WeatherTAP RadarLab® Weather Radar Image, 08/14/2021 @ 01:24 AM MST

Flood Control District of Maricopa County
Engineering Division, Flood Warning Branch

Storm Report : August 12-14, 2021

TABLE OF CONTENTS
Meteorology ............................................................................... 3
Precipitation .............................................................................. 7
Runoff & Impoundments ............................................................... 15
Public Outreach Summary ............................................................. 22
Data Sources ............................................................................. 24

TABLES
Table 1
Table 2
Table 3

Peak wind gusts measured at FCDMC Weather Stations ..................... 6
Rainfall Return Periods for Selected ALERT Gages .......................... 12
Summary of Impoundments at Fountain Hills Dams ......................... 19

FIGURES
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figs. 7-9
Figs. 10-15
Figs. 16a-d
Figure 17
Figure 18
Figure 19
Figure 20
Figure 21
Figs. 22-27
Figure 28
Figure 29
Figure 30
Figure 31
Figure 32

U of A WRF-RRGFS storm-total forecast graphic ending 8/14 5:00 am .... 4
4-Panel 00Z Synoptic Setup for 8/14/21 5:00 pm ............................ 5
Observed Skew-T/log P diagram at 8/13/21 5:00 pm ........................ 6
54-hour Rain Totals vs. # of Rain Gages ........................................ 7
54-hour gage-adjusted radar rainfall map, County-wide .................... 7
6-hour gage-adjusted radar rainfall map, Sand Tank, Bender Washes .... 8
Rain Maps: 1-day Totals Ending at 12 pm each day ....................... 9-11
Rain Statistics at Six Selected Gages ..................................... 13-14
Photos – Sand Tank Wash ....................................................... 15
Sand Tank Wash recorded hydrograph – Aug. 14 ............................ 16
Gila Bend area floodplain map ................................................ 16
Sand Tank Wash hydrograph adjusted to 14K cfs ........................... 17
Gila Bend area floodplains featuring Scott Ave. Wash ..................... 17
Sand Tank Wash at I-8 Flood Frequency Estimates .......................... 18
Hydrographs – Fountain Hills Dams ....................................... 19-21
Hydrograph – East Fork Cave Creek near 7th Ave. ........................... 21
Twitter Post Impressions ....................................................... 22
Sample Facebook Post .......................................................... 22
Facebook Post Performance Statistics ....................................... 23
ALERT Interactive Data Display Map – Total Pageviews ..................... 23

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METEOROLOGY
A high-grade Monsoon pattern led to the development of organized clusters of severe thunderstorms that
rolled off the higher terrain into the greater Phoenix Valley during the evening of August 13th before
continuing into Gila Bend during the early morning hours on the 14th. Maricopa County rain gauges
measured event totals anywhere from 1.00”-3.95”. There were reports of urban flooding in Fountain
Hills, Mesa, and Phoenix and the Town of Gila Bend experienced significant flash flooding due to runoff
along Sand Tank Wash and other local washes.
Synopsis:
There are four common large-scale patterns that develop during the yearly North American Monsoon,
which are historically quite favorable for producing active weather across southcentral AZ. The largescale setup on August 13th quite closely mimicked a Great Basin High pattern (commonly referred as a
Rim-to-Valley setup). It’s characterized by the summer ridge settling across the Great Basin region of
NV/UT. Clockwise flow around the high pressure center promotes deep northeasterly steering flow
across AZ. Thunderstorm activity tends to develop across the higher terrain regions (Mogollon Rim and
White Mountains) before moving west/southwest into the lower deserts as organized lines/clusters
causing severe weather outbreaks and/or flash flooding. Please refer to the following page for more
background information.
The synoptic setup on the evening of August 13th is shown in Figure 1 (below). The upper- level images (A
and B) depict a diffuse ridge of high pressure draped across NV/UT and a well-defined inverted trough
across southwestern NM. The positioning of the ridge and trough during this period created a large area
of upper-level divergence (rising motion, yellow lines in A) across eastern AZ, which likely enhanced
initial t-storm development across the Mogollon Rim and White Mountains regions. Both the 700mb and
850mb maps (images C and D) depict the moisture rich environment across AZ.
Mesoanalysis data across southcentral AZ in the near storm environment on Friday evening revealed a
favorable setup for organized t-storms capable of severe weather and heavy rainfall. Evidence of this
was shown by the 00Z, 5:00pm MST, KPHX sounding in Figure 2 below. Precipitable water was sampled at
1.90”, which is an incredibly high (wet) value for August. In fact, according to the Storm Prediction (SPC)
sounding climatology page this sounding set the period of record value for the date. Thermodynamic
variables of note included: high CAPE/low CIN profile (extremely unstable atmosphere), negative lifted
indices, and high DCAPE values indicative of strong downdrafts capable of damaging wind gusts. The
vertical wind profile showed strong 700mb-500mb 20kt steering-level winds out of the northeast and low
level (below 850mb) westerlies. The District’s hourly Convective Quantitative Precipitation Forecast
(QPF) at 8:06pm MST here was generated just before the first line of storms developed across the eastern
half of the County and showed an atmosphere primed for heavy rainfall (notable 60min and 30min rainfall
values highlighted in yellow). Another QPF generated at 11:06pm MST here shows the rainfall
environment across southwestern Maricopa County just before storms dropped torrential rains over the
area (discussed in detail in following sections). It’s also worth noting an additional description of the
setup for severe weather can be viewed on the Storm Prediction Center’s weather event archive here. A
full description of the heavy rain and flash flooding potential was conveyed by the Weather Prediction
Center through its mesoscale precipitation discussions here and here.
Storm Summary and Radar Imagery:
Organized clusters of thunderstorms rolled across Maricopa County spanning roughly seven hours while
bringing widespread heavy rainfall, numerous reports of severe weather, and flooding. The National
Weather Service Phoenix WFO created a radar loop covering the entire event which can be viewed here.
This loop will provide context to the following summary. As is customary with the Great Basin pattern,

