Area Forecast Discussion
Issued by NWS Las Vegas, NV

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FXUS65 KVEF 202232

Area Forecast Discussion
National Weather Service Las Vegas NV
332 PM PDT THU OCT 20 2016

.SYNOPSIS...High pressure will move through the Desert Southwest
bringing warming temperatures, light winds, and clear skies to the
region through Saturday.  A storm system will approach the area
Sunday and Monday with chances for showers and isolated storms
increasing, especially east of a Twentynine Palms to Tonopah line.

.SHORT TERM...through Saturday night.

Gusty winds have been quite slow to diminish across the Colorado
River Valley today, with area surface observations and buoys only
recently showing winds diminishing on/near Lake Mead with strong
north winds continuing on/near Lakes Mohave and Havasu.  Choppy
water evident on Lake Mohave, with significant wave heights around 2
feet again today.  Winds are expected to decrease this evening much
as they did yesterday.  However, unlike yesterday, the surface and
upper-level ridges will be moving through the area starting tonight,
which will act to reduce the pressure gradient and eliminate the
stronger winds.

The rest of the short term looks quiet as midlevel southwest flow
becomes established by Saturday.  Temperatures will continue to warm
on the upstream side of the ridge, with forecast maxes generally 7-
12 degrees above average.  May start to see some breezy south or
southwest winds in the southern Great Basin and the higher terrain
of Inyo County by Saturday afternoon.  Otherwise, a fine start to
the weekend is expected.

.LONG TERM...Sunday through Thursday.

The long term period looks active as one system is set to affect the
region Sunday through Tuesday and another looms on the horizon by
the end of the period.

Operational models are in fairly decent agreement in the depiction
of a trough moving through the area early next week.  A predecessor
vorticity maximum will progress rapidly northeastward through the
Southwest on Sunday and Sunday night.  12Z NAM/GFS/ECMWF all depict
precipitation breaking out in San Bernardino County by early Sunday
afternoon progressing northeastward into southern Nevada and
northwest Arizona through Sunday evening.  The NAM is the most
aggressive in this depiction but looks a little overdone given
relatively weak large-scale ascent and displaced dynamical support
from the stronger trough to the west.  Nevertheless, there is some
evidence of a tropical moisture tap (albeit not impressive), and
operational models do tend to underestimate the intensity of these
systems in the medium range.  For now, bumped up PoPs in the grids
Sunday afternoon/evening for areas south/east of I-15, but kept QPF
quite light.

As the main system moves into the region on Monday, moist advection
increases considerably, with a rapid increase in surface dew points
to 50+ F by Monday morning across eastern San Bernardino, eastern
Lincoln, Clark, and Mohave Counties.  As the larger-scale trough
approaches, temperatures will cool in the midlevels sufficiently to
develop at least marginal instability in these areas.  Included a
slight chance of storms in the grids for Monday.  Not expecting
particularly heavy/widespread precipitation, but do think chances
are fairly decent for scattered light to moderate showers,
especially in Mohave County.  Continued to trend PoPs upward on
Monday.  With increased cloud cover and decreasing thicknesses,
temperatures should cool to near average by Monday.

Thereafter, flow looks to become amplified as downstream ridging
occurs via substantial warm air advection/diabatic heating as a deep
trough develops in the eastern Pacific.  As this occurs, dry
conditions and slightly warmer than average temperatures should
occur as southerly/southwesterly flow becomes reestablished Tuesday
night and Wednesday.

Model agreement is improving regarding the aforementioned eastern
Pacific system as it approaches the West Coast on Thursday but
differs in its trek/timing thereafter, with the GFS breaking out
precipitation in the Sierra/southern Great Basin as early as
Thursday afternoon while the ECMWF is about 24 hours behind and the
CMC 24 hours behind that.  However, regardless of the timing/trek
differences, a major storm appears to be in the works for at least
portions of the West, with substantial contributions from jet streak
dynamics, large-scale lift, and substantial (subtropical) moist
advection. Certainly a system worth watching as the details become
clearer in the coming days.

.FIRE WEATHER...Near-critical fire weather conditions will continue
through early this evening in the Colorado River Valley before
lighter winds take over tonight and tomorrow.  Warming temperatures
and dry conditions will continue through Saturday, but winds should
stay light.  A system will approach the area Sunday and Monday, with
surface moisture and chances for showers and isolated storms

.AVIATION...For McCarran...Light, diurnally-driven flow is expected
for the next 24 hours under a clear sky.

For the rest of southern Nevada...northwest Arizona...and southeast
California...Strong north winds will continue to decrease slowly
through the evening hours near KIFP and KEED, though gusts to 30 kts
are possible through at least sunset.  North winds should continue
tomorrow, but speeds should be considerably lower (10-20 kts).
Outside of the Colorado River Valley, light/diurnally-driven winds
and clear skies are expected.



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