High Wind in California, November 17, 2020

Area Forecast Discussion National Weather Service Las Vegas NV 1010
AM PST Tue Nov 17 2020

.SYNOPSIS...Bouts of unsettled weather through the week as a
series of systems impacts the western US. The initial round through
midweek will bring warming temperatures with breezy to locally windy
conditions. Followed by cooler weather and breezes once again for
the end of the week. &&

.UPDATE...The main concern for the rest of the morning and afternoon
is the potential for strong winds in the eastern Sierra slopes and
downslope winds into the Owens Valley. Current satellite imagery
shows a broad longwave trough off the California coast moving
inland. Noted on water vapor is a decent jet streak moving into
north-central California. The overall set up of the trough and jet
streak would be favorable for high winds along the Sierra peaks and
then  downslope on the eastern side of the range, however, the
portion of the trough with the best setup is north of the area. The
base of the trough may brush far northern Inyo County, but overall
the area for favorable strong winds will be to the north. Forecast
models through the afternoon show this as well, with more of a
southwest flow instead of a cross boundary flow across the southern
parts of the Sierra. Could downslope still occur? Its not out of the
question. But overall confidence is low. Given all this, decided to
replace the High Wind Watch with a Wind Advisory. Gusts over 70 MPH
are possible still above 9000 ft, which could affect places such as
Aspendell and Whitney Portal, but overall wind impacts should be
limited. Will be monitoring through the afternoon to see how things
develop and will update if needed.

Otherwise, no significant changes were made to the forecast for
today. It will be mild, a few degrees warmer than yesterday, with
occasional clouds streaming overhead from the incoming trough to the
west. The wind advisory for Esmeralda and Nye Counties are in good
shape with gusts to around 40 mph expected today. May need to extend
the wind advisory for these areas into Wednesday, but will wait to
see how today unfolds before making any changes for tomorrow.  &&

.PREVIOUS DISCUSSION...250 AM PST Tue Nov 17 2020
.DISCUSSION...

The primary impacts over the short term will be winds and warmer,
near-record temperatures, followed by cooler weather and another
bout of winds to end the week.

Winds will be the biggest impact, and currently, hardest part of
the forecast. As shortwaves primarily impact the Pac NW, and just
graze the area, an interesting synoptic setup occurs. The region
will initially be on the warm side of tightening thermal
gradients, driving temperatures up through midweek. Both how much
warming and how much wind depend on a mid-level inversion. If the
inversion is able to be mixed from both mechanical and insolation
sources, temperatures could run a bit warmer, while also allowing
the stronger momentum to mix down to the surface - initially in
northwestern areas today, shifting eastward for Wednesday.

The focus for the winds today will be downslope wind potential in
the eastern Sierra/Owens Valley and White Mountains. The overall
synoptic setup is rather favorable, with Inyo and Esmeralda
counties positioned underneath right exit region of jet streak
rounding the base and ejecting ahead of the upper-level trough
axis. Such a pattern would generally be conducive to long-duration
downslope events. However, the differences between vertically
propagating waves vs trapped lee waves are incredibly subtle and
not usually well-handled by models. While no clear, stout
inversion is indicated above ridgetop in the models, they do tend
to under-do this feature, and the subsidence from the jet streak
circulation and warming 500mb temps would point to increasing
ridgetop stability today. It is rather difficult to imagine
impactful winds making it very far down the eastern slopes without
a downslope mechanism present, and stronger downslope winds could
push into the Owens Valley if they do develop. Thus, will remain
on the indecisive side and leave the watch as-is to see how things
start to unfold. Further east in the Great Basin, mixing is in
question, though confidence in areas like Tonopah and around the
Nevada National Security Site seeing afternoon gusts around 40 MPH
is high enough for a Wind Advisory today.

Similar story for Wednesday, just shifted eastward. For synoptic
winds, today will be the canary in the coal mine for the ability
for mid-level momentum to mix down. If it does, portions of
Clark, Lincoln, and Mohave county will likely see near- advisory
level winds tomorrow. Downslope winds could also be of concern for
the northeast slopes of the Spring Mountains. Will wait and see
what happens today before issuing anything for Wednesday.

The last shortwave in the series will swing the base of what's
left of the upper-level trough over the region Friday into
Saturday. Not really expecting much other than perhaps a belt of
westerly winds ahead of and northerly winds behind, and a
cooldown, cooling the region a good 10-15 degrees, down from the
near record highs earlier in the week.

For next week, some flavor of unsettled pattern looks likely, with
troughing somewhere near the northwest us continuing, likely
remaining dry for the region.
&&

.AVIATION...For McCarran...Light winds favoring typical daily trends
are expected through mid-afternoon Tuesday along with increasing SCT-
BKN high clouds after sunrise.  A push of southerly winds will
arrive late in the day, though timing is in question anywhere from
22z-02z. Favored an arrival closer to 00z though amendments are
likely.

For the rest of southern Nevada, northwest Arizona and southeast
California...Light winds through the morning hours will transition
to breezy south winds in the afternoon with speeds of 10-20 knots
and gusts to 25 knots possible. Stronger winds expected near KBIH
with gusty southeast winds exceeding 30 knots through the afternoon
and evening hours, along with moderate to strong turbulence downwind
of the Sierra. Gusty winds near Bishop and continued mountain wave
induced turbulence can be expected to linger overnight.
&&

.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
any significant weather or impacts according to standard operating
procedures.
&&

UPDATE...Nickerson
DISCUSSION...Steele
AVIATION...Outler

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