near Donner pass. Convective nature of
showers will mean potentially large variability in amounts from
place to place. Current snow forecast suggests amounts from 3 to 6
inches at those locations with higher amounts over peaks.

Trough will shift east into northern Nevada with cut-off low
developing on Friday. Second disturbance will drop south along
the Sierra late Friday with another quick shot of snow above 6500
feet with 2 to 3 inches. Precipitation amounts through Friday
afternoon could range from 0.25-0.75 inches over the Sierra.

Drier weather is expected this weekend as the trough shifts
eastward. A gradual warming trend is expected with Valley highs in
the 80s. However, temperatures will still remain below average for
this time of the year.

&&

.EXTENDED DISCUSSION (Monday THROUGH Thursday)

High pressure ridging spreads in from the west throughout the
extended period as the deep upper trough shifts eastward into the
Rocky Mountain region. Confidence is high for a warm and dry
pattern over NorCal at least through Thursday. Temperatures will
be above normal starting on Monday, and trending upward a few
degrees more on Tuesday. Valley temperature forecast in the high
80s on Monday and generally in the lower 90s for Tuesday through
Thursday.

Breezy north winds are expected to continue through mid week next
week. These winds will act to warm and dry the area, creating an
elevated fire risk. EK/HEC

&&

.AVIATION...

VFR conditions expected for Valley TAF sites for the next 24
hours. Scattered mountain/foothill rain showers and isolated
thunderstorms expected through tonight, along with snow showers
above 6000 feet. Northerly winds expected with gusts up to 14-18
kts at TAF locations this afternoon. EK

&&

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

&&

$$



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