Area Forecast Discussion
Issued by NWS Las Vegas, NV

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FXUS65 KVEF 230259

Area Forecast Discussion
National Weather Service Las Vegas NV
800 PM PDT SAT OCT 22 2016

.SYNOPSIS...Breezy to windy conditions, increasing cloudiness, and
chances for showers and isolated storms will occur across the area
tomorrow and Monday as a storm system moves through the region.
Temperatures will cool to near average.  A period of tranquil
weather is expected by the middle of next week before another
potentially more significant storm affects portions of the area
late next week or next weekend.

.UPDATE...Cloud cover encroaching on the area will allow
temperatures to say well above normal overnight.  This is reflected
in the going forecast and no changes are deemed necessary.

.SHORT TERM...through Monday night.

Complex short term forecast today, with main forecast concerns
associated with marginally windy conditions in the northwest CWA
tomorrow and considerable model disagreement associated with
timing/placement of scattered showers and isolated storms affecting
most of the region tomorrow afternoon through Monday night.

Currently, winds are beginning to increase in the southern Great
Basin and the lee side of the higher terrain in Inyo and Clark
Counties.  Winds will continue to pick up tonight in the southern
Sierra, White Mountains, and Spring Mountains but should remain
below advisory criteria.  As a deep system off the Pacific Northwest
Coast continues to approach the area tomorrow, winds will continue
to increase, with Esmeralda and Nye Counties likely to exceed wind
advisory criteria much of the afternoon.  Inyo County is a bigger
question mark.  Currently, aside from the higher terrain, high-
resolution models are not aggressive in developing widespread gusty
winds.  Given the variable terrain of Inyo County, always a tougher
call in figuring out what the winds will do.  Certainly think wind-
prone desert/valley locations (particularly in Owens Valley) will
see strong gusts, so I see no reason to remove the advisory here.

Meanwhile, the models are really struggling regarding the details of
a subtle predecessor vort lobe developing in southern California
tomorrow and quickly moving northeastward to Utah by tomorrow night.
Given the small nature of the perturbation and the relatively weak
larger-scale dynamical processes during its trek across the region,
predictability is likely to be below average right up to the event.
Nevertheless, some key trends exist in today`s operational
simulations.  The first is that there is a clear shift westward with
the precipitation on Sunday.  This is likely owing to a deeper
depiction of the strong low west of the Pacific Northwest Coast. The
second is that there is improved agreement that the precipitation
will increase in intensity in a relatively narrow NE-SW oriented
band on Sunday night.  The exact placement of this band is where the
discrepancies exist, with the 12Z ECMWF farthest west (eastern
Lincoln to western San Bernardino County) and the 12Z NAM farthest
east (mainly southeast of I-15).  GFS/CMC show reasonable agreement
and represent the consensus more or less at this point, so placed
the highest PoPs in the I-15 corridor Sunday night through Monday
morning with a gradual progression eastward through Monday evening.
Highest PoPs remain in northern Mohave and eastern Lincoln Counties,
where improving jet dynamics and localized orographic lift should
generate more widespread precipitation.

Models are also in disagreement regarding intensity of
precipitation, with the general trend toward heavier amounts
(particularly Monday) but considerable remaining disagreement. The
GFS/NAM are on the dry side, with the ECMWF/CMC on the wet side. The
trend is for heavier, and with the left front quadrant jet dynamics
in play given the favorable positioning of the 250-mb jet streak
Monday as well as a thermodynamic environment supportive of isolated
convection, felt the need to raise QPF a bit.  Right now, heaviest
amounts (0.4-1.0 inches) lie in eastern Lincoln and northern Mohave
Counties, with 0.1-0.4 inches generally east of a Twentynine Palms
to Rachel line (locally higher amounts in the higher terrain).
Confidence is below average given unusually large model
discrepancies so close to the event.

Chances for storms exist on Monday, with models trending a bit
stronger both with the vertical wind profile and with thermodynamic
instability.  As such, still think small hail is possible with
sustained updrafts, particularly in Mohave County.

Temp forecast on Sunday/Monday has substantial bust potential, with
impacts associated with cloud cover/precipitation not entirely clear
at this point.  Did not change much from inherited forecast, but
thinking is that guidance is a little on the warm side for
highs and the cold side for lows given the expected cloud cover.

System begins to move out Monday night, with PoPs confined to
Lincoln and Mohave Counties after midnight.

.LONG TERM...Tuesday through Saturday.

Extended forecast models are in good agreement Tuesday and Wednesday
as high pressure over the Desert Southwest gets shoved east by an
area of low pressure off the California coast. The result will be
continued warm temperatures with locally breezy conditions each
afternoon. Models really begin to differ starting Thursday and
continuing into the weekend. The GFS and Canadian models are in
fairly decent agreement with each other while the ECMWF is a
complete outlier. The ECMWF brings a closed low into northern
California Thursday afternoon while spreading precipitation into
western Nevada and the southern Sierra. Both the GFS/Canadian don`t
even have precipitation reaching the California coast until
overnight Thursday and the GFS really doesn`t bring it onshore until
Friday morning due to the high over TX/NM remaining fairly strong.
All the models have basically kept precipitation generally over the
northern and central portions of the CWA with only a quick shot
across the south. Due to the drastic changes in the newer model runs
will make little overall changes to the grids which has at least a
chance of showers over most of the area and wait for more
consistency. With the system potentially remaining further north
this would keep temperatures over the southern half of the area a
bit warmer along with locally breezy conditions. Most of the area
should be dry Saturday and Sunday, but left in a few pops based on
the forecast uncertainty.

.FIRE WEATHER...Upgraded fire weather watch for southern Nye County
to a red flag warning for tomorrow as winds and RH will easily meet
criteria.  In addition, added all of Inyo County to the red flag
warning as well, as at least localized critical fire weather
conditions are expected in the deserts/valleys with more widespread
critical fire weather conditions expected in the higher terrain.
Continued the red flag warning for the Spring Mountains, despite the
marginal environment.  Though winds are not reaching critical
thresholds near/below 7000 feet at this time, observations near the
October fire this week have suggested strong upslope channel flow
during the afternoon (which is often not sampled by permanent
surface obs). In addition, proximity observations from ridgetops in
southern Nevada and northwest Arizona suggest stronger winds are
likely at the higher elevations.  With critical fuels and recent
fire starts in the Springs, feel the red flag warning is warranted.
Conditions may actually worsen this evening as stronger winds kick
in near/after sunset with poor moisture recovery expected.  However,
moisture will be on the increase in the Spring Mountains tomorrow
afternoon and farther west tomorrow evening, which should conclude
the critical fire weather concerns across the region.  A decent
chance of precipitation exists for much of the area Sunday night and

.AVIATION...For McCarran...No significant weather is expected
through tonight. Winds will be light and follow typical diurnal
trends. Clouds will thicken and lower overnight with cigs down to
10K feet by late Sunday morning. Shower chances will also increase
during the afternoon.

For the rest of southern Nevada...northwest Arizona...and southeast
California...Generally light winds overnight. South winds are
expected to increase over most areas Sunday afternoon especially
across Inyo, Esmeralda and Nye Counties. Wind gusts up to 30 mph
possible. Clouds will lower and thicken overnight and early Sunday
with cigs down to around 10k feet by Sunday afternoon. Shower
chances will be highest Sunday afternoon over Mohave, Clark, and San
Bernardino Counties.

.SPOTTER INFORMATION STATEMENT...Chances for rain and perhaps
isolated storms will increase on Sunday and Monday. Spotters should
report significant weather according to standard operating


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

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