Flash Flooding and Flooding in Arizona, August 2, 2016

Name: Flash Flooding and Flooding in Arizona, August 2, 2016
Start: 2016-08-02T16:00
End: 2016-08-02T23:15
Location: Arizona

August 2, 2016 · NWS PSR · 6 events in 2 counties and zones

Where it happened

Every event with a place to put it. Tap one for its entry below.

Tornadoes
Tropical storms and surge
Floods
Wind
Hail
Winter weather
Heat and drought
Cold
Fire, smoke and dust
Marine and coastal
Other

Open the live radar over this ground

The toll

Deaths: 0
Injuries: 0
Property damage: $110,000
Crop damage: None recorded
Events: 6

Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.

What happened

The episode as NWS PSR wrote it up.

Scattered to numerous showers and thunderstorms overspread much of south central Arizona during the afternoon and evening hours on August 2nd. Due to very moist and unstable conditions, the stronger thunderstorms that formed produced a variety of significant and severe weather, including damaging winds, large hail, very heavy rain, flooding and flash flooding. Public reports along with mesonet station readings indicated rainfall totals between three and four inches; the heavy rain led to road closures, flooding of underpasses and necessitated swift water rescues near the community of Cave Creek. Multiple Urban and Small Stream Advisories as well as Flood and Flash Flood Warnings were issued during the afternoon and evening hours. During the late afternoon, gusty thunderstorm outflow winds downed several power poles near Fountain Hills, and large hail was reported in central Phoenix.

What forecasters said that day

The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.

NWS Phoenix, AZ wrote its last area forecast discussion before the first event at Aug 2, 2:10 PM MST, 1 hour 50 minutes earlier.

Area forecast discussion, Aug 2, 2:10 PM MST, 1 hour 50 minutes before

Area Forecast Discussion
National Weather Service Phoenix AZ
210 PM MST TUE AUG 2 2016

.SYNOPSIS...
An excessively humid airmass will continue to cover the region the
next couple days, with the best chance of showers and thunderstorms
over Arizona. While some areas may not see rain, locally heavy
rainfall is possible in some locations. A drier and more stable
airmass will begin to spread into the area during the end of the
week, pushing thunderstorm activity into eastern Arizona. An even
drier and warmer airmass will then cover the region over the weekend
and into early next week with very limited thunderstorm activity.

&&

.DISCUSSION...
An unusually moist airmass continues to blanket virtually all of
southern and central Arizona this morning with 12Z KPSR and KTWC
soundings sampling near sfc mixing ratios at least 15 g/kg and mean
sfc-H7 mean mixing ratios at least 13 g/kg. Throughout the total
atmospheric column, PWATs measured at 2.0 inches are easily a record
for the date, and some of the highest historical values ever
recorded at these stations. Sounding data showed nearly moist
adiabatic lapse rates; and given the moist environmental profiles,
its not surprising that scattered showers/storms continued to
percolate throughout the region even during non-traditional times.

Meanwhile, a weak H7-H5 vorticty center earlier over Pima County
was becoming more diffuse and possibly lifting into the White
Mountains. At the same time, a more well defined inverted trough
over the northern Gulf of California leading to general difluence
aloft amid extremely weak tropospheric wind profiles. Moderate CAPE
in the 0-3km layer was still observed in objective analysis with
excessive moisture profiles below the freezing level yielding
efficient rain producers. Thus while cloud cover has largely limited
growing instability over parts of central Arizona, inhibition was
minor to begin with and areas to the west of the thickest cloud
coverage (i.e. a line roughly from Gila Bend-Buckeye-Wickenburg)
would conceptually stand the most likely chance to experience rapid
convective growth this afternoon with locally heavy rainfall.

Much like early this morning, convective activity may continue to
pulse through the overnight hours into Wednesday as any small
perturbation or weak boundary would easily be capable of releasing
modest instability. Overall coverage tonight should generally wane
with little large scale forcing mechanisms, but confidence in any
given model output is quite low. HRRR had earlier depicted the most
reasonable outcomes, though recent iterations have a surprisingly
lack of convection given the current observational trends. High
resolution NMM cores seem to be overly aggressive with convection,
and possibly the 00Z NSSL ARW core shows slightly better realism.

