Weather Service Report

FXUS65 KVEF 230231 AAA

Area Forecast Discussion
National Weather Service Las Vegas NV
740 PM PDT Wed Sep 22 2021

.SYNOPSIS...Above normal temperatures will continue through the 
weekend and into early next week. An area of low pressure is 
forecast to set up south of the Mojave Desert late this week, which 
could lead to spotty showers and thunderstorms southeast of 
Interstate 15 as early as Thursday, with better chances on Sunday.

.UPDATE...Thick smoke from the Windy and KNP Fires in the southern 
Sierra will continue to make its way east tonight through Thursday 
morning bringing areas of reduced visibilities and likely air 
quality issues. Some areas in Inyo, Nye and, northern San Bernardino 
Counties could see visibilities reduced to less than 2 miles at 
times, however smoky conditions will be seen across much of southern 
Nevada and northwest Arizona. Made some minor adjustments to the 
forecast as the latest smoke data arrived.  

105 PM PDT Wed Sep 22 2021

.DISCUSSION...Midday water vapor loop showed mid level trough axis 
over California, with only a thin band of high clouds to announce 
its arrival. Skies were clear in our CWA, with smoke slowly 
advecting from Inyo County into central Nye and southern Esmeralda. 
The behavior of the smoke will be the most noticeable effect of the 
wind shift associated with the incoming trough. As westerly winds 
increase this afternoon and evening, smoke plumes from the fires in 
central California will be pushed over the Sierra and across the 
Mojave Desert, reaching the sky over Las Vegas early this evening. 
HRRR-Smoke runs suggest heavy smoke concentrations aloft over Clark 
County overnight into Thursday morning, with the worst conditions 
slowly shifting southward across San Bernardino County Thursday 
afternoon and evening as winds become more northerly behind the 
trough. The next item of concern will be precip chances. There has 
been, and still is, general model agreement that the aforementioned 
trough will split, with the northern piece continuing east along the 
Canadian border, and the southern piece forming a cutoff low. 
However, models often have a hard time with the details of a trough 
split, and this one is no exception. Different perturbations show 
different placements and strengths of the cutoff low, and small 
differences in its track and timing will make big differences in 
whether and to what degree rain chances will make it into the Mojave 
Desert. Thus, there is a lot of uncertainty in the forecast details, 
and confidence is below average. A few models show some weak upslope 
showers developing over the Morongo Basin in the northerly flow as 
early as tomorrow afternoon. By Friday afternoon, when the cutoff 
low should have formed somewhere in the vicinity of the CA/AZ/MX 
triple point, weak easterly flow and lift in its northern semicircle 
could lead to spotty showers and storms breaking out southeast of 
Interstate 15. Over the weekend, model spread increases and 
confidence decreases, but the best estimate is that chances for 
showers and thunderstorms should remain mostly southeast of I-15, 
with a few exceptions possible near Mountain Pass and Mount 
Charleston where upslope flow is maximized. Monday looks like the 
most likely dry day (although confidence remains low) under 
shortwave ridging as the cutoff low finally kicks east, with the 
next trough possibly approaching Tuesday. Confidence has increased 
in one area at least, as the probability of Las Vegas reaching 100 
degrees on any of the weekend days has been steadily decreasing with 
each successive model run for a couple of days. Thus, barring a 
sudden change, the number of official 100 degree days at KLAS will 
be 87 this year, down from 97 in 2020, and well above the average of 
69.7 based on 1991-2020 data.

.AVIATION...For McCarran...Light winds are expected at the terminal, 
with easterly components during the afternoons becoming 
southwesterly at night. Smoke from California wildfires will begin 
moving into the area late this afternoon which is not expected to 
have any impacts on surface visibility, but may result in reduced 
slant range visibility at times. Outside of the hazy skies, no 
impactful weather is expected at or near the terminal.

For the rest of southern Nevada, northwest Arizona and southeast 
California...Winds under 20 knots are expected for the vast majority 
of the region through Thursday. Smoke could cause surface visibility 
reduction in Inyo County today, spreading to San Bernardino and 
Clark counties Thursday, with slant range visibility reduction 
possible areawide. There is an outside chance of showers developing 
in the Morongo Basin Thursday afternoon, with erratic, gusty winds, 
sudden visibility reduction, and terrain obscuration being the main 

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



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