Flash Flooding in Arizona, September 23, 2019

Name: Flash Flooding in Arizona, September 23, 2019
Start: 2019-09-23T05:00
End: 2019-09-23T12:15
Location: Arizona

September 23, 2019 · NWS PSR · 5 events in 3 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: $312,000
Crop damage: None recorded
Events: 5

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.

During the early morning hours on September 23rd, copious tropical moisture began to spread north and into south-central Arizona. A weather system dropping south out of Nevada began to act on the moisture, resulting in the development of numerous showers and thunderstorms. Some of the storms produced copious amounts of rain with peak rain rates in excess of 2 inches per hour. Many locations across the high terrain east of Phoenix measured more than 3 inches of rain during the morning hours. Multiple events of flash flooding occurred in locations such as Apache Junction, Tortilla Flat, Fountain Hills and terrain to the south and east of Lake Roosevelt. As an example, at 0730MST a local emergency manager reported that 7 homes were flooded in Roosevelt Estates as Campaign Creek overflowed its banks. Several Flash Flood Warnings were issued for areas to the east of Phoenix during the morning hours. In 2020, the Arizona Department of Transportation was awarded $10.7 million from the Federal Highway Administration to support the repair of roads and bridges damaged due to the numerous flash flooding events that occurred across Arizona on the 23rd.

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 Sep 23, 3:59 AM MST, 1 hour 1 minute earlier.

Area forecast discussion, Sep 23, 3:59 AM MST, 1 hour 1 minute before

Area Forecast Discussion
National Weather Service Phoenix AZ
359 AM MST Mon Sep 23 2019

.UPDATE...aviation discussion.

.SYNOPSIS...
Multiple periods of scattered to widespread showers and
thunderstorms are expected through Tuesday morning across south
central and southwest Arizona with isolated storms over eastern
Riverside and Imperial Counties. Locally heavy rainfall capable of
producing flash flooding along with isolated severe thunderstorms
capable of producing large hail and damaging winds are expected.
Although rain chances will begin to decrease on Tuesday, elevated
rain chances will linger through Thursday as the upper level
disturbance remains near the region with the best rain chances
Wednesday night into Thursday. This disturbance will move east of
the region Friday, with drier conditions expected this weekend.

.DISCUSSION...
Tropical moisture associated with the remnants of what was
Pacific TC Lorena has moved into south central Arizona with
supplemental moisture from a Gulf surge moving into southwest
Arizona. Dew points are already in the upper 60s over Phoenix and
south central Arizona with mid 70s dew points pushing into Yuma
County. Much drier air is currently present just west of the
Colorado River over southeast California where dew points are in
the 40s, resulting in a very sharp moisture gradient. A very
strong mid- to-upper level trough with 571 dm H500 heights digging
into the Great Basin will dig farther south into the Colorado
River Valley by this evening as it fills to some extent. This will
maintain very favorable dynamics near and east of the cold core
low, with significant diffluence aloft, upper-level divergence in
the right entrance region of a jet streak, and isentropic ascent
in the presence of near record level 1.4-1.8 inch PWATs through
Tuesday morning. These conditions will support multiple periods of
scattered to widespread showers and thunderstorms through Tuesday
morning across south central and southwest Arizona with isolated
storms over eastern Riverside and Imperial Counties. Locally heavy
rainfall capable of producing flash flooding is expected, so we
have issued a Flash Flood Watch for most of La Paz, Maricopa,
Gila, and Pinal Counties from 5 AM MST this morning through 11 AM
MST Tuesday morning. Isolated severe storms capable of producing
large hail, damaging winds, and possibly even an isolated tornado
or two are expected by late morning through this evening.

Three main periods or focus areas of convection are expected
today with varying threats, so we cover them individually here:

1) Scattered to widespread convection is expected to continue for
the rest of the overnight hours through mid-morning primarily
from the a line along and east of a Maricopa-Phoenix-Anthem line
with more isolated convection over southwest Arizona. A weak
shortwave rotating out ahead of the stronger shortwave over the
Great Basin may be enhancing dynamic lift locally right now. Most
of this convective activity should be relatively shallow given we
still have lingering midlevel stability, but this should rapidly
erode from west to east as height falls and cooler air moves in
aloft to steepen lapse rates and allow for deeper convection and a
greater coverage of thunderstorms. Locally heavy rainfall will be
the primary threat with these storms, particularly over eastern
Maricopa and Gila Counties where S-SW- to-N-NE oriented isentropic
ascent is best aligned with terrain. Rain rates should initially
remain below 1 inch per hour, but as convection intensifies over
the next few hours with the assistance of terrain and increasing
instability, 1-2 inch per hour rain rates will be possible. For
this reason, we have decided to start the Flash Flood Watch at 5
AM MST this morning as that is when we really expect the heavier
rainfall and the cumulative effects of ongoing shower activity to
accentuate the flash flood threat, particularly for Gila and
northeast Maricopa Counties where WPC has issued a moderate risk
of excessive rainfall in collaboration with local Arizona offices.