Overall, a blend of SREF and high resolution ensemble output was
utilized to depict rainfall chances through Wednesday afternoon with
most guidance packages suggesting convective activity less robust
over lower elevations. However, given the very high moisture content
currently over the forecast area, there is serious reservation on
how rapidly this could be scoured out. Numerical guidance seems to
identify this fact while raw model output is more pessimistic about
organized activity. Have blended these guidance packages to achieve
a moderately confident forecast which doesn't completely discount
storm chances through Thursday.

Friday through Tuesday: A warmer and drier airmass will continue to
spread across the forecast area as deeper westerly flow envelops the
SW Conus. Afternoon temperatures at lower elevations will gradually
warm into 108 to 110 degree range, though some uncertainty exists
given how rapidly low level moisture advects eastward. Only a slight
chance of afternoon and evening showers and thunderstorms are
forecast over the mountains of southern Gila County zone 24 near the
higher elevations of the White Mountains.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL:
Abundant monsoon moisture will keep showers and isolated
thunderstorms near Phoenix terminals through the period, with the
highest coverage in the western valley, especially over the White
Tank mountains. This moist atmosphere will also aid in keeping
clouds aoa 8kft in place. In addition, this abundant cloud cover
will allow easterly winds to take dominance through the entire
period, with a more southerly component between 21Z and 02Z.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds will remain primarily out of the south-southwest at both TAF
sites with ceilings aoa 10kft. With a very moist atmosphere in place
showers and thunderstorms, especially over the KBLH airport, cannot
be ruled out this afternoon into tomorrow morning.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Friday through Tuesday...
A drying trend will commence with minimum humidities on Friday
around 10 to 20 percent, slowing dropping into the 8 to 15 percent
range by Tuesday. Light winds and a very moist environment in place
prior to this period will really aid in keeping the fire threat low
Friday through Tuesday.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation is not likely.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.

&&

Visit us on Facebook...Twitter...and at weather.gov/phoenix

DISCUSSION...MO/Vasquez
AVIATION...Hernandez
FIRE WEATHER...Hernandez

The radar

The Phoenix radar (KIWA), 68 km away, was scanning. The volume nearest the first event began at 22:57 UTC.

Open the radar archive at this moment

Event by event

2 flash flood, 2 flood, 1 hail, 1 thunderstorm wind. Local standard time, as filed.

Flash Flood, Maricopa

When: August 2, 4:00 PM MST to 7:15 PM MST
Where: Maricopa County, 7.76 mi WSW of Cave Creek to 7.02 mi WSW of Cave Creek
Cause: Heavy Rain
Property damage: $30,000
Reported by: Broadcast Media
Coordinates: 33.793, -112.077

Strong thunderstorms developed across the northeastern portion of the greater Phoenix metropolitan area during the afternoon hours on August 2nd and they affected communities such as Carefree, Cave Creek and Paradise Valley. The storms produced intense rainfall with peak rain rates in excess of one inch per hour; this resulted in flash flooding near Cave Creek. According to a report from local broadcast media, at 1729MST flash flooding 6 miles southwest of Cave Creek necessitated a swift water rescue for a vehicle stuck in water. The rescue was trapped in the vicinity of East Carefree Highway and North 3rd Street.

Flash Flood, Maricopa

When: August 2, 4:45 PM MST to 8:15 PM MST
Where: Maricopa County, 4.18 mi ENE of Glendale Muni Arpt to 0.59 mi NNW of Paradise Vly
Cause: Heavy Rain
Property damage: $50,000
Reported by: Public
Coordinates: 33.600, -112.157

Numerous thunderstorms developed across the greater Phoenix metropolitan area during the afternoon hours and they persisted into the evening; the stronger storms produced intense rain with peak rain rates in excess of 4 inches per hour. The heavy rain led to episodes of flash flooding which impacted the central portion of the Phoenix area including the Interstate 17 corridor north of Grand Avenue. According to a report from the public, at 1753MST flash flooding along Interstate 17 at the Indian School underpass resulted in a water rescue. No injuries were reported. Additionally, a mesonet weather station 3 miles northeast of central Phoenix, near the Phoenix County Club, measured 2.1 inches of rain within 30 minutes ending at 1739MST. The mesonet station was part of the Maricopa County Flood Control gauge network.

