Flash Flood and Dust Storm in Arizona, August 13 to 15, 2023

Area Forecast Discussion
National Weather Service Las Vegas NV
105 PM PDT Sun Aug 13 2023

.SYNOPSIS...Scattered thunderstorms are expected through this
evening as a low pressure system sits over southern California.
Drier conditions return tomorrow but will be short-lived as
monsoonal moisture will spill back into the eastern Mojave Desert
and eastern Great Basin Tuesday through the remainder of the week
with daily thunderstorm chances, especially in the higher terrain.

&&

.SHORT TERM...through Monday night.

Convection through early afternoon has begun to unfold about as
expected, with the greatest coverage focused across central Mohave
county into far southern Clark and northern San Bernardino
Counties. This particular zone is under an area of maximized
upper level divergence allowing for stronger ascent within a
marginally unstable environment of 500-1000 J/KG of CAPE. While
the instability is not particularly impressive, steep lapse rates
with H500 temps of -11C is promoting some pulse severe hail
signatures on radar and we have had a report of some dime sized
hail near Hualapai Mountain Park late this morning.

Expecting storms to persist in this general zone through early
evening and perhaps expand into the adjacent deserts through the
late afternoon on boundary collisions. Late afternoon and early
evening convection will likely shift into central and western San
Bernardino county later this evening as the best forcing shifts
southwestward. Main threats through this evening will be sporadic
strong updrafts capable of hail up to penny size, along with
strong wind gusts. Slow movement of storms and potential for
training also poses some potential for isolated flash flooding.

Outside of the storm risks, temperatures today will remain
slightly cooler than normal thanks to the cool thermal profile in
the vicinity of the nearby upper level low. The low will begin to
retrograde westward tomorrow and also advect some drier and more
stable air in from the south. This will tend to limit convective
coverage tomorrow to exclusively far eastern Mohave county, as
well as allow temperatures to climb closer to daily averages.

.LONG TERM...Tuesday through next Monday.

High pressure will build over the Four Corners states much of next
week allowing for a slow climb of temperatures to slightly above
normal along with an open door to increasing monsoonal moisture.
This will allow for daily chances of thunderstorm activity to
return Tuesday onward, particularly for the higher terrain of the
eastern Mojave Desert, Northwest Arizona, and Lincoln County. Day
to day variations in storm coverage and intensity can be expected,
but at least low-grade monsoon activity is anticipated each day.

Confidence in the forecast details decreases considerably by the
weekend onward. This is due to tropical activity and uncertainty
in the placement of the Monsoonal High versus an offshore low
pressure system. The way these features come together leads to an
assortment of possible weather outcomes, ranging from a return of
dry conditions within a southwesterly flow, or a very wet pattern
emerging as tropical moisture gets directed into the Southwestern
US. The NBM PoPs seem to have latched onto some of these wetter
outcomes with widespread PoPs of 30-60% advertised Sunday and
Monday across our eastern zones and a sharp cooling trend,
however, drier solutions remain possible as the ECMWF ensemble
envelope remains on the drier side of things.

&&

.AVIATION...For Harry Reid...Outside of any thunderstorm
influence, look for light to occasional northeast winds through the
afternoon. Speeds are generally expected to remain around 8 kts or
less. However, there will be thunderstorms developing around the Las
Vegas Valley through the afternoon and early evening that could
bring gusty and erratic outflow winds to the area. Although
unlikely, it is possible that a thunderstorm does develop in the
valley, but overall most of the storms will remain out of the
valley. Storm activity will wane this evening with a notable
decrease in storm coverage Monday. Look for light northeast to east
winds along with a FEW clouds around 12k feet.

For the rest of southern Nevada, northwest Arizona and southeast
California...Scattered showers and thunderstorms will develop across
much of the region this afternoon with the greatest likelihood of
impacts near KIFP, KEED, KDAG, and the Las Vegas Valley. Most of
these impact's will be in the form of gusty thunderstorm outflow
winds, but it's possible that a storm could directly form over and
near these TAF sites into early evening. Outside of any thunderstorm
influence, most areas will maintain a diurnal wind pattern.
Thunderstorm coverage on Monday is not expected and winds will
generally be less than 10 kts over most areas.
&&

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

DISCUSSION...Outler
AVIATION...Gorelow

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