2) By late morning, the ingredients for strong to severe storms
will be present as the trough provides atmospheric forcing via
upper level diffluence and a strong jet streak aloft. Gradually
veering winds and bulk shear values in the 40-50 knot range in the
presence of MLCAPE and CAPE values in the 1500-2500 J/kg range if
not even higher over Yuma County will support discrete supercells
depending on how convection evolves. The moisture gradient
currently present near or just west of the Colorado River Valley
should continue to sharpen further to act like a pseudo-dryline
and provide an initial focus for convection, so there is highest
confidence in severe storms over Yuma, La Paz, and Maricopa
Counties. SPC has this area and the Phoenix metro in a slight risk
of severe storms, so a Severe Thunderstorm Watch may be issued
later this morning or afternoon once confidence is high enough.
Further east towards the Phoenix metro, there are significant
uncertainties in how much the ongoing convection and cloud cover
may temporarily stabilize the atmosphere locally, so we may see a
brief lull in the action through late afternoon. However,
convective temperatures are only in the upper 80s so it will not
take too many breaks in the clouds for the Phoenix metro to
realize strong instability and favorable wind profiles to get
strong to severe storms. Large hail up to golf ball size will be
the biggest threat with these storms, followed by damaging winds.
Very steep low- level lapse rates with relatively low LCLs and
cloud bases down to about 5000 feet will support a very isolated
tornado threat, but any that do occur should be weak. Westerly
winds expected by this afternoon should keep storm- relative
helicity values low to keep this threat minimal, but winds backing
to southeasterly by early evening could briefly increase this
threat depending on how convection evolves.

3) There is perhaps stronger model agreement in that another round
of storms is expected late this evening into the overnight hours
as the synoptic front pushes through the region. This should
result more linear storm structures in the the from multicellular
line segments, for which damaging straight line winds will be the
primary threat. Hail should generally be smaller since rotating
updrafts will generally not be expected and instability should
begin to decrease. However, any cells that are able to train or
occur over areas that have already experience heavy rainfall will
have the potential to cause flash flooding. Although agreement for
this third period of storms is strong, there is considerable
uncertainty in how widespread storms will be and what time they
will actually occur. Fortunately, this uncertainty should decrease
over time as hi-res models come into clearer focus based on how
the aforementioned two periods of convection evolve.

Activity should wane in our area by late Tuesday morning although
enough moisture will be present that lingering showers will be
possible. However, given the close proximity of the upper-level
disturbance that should eventually stall over the northern Gulf of
California and uncertainties in when third more linear period of
storms will occur, we elected to keep the Flash Flood Watch in
place through 11 AM MST Tuesday. Some hi-res guidance shows
isolated showers and storms over Gila and eastern Maricopa
Counties Tuesday afternoon as a cutoff low takes shape, but there
is still considerable uncertainty there. Residual moisture and a
second Pacific trough later in the week justify elevated PoPs in
the forecast, primarily for Maricopa County eastward, late
Wednesday and Thursday as the trough finally opens up and ejects
to the east as it is kicked out by a much stronger longwave trough
upstream. As for temperatures, the persistent troughing and
shortwaves will keep a cooler airmass in place resulting in
daytime temperatures below seasonal normal values through the
forecast period.