Hail, Maricopa

When: August 2, 5:25 PM MST
Where: Maricopa County, 1.86 mi ESE of Phoenix
Measured: 0.75 in hail
Reported by: Trained Spotter
Coordinates: 33.470, -112.040

Strong thunderstorms moved across the greater Phoenix area during the afternoon hours on July 2nd and in addition to very heavy rain, some of the storms produced large hail. At 1725MST a trained weather spotter located 2 miles northeast of central Phoenix measured a hailstone with a diameter of 3/4 inches. The hailstone was observed just to the northeast of the intersection of East McDowell Road and Highway 51.

Thunderstorm Wind, Maricopa

When: August 2, 6:00 PM MST
Where: Maricopa County, 4.89 mi SW of Fountain Hills
Measured: 61 kt (70 mph) estimated gust
Property damage: $30,000
Reported by: Trained Spotter
Coordinates: 33.580, -111.790

Thunderstorms moved across the greater Phoenix area, including the community of Fountain Hills, during the afternoon hours on July 2nd. Some of the stronger storms generated gusty and damaging outflow winds estimated to be as high as 70 mph. At 1800MST a trained spotter located about 5 miles southwest of the town of Fountain Hills reported that four power poles were blown down. The downed poles were located at the intersection of 136th Street and Shea Boulevard.

Flood, Pinal

When: August 2, 6:30 PM MST to 11:00 PM MST
Where: Pinal County, 9.98 mi W of Lirim to 12.03 mi WNW of Lirim
Cause: Heavy Rain
Reported by: Department of Highways
Coordinates: 33.044, -112.150

Thunderstorms with very heavy rain moved across portions of the south central deserts including communities to the south of the greater Phoenix area including Maricopa. The storms produced locally heavy rain and areal flooding during the evening hours on August 2nd. According to the Arizona Department of Highways, flooding caused State Route 238 to become closed between the town of Maricopa and Gila Bend at mile marker 40 at 2103MST. An Urban and Small Stream Flood Advisory had been issued for the area starting at 1826MST and it was in effect through 2130MST. No additional flood warnings or advisories were issued for the area.

Flood, Maricopa

When: August 2, 8:15 PM MST to 11:15 PM MST
Where: Maricopa County, 3.27 mi NE of Phoenix Farm Aero Ar to 1.39 mi ENE of Glendale
Cause: Heavy Rain
Reported by: Law Enforcement
Coordinates: 33.466, -112.133

Thunderstorms with very heavy rainfall moved across the greater Phoenix metropolitan area during the evening hours on August 2nd and they especially impacted the Interstate 17 corridor north of central Phoenix. Intense rain was observed with peak rain rates in excess of 4 inches per hour at times; this initially led to flash flooding which was followed by areal flooding. A mesonet rain gage 5 miles north of central Phoenix, near the intersection of Interstate 17 and West Camelback road, measured 2.91 inches of rain ending at about 1900MST. This led to flash flooding and water rescues at the Indian School underpass and Interstate 17. The heavy rains then resulted in continued areal flooding along the Interstate 17 corridor and an Areal Flood Warning was issued at 2015MST which remained in effect through 2315MST. Local law enforcement reported the continued flooding along Interstate 17 at Indian School Road and this was the basis for the Areal Flood Warning.

What these are

Flash Flood: Flooding that begins within six hours of the rain that causes it, or of a dam or levee failure or ice jam release, often with deadly speed.
Hail: Hail three-quarters of an inch or larger, or smaller hail that caused damage, filed with the largest stone measured.
Thunderstorm Wind: Winds from a thunderstorm of 50 knots (58 mph) or more, or lower winds that caused damage or casualties. Measured gusts are marked MG, estimated EG.
Flood: Flooding that develops more slowly than a flash flood: rivers leaving their banks, or water standing over land for an extended time.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

Every storm in Arizona in 2016 Every state in 2016 This day in weather history Warnings in Arizona, August 2016

Sources

NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 646606, 646611, 646594, 646596, 646639, 646619.