Flash Flood in Nevada, September 20, 2023

Area Forecast Discussion
National Weather Service Las Vegas NV
130 PM PDT Wed Sep 20 2023

.SYNOPSIS...An area of low pressure will dig south out of the
Pacific Northwest through Thursday ushering in cooler temperatures
along with stronger winds with widespread gusts of 30 to 40 mph. Any
precipitation associated with this system will be limited to the
northern and central Great Basin. Cooler than normal temperatures
are expected on Friday, with temperatures rebounding back to normal
Sunday through Tuesday.
&&

.SHORT TERM...through Thursday night.

Weak upper disturbance currently located over the deserts of
southern California will continue to track east through the day.
This will help bring increased instability this afternoon across
portions of southern Nevada and northwest Arizona. There is rather
limited moisture to work with, but it's possible for a thunderstorm
or two to develop over the higher terrain this afternoon with the
latest CAM's showing the greatest probability across Clark and
southern Lincoln County. Outside of lightning, the greatest threat
will be outflow winds are the low levels remain rather dry, but a
quick shot of heavy rain isn't out of the question.

A second stronger low pressure will dig south over the next 24 hours
and move into the northern Great Basin. There is still a bit of
uncertainty, but it looks like this system is not forecast to dig as
far south as previous model runs. We will still see increasing winds
across San Bernardino, Clark, Lincoln, and Mohave Counties, but
widespread wind gusts over 40 mph looks unlikely. There will be a
few favored areas that will see gusts over 40 mph, but impacts
should remain minimal. With that said, winds over Lake Mead and
Mohave are expected to see sustained winds around 20-25 mph along
with gusts between 30-40 mph. These winds combined with forecasted
wave heights of 1-3 feet have prompted the issuance of a Lake Wind
Advisory for the Lake Mead National Rec Area for Thursday afternoon.

Along with the winds, a few showers and storms will develop Thursday
afternoon and evening across central Nye and northern Lincoln
Counties. Amounts should generally remain less than a tenth of an
inch of most areas, but areas across northern Lincoln County could
see between 0.10-0.25" by Thursday evening.

Temperatures will be some 3-5 degrees below normal today, falling to
6-10 degrees below normal Thursday.

.LONG TERM...Friday through Wednesday

The position of the incoming upper level low will advect cool
Canadian air into the western United States, with temperatures
bottoming out on Friday, the autumnal equinox. The question becomes
how much lower will temperatures become. The latest NBM has backed
off somewhat, now only giving Las Vegas a 25 percent chance of a
high of 80 degrees on Friday compared to a 42 percent chance 24
hours ago. This may be because the latest model runs do not take the
low as far south into Nevada as previous runs. However, the
probability of Las Vegas having a morning low of 60 degrees has
remained steady at 38 percent. In general, temperatures on Friday
should be around 10 degrees below average across the area.
Temperatures begin to increase over the weekend and early next week
as ridging builds over the area, still remaining near or below
seasonal averages. Another decrease in temperatures appears possible
in the middle of next week as a trough with a low centered over the
Gulf of Alaska begins to push into the West Coast.

&&

.AVIATION...For Harry Reid...Southeasterly winds will swing to the
south-southwest and increase further with 15-20 knot gusts beginning
around mid-afternoon. It is likely that these breezier winds will
wobble between 150 and 210 before finally settling in from the south-
southwest. These southwesterly wind gusts will continue into the
evening hours, dropping off around a few hours after sunset.

Due to low-to-moderate probability of occurrence and low confidence,
vicinity showers and thunderstorms are not currently in the TAF.
However, high resolution models have begun hinting at the potential
for convection in and around the Las Vegas valley this afternoon.
There is currently a 15-30% chance for vicinity precipitation from
19Z to 04Z. Of the high resolution models that have convection
occurring during that window, all but one of them have lightning
occurring with said convection. Variable gusty outflow winds will be
possible with any convection that develops.

For the rest of southern Nevada, northwest Arizona and southeast
California...VGT and HND will see light southeasterly winds
transition to a more south-southwesterly direction during the
afternoon hours, with 15-20 knot southwesterly gusts expected at
HND. TAFs for VGT and HND do not have mention of vicinity showers
and thunderstorms. However, there is a 15-30% chance for afternoon
convection capable of producing variable gusty outflow winds from
19Z to 04Z. Winds at BIH will remain relatively light, 10 knots or
less, and will follow typical diurnal directional trends. DAG will
favor a westerly direction with 20-25 knot wind gusts picking up
around mid-morning and continuing through the afternoon. IFP and EED
will see winds swing to the south, with 15-20 knot gusts beginning
around mid-to-late morning and continuing through the afternoon.
&&

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

UPDATE...Gorelow
SHORT TERM...Pierce
LONG TERM...Meltzer
AVIATION...Stessman

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