.AVIATION...Updated at 0527Z

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Light diurnal winds will continue through the early part of the
evening with speeds mainly below 6 kts. As we head into the
overnight hours, increasing tropical moisture will result in
increasing low and mid-level cloud cover with BKN-OVC cloud decks
currently near 8-10 kft, dropping closer to 5 kft after 10Z.
Going into tomorrow morning, shower and storm chances will
continue to ramp up and hi-res models are now indicating a
slightly earlier onset over terminals and this has been reflected
in the most recent TAF package. Latest model guidance is
now suggesting that isolated showers could begin to develop
near terminals as early as 9Z, with better coverage after 12Z.
There then seems to be a hint at a break during the early part of
the afternoon, however there very much so could be showers and
thunderstorms still lingering near or over terminals throughout
the afternoon hours. Thereafter, models continue to indicate a
line of strong thunderstorms approaching the Phoenix terminals
from the west after 22Z. These storms will be the strongest storms
of the day and could become severe with gusty winds, frequent
lightning, and severe hail possible. CIGs may get as low as 4 kft
with any storms that pass through.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds will continue to be light and at times variable through
the morning hours before becoming gusty out of the south-southwest
in the early afternoon. In addition, low-level cloud cover is
expected to increase through the TAF period with FEW-SCT decks
around 5-8 kft. There is a little bit more confidence that
showers and storms could be possible over KBLH in the early to
mid afternoon. Any storm that develops will have the potential to
produce gusty winds and frequent lightning. Due to this, it is now mentioned in the latest TAF
package.

Aviation Discussion not updated for amended TAFs.

.FIRE WEATHER...
Wednesday through Sunday:
No changes to the outlook forecast...Lingering isolated shower
Aside from gusty thunderstorms, winds should generally remain
below 15-20 mph until Friday and Saturday when stronger winds are
expected with the approaching disturbance. Minimum relative
humidity values will remain in the 20 to 30 percent range for most
locations except a few spots in southeast California that may drop
into the upper teens. Overnight recoveries will remain decent as
long as the moisture remains in the area.

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through Tuesday morning for AZZ531-533-534-537-
538-540>558-560>563.

CA...None.
&&

DISCUSSION...Hopper/Deems
AVIATION...Hernandez
FIRE WEATHER...Deems/Hopper

The radar

The Phoenix radar (KIWA), 34 km away, was scanning. The volume nearest the first event began at 12:00 UTC.

Open the radar archive at this moment

Event by event

5 flash flood. Local standard time, as filed.

Flash Flood, Maricopa

When: September 23, 5:00 AM MST to 10:00 AM MST
Where: Maricopa County, 2.81 mi SSW of Tortilla Flat to 11.79 mi NW of Tortilla Flat
Cause: Heavy Rain
Property damage: $40,000
Reported by: Department of Highways
Coordinates: 33.496, -111.407

Copious amounts of tropical moisture spread northward and into south-central Arizona during the early morning hours on September 23rd, leading to the development of numerous showers and thunderstorms over higher terrain areas east of Phoenix. Locally heavy rain developed, with peak rain rates in excess of 2 inches per hour. By mid morning, a number of locations had already received more than 3 inches of rain. The heavy rain led to episodes of flash flooding from Tortilla Flat eastward. At 0605MST, the Fish Creek USGS stream gage 5 miles east of Tortilla Flat reported an increase from 5 feet to 9.5 feet. At 0615MST, Arizona Department of Highways reported that State Route 88 was closed indefinitely between Tortilla Flat and State Route 188. The closure was due to flash flooding as well as debris across the road. Additionally, at 0748MST a member of the public reported flash flooding along Lewis and Pranty Creek near State Route 88. This occurred roughly 7 miles east of Tortilla Flat and south of Apache Lake. The report also indicated that water had jumped out of the creek's banks and was inundating one of the properties in the area. Multiple Flash Flood Warnings were issued during the morning and were in effect at the time of the flooding. No injuries were reported fortunately.

Flash Flood, Maricopa

When: September 23, 5:00 AM MST to 9:45 AM MST
Where: Maricopa County, 2.36 mi ESE of Desert Sands to 4.82 mi E of Twin Knolls
Cause: Heavy Rain
Property damage: $30,000
Reported by: Trained Spotter
Coordinates: 33.393, -111.630

Copious amounts of tropical moisture spread north and into south central Arizona during the early morning hours on September 23rd, resulting in scattered to numerous showers and thunderstorms. Peak rain rates with the stronger convective cells was in excess of 2 inches per hour; the heavy rain resulted in episodes of flash flooding east of Phoenix, affecting communities such as East Mesa and Apache Junction. According to a trained spotter located about 4 miles southeast of East Mesa, at 0525MST flash flooding caused several streets to flood and an area wash was filled with several feet of water. The flooding was west of Crismon Road and between Broadway and Southern. At 0630MST, local broadcast media reported that flash flooding 1 mile southwest of Apache Junction resulted in the closure of 16th Avenue between Delaware and Ironwood Drive. The flooding occurred when a wash flowed across the road. Finally, at 0700MST local broadcast media reported an episode of flash flooding 3 miles west of Apache Junction. The flash flooding necessitated a water rescue of a stranded vehicle just north of Main Street and near Signal Butte Road. No injuries were reported for any of the flash flooding incidents. A Flash Flood Warning was issued at 0541MST and the warning covered most of the flooding incidents.

