487
FXUS65 KVEF 110828
AFDVEF

Area Forecast Discussion
National Weather Service Las Vegas NV
128 AM PDT Wed Sep 11 2024

.SYNOPSIS...Breezy to locally windy conditions are expected today,
leading to conditions favorable for fire spread. Winds will decrease
late this evening, except in the Barstow area. Temperatures will
also drop several degrees today and Thursday. Thunderstorm chances
could return to portions of the area Sunday and Monday, and further
cooling is possible next week.
&&

.SHORT TERM...through Thursday night. Early morning satellite loop
showed a slug of mid and high level clouds rolling east across
Lincoln, Clark, and Mohave counties. A large pall of smoke from
multiple large wildfires in California blanketed much of San
Bernardino and Clark counties and was drifting east. Surface obs
showed increased southwest winds compared to yesterday, similar
temperatures, and dewpoints a few degrees lower. The main feature of
concern today will be low pressure coming onshore in the Pacific
Northwest. This will increase both surface pressure gradients and
upper level thermal support for winds this afternoon. The Red Flag
Warnings look good and will remain in place, as even where winds
fall just short, the relative humidity will be extremely low. Winds
will also be a concern in western Inyo and San Bernardino counties.
Conditions will be favorable for downslope winds on the eastern
slopes of the Sierra to push across the Owens Valley floor to
Highway 395 for a few hours this afternoon. The threat will be quite
limited in both space and time, so rather than issuing a headline,
will address it directly with partner agencies. Farther south,
westerly winds accelerating through the Tehachapi Gap will affect
the Barstow area this afternoon through tonight. Since this threat
is of a longer duration, hoisted a Wind Advisory. As the parent low
tracks east across the intermountain West tonight, the associated
cold front will push south through the southern Great Basin and much
of the Mojave Desert. Since the low is tracking east rather than
south, the post-frontal winds will be weaker than the pre-frontal
winds, and any stronger northerly winds should be limited to areas
of favorably oriented terrain. Temperatures will come down a few
degrees today and several more degrees tomorrow. The question still
remains: will KLAS reach 100F today and break the record for most
100F days in a year? NBM probabilities have been steady at 27-28%
over the past 24 hours, suggesting that it is unlikely but not out
of the question. The thick smoke in the sky could tilt the odds even
more against it happening.

.LONG TERM...Friday through Tuesday.

The long term forecast features below-average temperatures across
the region as back-to-back troughs of low pressure work to reinforce
a fall-like weather pattern heading into next week.

The midweek trough will finally push northeastward out of the Desert
Southwest through the day on Friday. Pressure gradients will ease as
a zonal flow sets up aloft, which will decrease wind speeds across
the forecast area. Temperatures will initially begin to rebound back
toward seasonal normals; however, both the ECMWF and GEFS ensemble
means indicate a shortwave passage through the zonal flow on
Saturday will help keep temperatures 3-5 degrees cooler than
seasonal averages.

The interesting / least confident part of the forecast comes late-
weekend into the start of next week. There continues to be an
indication among ensemble members that tropical moisture will push
northward up the Gulf of California over the weekend. There are a
lot of variables to consider before deciding if this will have any
direct impact on the forecast area, as models remain uncertain how
organized this moisture will become as it pushes northward as well
as what exact track it will take. That said, the NHC has formally
noted this tropical disturbance off the western coast of Mexico by
Colima with a 40% chance of cyclone formation in the next 48 hours.
Both the ECMWF ensemble and GEFS means show an associated increase
in PWATs up the Colorado River Valley on Sunday and Monday, with the
NBM subsequently increasing PoPs over Mohave County Sunday and the
southern Great Basin and eastern Mojave Desert on Monday. That said,
the ECMWF ensemble mean is not as enthused about bringing moisture
as far north as Las Vegas as it was 24 hours ago. Timing will be key
in this scenario as an additional trough will usher dry air into the
southwestern United States late Monday into early Tuesday.
&&

.AVIATION...For Harry Reid...Southwest winds prevail through most of
the TAF period. Speeds strongest from late morning through the
evening hours with sustained winds of 15-20 knots and peak gusts of
30-35 knots. Up until ~16z this morning and after ~08z tonight,
speeds are expected to be in the 10-15 knot range with periods of
gusts around 20 knots. Late in the TAF period (Thursday morning),
winds shift to the north-northwest with gusts 15-20 knots. Skies
will be mostly free of clouds, but a persistent smoke layer is
likely to be present over or just south of Las Vegas. However, there
is only a ~10% chance that this smoke reduces surface visibility
below 6SM.

For the rest of southern Nevada, northwest Arizona and southeast
California...Gusty winds anticipated across the area today with
direction at most TAF sites ranging from southerly to westerly. The
exception to this looks to be KBIH, where gusty winds are likely to
be from the west or northwest this afternoon and evening. In terms
of wind speeds, guidance shows 10-20 knots at most sites (locally
stronger at KDAG) with peak gusts in the 25-35 knot range. Skies
will be mostly free of clouds, but a persistent smoke layer is
likely to be present over areas along/south of I-15. However, there
is only a ~10% chance that this smoke reduces surface visibility
below 6SM.
&&

.FIRE WEATHER...Winds will pick up this morning, with gusts around
35 to 40 mph expected in Lincoln, Clark, and Mohave counties, where
Red Flag Warnings are in effect from late this morning through this
evening. Winds will relax later this evening, lessening the threat.
&&

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

$$

SHORT TERM...Morgan
LONG TERM...Soulat
AVIATION...Woods
FIRE WEATHER...Morgan

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