Flash Flooding and Damaging Winds in Arizona, August 17 to 18, 2023

Area Forecast Discussion
National Weather Service Las Vegas NV
948 PM PDT Thu Aug 17 2023

.SYNOPSIS...Storm coverage will increase in Mohave County on
Friday, with isolated showers and storms in parts of San
Bernardino, Clark, and Lincoln counties. A major surge of
moisture will arrive over the weekend driven by Hurricane Hilary,
which will bring widespread rain and flash flood concerns to the
entire region through early next week.

&&

.UPDATE...Convective activity has diminished this evening but at
930 pm, radar indicated isolated thunderstorm cells from near
Mohave Valley AZ to the Laughlin/Bullhead City area producing
brief heavy rain. Rain gauges and a spotter in the area reported
up to .4 inches of rainfall in 30 minutes. Additional
thunderstorm activity is percolating south of Mohave County in the
Phoenix CWA and these storms are moving/building slowly
northward. If this activity holds together, there could be some
thunder in parts of southern Mohave County later this evening.
Output from a few of the hires models have shower or thunderstorm
flareups in different parts of the area overnight, depending on
the model. Thus, we can't completely rule it out for tonight. For
the most part, any activity should be generally isolated through
early Friday morning followed by a potential ramp up in activity
later in the night to around sunrise.

&&

.PREV DISCUSSION...238 PM PDT Thu Aug 17 2023...

.SHORT TERM...Today and Friday.

There remains abundant monsoonal moisture across the eastern Mojave
and southeastern Great Basin this afternoon, with PWAT values
between 1.00 and 1.50 inches. Surface-based CAPE this afternoon is
meager, generally around 250 J/kg, with 15-20 kts of vertical wind
sheer across eastern Clark and Mohave counties. With limited
instability, convection will largely be confined to the higher
terrain this afternoon. Primary impacts will be cloud-to-ground
lightning and locally heavy rainfall.

Moisture will briefly retreat west-to-east, with the western Mojave
Desert and southwestern Great Basins experiencing mostly dry
conditions today and Friday. However, activity ramps up
substantially for Mohave County on Friday.

A closed area of low pressure will approach the central California
coast, which will interact with the upper-level monsoonal high, 2
resulting in a southerly flow aloft across northwestern Arizona.
This will allow monsoonal moisture to funnel up from northern Mexico
into Sonora, resulting in shower and thunderstorm chances across
Mohave County Friday. A 40-50kt 250 mb jet will enhance vertical
wind shear, which will increase the severity of storms that form.
Due to widespread monsoonal thunderstorm coverage, we have issued a
Flash Flood Watch for Mohave County starting 11am on Friday. While
shower and thunderstorm chances pick up as early as 4am, flash
flooding does not become a widespread concern until later in the
morning, and will last through the evening due to several rounds of
precipitation. Forecast PWAT values will range from 1.25 to 1.75
inches, so storms that form will be heavy rain-makers.

The Flash Flood Watch for Mohave County will continue into early
next week. Details will be highlighted in the next section.

.LONG TERM...Saturday through next Wednesday.

Tropical Storm Hilary has strengthened to Hurricane Hilary. Hilary
is forecast to continue to rapidly intensify over the next 24-48
hours as it strengthens into a Major Hurricane (Category 3 or
higher). Confidence continues to increase with regard to the
forecast track that Hilary will take. That said, minor fluctuations
in the track could result in strong gradients in forecast
precipitation amounts across our forecast area, so we will continue
to refrain from referencing exact rainfall amounts at this time.

[What We Know]
- Precipitation will be highest in southeastern California,
particularly over the higher terrain. This includes the Transverse
Range through San Bernardino County as well as the Panamint Range in
Death Valley National Park. To put "highest" into context... we're
talking about rainfall amounts of potentially 75 to 125% of annual
averages.
- Rain will fall in light-to-moderate rain rates through the weekend
into early next week with numerous bands of precipitation. Breaks
between bands will be limited. Though rain rates in and of
themselves won't be impressive, the consistent rain is what will be
impressive. Soils will quickly saturate, resulting increased flood
concerns and increased flood severity as the event unfolds.
- Locations such as the Sierra Nevada Mountain Range where snowpack
still exists can expect swollen creeks and rivers. Warm rain will
work to melt existing snow, but existing snow and ice will also
prevent rain from soaking into the ground. Where there is snow, rain
will flow. Flooding impacts will be experienced down the eastern
slopes of the Sierra into the Owens Valley.
- Areas vulnerable to rainfall are of particular concern including
Death Valley National Park and the York Fire Burn Scar in the Mojave
National Preserve.

[Our Headlines]
- Flash Flood Watches are in effect for our entire forecast area
throughout this event.
- Mohave County, Arizona [Friday, August 18th at 11am - Monday,
August 21st at 5pm]. This watch begins on Friday due to monsoonal
activity and will continue through Hilary's impacts. We felt
maintaining a longer Flash Flood Watch through both events was less
confusing than canceling one Friday night just to issue another
Saturday morning.
- San Bernardino County, California [Saturday, August 19th at 11am -
Monday, August 21st at 5pm].
- Clark & Southern Nye Counties, Nevada [Saturday, August 19th at
11am - Monday, August 21st at 5pm].
- Inyo County, California and Esmeralda, Central Nye, & Lincoln
Counties, Nevada [Saturday, August 19th at 11pm - Tuesday, August
22nd at 5am].
- The timing of these watches helps to paint the picture of moisture
pushing northward into our forecast area, then northwestward as the
event progresses.

[Temperatures] In addition to widespread rainfall and flash flood
impacts, temperatures will drop substantially this weekend, which
will threaten several daily record "cool" max temperatures across
our forecast area. The forecast high temperature for Las Vegas on
Sunday is 79F. The record coolest maximum temperature for August
20th is 78F, which was set in 1957. According to the NBM, Las Vegas
has a 34 percent chance of tying or breaking this record.

[Rest of Next Week]
Remnant tropical moisture as well as monsoonal moisture will keep
thunderstorm chances in the forecast through the middle of next week,
though confidence in details is low in the extended forecast.

&&

.AVIATION...For Harry Reid...Convective activity to the east of the
terminal will result in persistent east gusts between 15 and 25 kts.
In general, lightning is not expected with vicinity showers, but an
isolated lightning strike cannot be ruled out. Otherwise, winds will
fall into diurnal patterns, with southeasterly winds through the
afternoon and southwesterly winds through sunset with speeds below
10 kts. Gusty south-southeast winds will pick up late Friday
morning, with gusts between 20 and 30 kts. Through the rest of
Friday afternoon, gust speed and direction will be relative to
vicinity thunderstorm activity, with best chances of VCTS between 20
and 01Z.

For the rest of southern Nevada, northwest Arizona and southeast
California...Convective activity to the east of the Las
Vegas Valley will result in persistent east gusts between 15 and 20
kts at KLAS, KVGT, and KHND. KBIH and KDAG will experience afternoon
wind gusts between 20 and 25 kts from the south and the west,
respectively. Winds will fall into diurnal patterns after sunset
with speeds generally below 10 kts. Convection returns to the Mojave
Desert on Friday, with thunderstorm-related gust impacts expected at
the Las Vegas and Colorado River Valley TAF sites.

&&

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

&&

DISCUSSION/AVIATION...Varian

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