Area Forecast Discussion
Issued by NWS Las Vegas, NV

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FXUS65 KVEF 212220 AAA

Area Forecast Discussion
National Weather Service Las Vegas NV
320 PM PDT FRI OCT 21 2016

.SYNOPSIS...The weekend will begin with a warm and increasingly
breezy Saturday before a storm system moves into the region Sunday
and Monday.  Continued breezy conditions and scattered showers and
isolated storms are likely as the system passes through.  A period
of tranquil weather will resume for the middle of next week before a
potentially significant storm affects portions of the area by the
end of the work week.

.SHORT TERM...through Monday night.

A nice day is in progress across the area with light winds, clear
skies, and slightly above average temperatures.  An upper-level
ridge is retreating quickly into the southern United States today,
allowing for deep southwesterly flow to become established in our
area.  This flow will increase as a shortwave trough approaches the
California Coast Sunday and Sunday night and a deep low edges toward
the Pacific Northwest.  The approach of these systems will begin to
increase the surface winds in the southern Great Basin and the
higher terrain of southeast California and southern Nevada this
weekend.  Residual dry air in place will create another period of
critical fire weather conditions in portions of southern Nevada this
weekend (see the fire section for details) before stout moist
advection occurs as the aforementioned shortwave trough approaches
the area Sunday afternoon/night.  Currently, wind speeds do not look
to exceed advisory criteria, but the southern Great Basin is
particularly susceptible to south wind regimes, so this will
continue to be monitored.  The increase in surface winds and dry
boundary layer will allow for very warm daytime temperatures, with
forecast highs around 7-12 degrees above climatology on Saturday.
However, increased cloud cover on Sunday should result in
temperatures about 3-8 degrees cooler than Saturday`s values.

The system that affects the area late this weekend into early next
week looks to have two distinct phases.  The first involves a
predecessor vort lobe that progresses rapidly northeastward from
just off the coast of Baja Saturday night to Utah by Sunday night.
Scattered showers will develop in association with modest large-
scale lift this system produces.  Instability looks marginal to
negligible with this first system, so kept thunder out of the
forecast for now.  Nevertheless, this system will act to increase
near-surface moisture rapidly Sunday afternoon/night in Lincoln,
Clark, much of San Bernardino, and Mohave Counties, with forecast
soundings showing 20+ F dew point increases in 1-3 hours.  With
surface dew points nearing/exceeding 50 F by Sunday night and
Monday, sufficient moisture will be in place in advance of the
stronger shortwave trough moving through the area Monday.  With
stronger large-scale lift owing to improving jet dynamics and
stronger differential cyclonic vorticity advection, more widespread
showers and isolated storms are expected on Monday.  Cold midlevel
air overspreading the region will combine with the increased near-
surface moisture to generate a large region of marginal instability
(generally east of a Rachel to Twentynine Palms line), which should
be sufficient to produce isolated storms.  Combined with increased
cloud-layer shear as the system approaches on Monday, thermodynamic
profiles are favorable for small hail wherever updrafts can become
sustained, given the sufficiently low wet bulb zero heights and
modestly deep (albeit thin) CAPE.

Operational models are clearly suggesting banded precipitation
(though considerable disagreement remains on where these bands
develop).  At the very least, this suggest mesoscale lift may be
sufficient to produce locally moderate precipitation amounts. Bumped
QPF up with this afternoon`s package.  Thinking the heaviest amounts
will be located in eastern Lincoln and northern Mohave Counties and
in the higher terrain of Clark and San Bernardino Counties (0.15-
0.75 inches) with lower amounts elsewhere.  Locally higher amounts
are possible with isolated storms.  Most precipitation should exit
to the east of the area by late Monday night as the next
progressive ridge moves in.

.LONG TERM...Tuesday through Saturday.

High pressure will remain over the area Tuesday and Wednesday
providing above normal temperatures along with generally light winds
over the region. A deepening area of low pressure off the California
coast will begin to shift east Wednesday and Thursday helping to
push the ridge east. This will bring an increasing southwest flow to
the area each afternoon with most areas seeing breezy to locally
windy conditions. Temperatures will also be several degrees above
normal with the increase in the southwest flow. Moisture is expected
to increase significantly by Thursday night through Saturday as
ample Pacific moisture begins to work toward the area. Models are
showing PW values between 1.75-2.00 inches reaching southern
California with the 1.00 inch line reaching as far north as southern
Lincoln County. With the upper low pushing into central California
by Friday afternoon much of our area will be in a favorable position
to see widespread precipitation Friday and Saturday. The Sierra
could see high precipitation totals beginning Thursday night and
continuing through Friday night with snow levels expected to be
around 9-10k feet. Widespread precipitation could potentially spread
across the remainder of the area Friday and Saturday and have gone
ahead and increased pops quite a bit. Precipitation should mainly be
stratiform and will not include any mention of thunderstorms. Things
should quickly dry out late Saturday afternoon as the low moves
quickly east as weak high pressure moves in.

.FIRE WEATHER...With the approach of a storm system late this
weekend, winds will increase in advance of it across southern
Nevada.  Antecedent dry air will promote locally critical fire
weather conditions in the Spring Mountains Saturday afternoon
through Sunday morning.  However, moisture is expected to spread
rapidly northward into the Spring Mountains by Sunday afternoon.
Issued a red flag warning for this area from 11 am Saturday to 2 pm
Sunday, though the most likely time critical fire weather conditions
will occur is Saturday afternoon.  With winds expected to stay up
during the night and little moisture recovery expected, decided to
keep the warning through early Sunday afternoon despite its more
marginal nature by this time.

Meanwhile, winds are expected to reach critical fire weather
thresholds on Sunday across the southern Great Basin.  Little to no
near-surface moisture advection will reach Nye County, so hoisted a
fire weather watch during the day Sunday.  Lower confidence in
critical fire weather conditions affecting Inyo County, and fuels
are not critical in Esmeralda County.  Nevertheless, there is an
elevated risk of rapid fire spread in these areas on Sunday as well.

.AVIATION...For McCarran...No significant weather is expected
through Saturday. Winds will be light and follow typical diurnal
trends. The next storm system will move through Sunday and Monday
bringing enhanced southwest winds and chances of rain/lower CIGS to
the area.

For the rest of southern Nevada...northwest Arizona...and southeast
California...Generally light winds through tonight. South winds will
increase slightly on Saturday. The next storm system will bring
further increases in wind speeds as well as chances for rain and
lower CIGS to portions of the area Sunday and Monday.

.SPOTTER INFORMATION STATEMENT...Spotter activation will not be
needed through Saturday. Chances for rain and perhaps isolated
storms will increase on Sunday and Monday. Spotters should report
significant weather according to standard operating procedures.


Short Term/Fire Weather:  Shafer
Long Term/Aviation:  Gorelow

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