Flash Flood in Arizona, August 22, 2024

Area Forecast Discussion
National Weather Service Flagstaff AZ
1048 AM MST Thu Aug 22 2024

.SYNOPSIS...Very active monsoon conditions will persist through
today as moisture surges northward into the state. After another
round of storms on Friday, the weekend should see a return of
drier conditions across most of northern Arizona.

&&

.UPDATE...Showers have diminished this morning besides for a few,
isolated thunderstorms in southwestern Yavapai County. As cloud
cover continues to clear for the remainder of the morning, expect
scattered showers and thunderstorms to begin to development by
early this afternoon, especially for locations where it was mostly
clear overnight. With this morning's upper air sounding, taken
here at Bellemont, indicating a PWAT value of 0.99" and
saturation from the surface to ~350 mb, atmospheric moisture
remains abundant for today. With decent sfc-6 km shear and CAPE
values, conditions remain unstable as well. Just need some daytime
heating to kick in as clouds continue to clear this morning.
Coverage should become numerous to widespread later this afternoon
with the greatest coverage and chances along and north of the
Mogollon Rim and White Mountains. Prepare for flash flooding,
especially after yesterday's rainfall resulting in saturated
surfaces. A few severe storms are not out of the question as well
with 1.00" hail and strong winds possible. The current forecast is
in good shape with only minor changes to the POPs needed at this
time.

&&

.PREV DISCUSSION /458 AM MST/...Today...Hi-res guidance has a
brief lull in activity for the first part of the morning hours,
before the second round of widespread showers and thunderstorms
develops by the late morning and early afternoon. Remaining debris
cloud from the overnight convection may delay initiation, but
added dynamics from an approaching trough will help to alleviate
this. Nevertheless, the greatest coverage looks to largely be from
the Mogollon Rim and northward with more scattered coverage to
the south over much of Yavapai and Gila counties.

With ample moisture in place, flash flooding will be the primary
threat from these storms. With 0-6 km flow around 15-20 kts, the
residency time of the storms are reduced, outside of training
storms. However, model soundings show good potential very efficient
rainfall rates with a tall skinny CAPE profile and deep warm cloud
layer. Additionally, an already primed environment from prior
rainfall will only increase the risk. Latest HREF PMM guidance hits
at the mainly the Grand Canyon and Glen Canyon areas as the greatest
risk for flash flooding, and to a lesser extent areas to the south
and east along the Mogollon Rim and into portions of Navajo Nation.

Outside of the flash flood threat, a few strong to severe storms
will be possible given the added dynamics from the trough descending
into the Great Basin. Sufficient instability around 1000-2000 J/Kg
combined with modest 0-6 km bulk shear values around 20-30 kts
should help to promote a few stronger storms capable of producing
large hail and damaging winds.

Friday and Saturday...As the trough continues it's approach to
Arizona, moisture is pushed further eastward. Showers and
thunderstorms look to remain, but the greatest coverage will shift
further east each day. Localized flash flooding and a stronger storm
or two will remain a possibility, but certainly not as likely as
today. Cooler temperatures and breezy southwesterly winds are also
expected.

Sunday through Wednesday...For next week, global ensembles continue
indicate a brief downturn in moisture continuing into at least the
start of the week behind the trough. Now for when the moisture
returns, uncertain still remains. High pressure looks to build over
Nevada, which would continue to bring drier air into northern
Arizona and keep precipitation chances at a minimum. The EPS and
CMCE ensembles seem to support this, while the majority of the GEFS
seem to gradually increase moisture through the week (at least
briefly). Nevertheless, as rising heights return to Arizona, warmer
and slightly closer to normal temperatures should return.

&&

.AVIATION...Thursday 22/18Z through Friday 23/18Z...Scattered to
numerous SHRA/TSRA are expected from 19Z-02Z, becoming isolated to
scattered -SHRA thereafter through 06Z. Periods of MVFR/IFR VIS
and CIGS are likely for much of this afternoon and evening. MVFR
CIGS could linger overnight due to scattered BR/FG. Widespread VFR
conditions should return by 15Z with SCT-BKN cloud cover above
8,000 feet AGL. Outside of erratic outflow winds of up to 35 kts
from any storms, anticipate south-southwest winds 10-20 kts,
becoming light southeast winds 5-10 kts after 03Z.

OUTLOOK...Friday 23/18Z through Sunday 25/18Z...Shower and
thunderstorm coverage decreases for Friday and into the weekend
with activity mainly confined to locations along and east of a
KPGA-KFLG-KPAN line. Enhanced southwest winds, 20-30 kts, are
forecast for Friday and Saturday afternoons.

&&

.FIRE WEATHER...Today through Friday...Showers and thunderstorms
will continue this morning and then increase again late morning
through early evening and then decreasing overnight. Heavy rain and
gusty, erratic winds will be the main threats. Otherwise, expect
southerly winds around 5-15 mph. Slight downturn in storm activity
on Friday, but increasing southwest winds with gusts around 20-30
mph.

Saturday through Monday...Scattered showers and thunderstorms
coverage largely confined to eastern Arizona and only isolated
precipitation elsewhere. Southwest winds with 20-30 mph gusts on
Saturday, decreasing to 5-15 mph on Sunday and Monday.

&&

.FGZ WATCHES/WARNINGS/ADVISORIES...

Flood Watch through this evening for AZZ004>007.

&&

PUBLIC...LaGuardia/Humphreys
AVIATION...LaGuardia
FIRE WEATHER...MAS

For Northern Arizona weather information visit
weather.gov/flagstaff

Preliminary Local Storm Report
National Weather Service Flagstaff AZ
903 AM MST Thu Aug 22 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0700 PM     Flash Flood      1 S Carrizo             33.99N 110.29W
08/21/2024                   Gila               AZ   Official NWS Obs

            The CRZA3 gauge reported a rapid 2 foot
            rise.

&&

RJR

Preliminary Local Storm Report
National Weather Service Flagstaff AZ
907 AM MST Thu Aug 22 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1211 PM     Hail             5 W Chino Valley        34.74N 112.53W
08/21/2024  M1.00 Inch       Yavapai            AZ   Public

            Report from mPING: Quarter (1.00 in.).

&&

RJR

Special Weather Statement
National Weather Service Flagstaff AZ
1157 AM MST Thu Aug 22 2024

AZZ006-007-221945-
Coconino Plateau-Grand Canyon Country-
1157 AM MST Thu Aug 22 2024

...A STRONG THUNDERSTORM WILL IMPACT COCONINO COUNTY THROUGH 1245 PM
MST...

At 1154 AM MST, Doppler radar was tracking a strong thunderstorm
near Supai, or 30 miles west of Hermits Rest, moving northeast at 10
mph.

HAZARD...Torrential rain over Hualapai and Havasu Creeks, wind gusts
         up to 40 mph and penny size hail. Flash flooding is
         possible.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor hail damage to vegetation is
         possible.

Locations impacted include...
Hualapai Hilltop, Beaver Falls, Little Navajo Falls, Mooney Falls,
Supai, and Havasupai Reservation.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 3612 11277 3614 11280 3617 11280 3623 11277
      3627 11276 3631 11276 3631 11273 3618 11264
      3615 11268
TIME...MOT...LOC 1854Z 235DEG 10KT 3619 11274

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...40 MPH

McCollum