Flash Flood, Pinal

When: September 23, 5:30 AM MST to 9:45 AM MST
Where: Pinal County, 1.28 mi SW of Goldfield to 1.23 mi NW of Apache Jct
Cause: Heavy Rain
Property damage: $40,000
Reported by: Department of Highways
Coordinates: 33.437, -111.496

Copious amounts of tropical moisture spread north and into south-central Arizona during the early morning hours on September 23rd allowing for the development of numerous showers and thunderstorms. Many of the storms had heavy rain with peak rain rates in excess of 2 inches per hour. The heavy rain led to episodes of flash flooding across areas to the east and southeast of Phoenix including the communities of Apache Junction and East Mesa. According to the Arizona Department of Highways, at 0630MST flash flooding 3 miles northeast of Apache Junction resulted in State Route 88 becoming impassable at Mile Marker 199 along Weekes Wash. Flooding was also occurring along Palm Wash. In addition, at 0700MST local broadcast media reported a swift water rescue underway at the intersection of Main Drive and Mckellips Boulevard, about 3 miles northwest of Apache Junction. A Flash Flood Warning was issued for the area at 0541MST and was in effect during the flooding. No injuries were reported due to the flooding.

Flash Flood, Gila

When: September 23, 6:00 AM MST to 10:45 AM MST
Where: Gila County, 6.29 mi SE of Roosevelt Grpvn Arpt to 3.99 mi E of Roosevelt Grpvn Arpt
Cause: Heavy Rain
Property damage: $200,000
Reported by: Emergency Manager
Coordinates: 33.572, -110.966

Copious amounts of tropical moisture spread northward and into the high terrain areas east of Phoenix during the early morning hours on September 23rd and as a result, numerous showers and thunderstorms developed. Many storm produced heavy rain with peak rain rates in excess of 2 inches per hour. The heavy rain resulted in episodes of flash flooding, affecting areas to the south and east of Lake Roosevelt including Tonto National Monument. At 0700MST a federal agency reported flash flooding on the upper cliff dwelling trail about 2 miles southeast of Roosevelt. Shortly thereafter, at 0730MST, a local emergency manager located 8 miles east-southeast of Roosevelt reported that 7 homes were flooded in Roosevelt Estates as Campaign Creek overflowed its banks. As much as 18 inches of water was reported in one of the houses. Fortunately there were no reports of injuries. Two Flash Flood Warnings were issued for the area, the first was issued at 0609MST and the second at 0652MST.

Flash Flood, Maricopa

When: September 23, 8:00 AM MST to 12:15 PM MST
Where: Maricopa County, 1.71 mi S of Ft McDowell to 0.94 mi SW of Fountain Hills
Cause: Heavy Rain
Property damage: $2,000
Reported by: Fire Department/Rescue
Coordinates: 33.605, -111.678

Copious amounts of tropical moisture spread north and into south-central Arizona during the morning hours on September 23rd, allowing for the development of numerous showers and thunderstorms. Locally heavy rain occurred with peak rain rates in excess of 2 inches per hour at times, and the heavy rain led to flash flooding affecting areas such as Fountain Hills. As an example of the heavy rain that fell, at 0820MST a trained spotter located about 2 miles northeast of Fountain Hills indicated that 3.5 inches of rain had fallen over the past one hour and forty minutes resulting in minor flooding. Flash flooding soon developed and at 0900MST local fire and rescue reported that Ashford Wash in Fountain Hills had exceeded its banks. Flooding occurred at the intersection of El Pueblo Boulevard and Calico Drive. A Flash Flood Warning was issued for the area at 0818MST and was in effect at the time of the flash flooding.

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.

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

The same day elsewhere

Every storm in Arizona in 2019 Every state in 2019 This day in weather history Warnings in Arizona, September 2019

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: 849957, 850051, 850042, 849955, 849961.