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storms initiated across the Mogollon Rim and White Mountains during the afternoon/early evening hours
before moving west/southwest into portions of the lower deserts of Maricopa County.
A lead complex of storms exiting Payson grew upscale into a bow echo that ultimately traversed west
along the Yavapai/Maricopa County line. Multiple reports of 60+mph winds were observed along this line
along with heavy rainfall in Wickenburg and Aguila. Further south multiple strong/severe t-storms
initiated along a line from the lower Salt River Lakes into east Mesa just after 8:00pm MST before
progressing west into Fountain Hills, Scottsdale, and Paradise Valley. Fountain Hills and East Mesa
received the heaviest rainfall with multiple gauges picking up 2.00” or more. There were multiple
reports of flash flooding in these areas and impoundments were measured behind Spookhill FRS, Golden
Eagle Park Dam, and North Heights Dam. By 10:00pm MST additional clusters of strong t-storms rapidly
developed across central Phoenix, Glendale, Peoria, Surprise, and Estrella bringing anywhere from 0.75”2.95”. Heavy rain lead to severe water ponding which closed multiple sections of I-17 heading north out
of metro Phoenix. Noteworthy runoff was also measured in the ACDC, East Fork Cave Creek, Cave Buttes
Outlet, and Deadman Wash.
Multiple clusters of strong/severe t-storms, covered above, continued to grow upscale into a Mesoscale
Convective System as they moved southwest through the greater Phoenix Valley and into the SR85/I-8
corridors. As of 12:00am MST, a nearly uniform line of storms could be seen stretching from Wenden
east/southeast into Gila Bend and through Casa Grande. Of note, areas to the east and south of Gila
Bend were hit with torrential rainfall. Multiple District gauges picked up anywhere from 1.50”-3.94” with
the majority of that falling in a two-hour period. Heavy rain triggered significant flash flooding in area
washes through the Town of Gila Bend. A period of record flow was measured along Sand Tank Wash @ I8. Unfortunately, two people lost their lives and numerous homes were destroyed/damaged due to the
flooding. An Emergency Declaration was issued for Gila Bend later in the morning on the 14th. One day
rainfall totals (ending 8/14 6:00 am) around the County from the ALERT network can be viewed here;
also local NWS storm reports can be viewed here.

FIGURE 1

Figure 1: depicts a forecast of storm-total rainfall ending at 5:00 AM on 8/14/2021. The forecast model run was
completed at 5 AM on August 13th. The model suggests a large area of rainfall greater than 1.25 inches through westcentral Maricopa County, which was for the most part verified. It missed an area of heavy rainfall south of I-8.

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4-Panel 00Z Synoptic Setup 08/14/2021

FIGURE 2

Figure 2: depicts the synoptic setup at 00Z (5:00 PM MST) on September 13th ,2021. The images are from the Storm
Prediction Center upper air maps archive. A) is the 300mb map: the black lines are stream lines, the yellow lines are
divergence, and station obs. are plotted at each available location B) is the 500mb map: the red dashed lines are
isotherms, the other variables are colored the same as in A. C) is the 700mb map: variables are colored the same as
D. D) is the 850mb map: the black lines are heights, the red dashed lines are isotherms, the green lines are
isodrosotherms, blue barbs are wind speed, and station observations are plotted at each available location.

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FIGURE 3

Figure 3: Observed Skew-T/log P (vertical profile of the atmosphere above Phoenix) diagram at 5:00pm MST on August
13th, 2021. Sounding generated from NOAA/SPC. The vertical axis is pressure in (hPa) and the horizontal axis is
temperature in (°C). The thick solid red line is the temperature profile. The thick solid green line is the moisture profile.
The dashed grey and orange lines are the atmospheric parcel profiles. The vertical axis on the right displays the wind
speed and direction at each level in knots. Gray solid lines sloping from upper left to lower right are constant potential
temperature (°K). Thin dashed blue and gray lines sloping from right to left are isotherms (°C). Thin dashed green lines
sloping from right to left are mixing ratio (g/kg). The table in the bottom left displays various parcel attributes and
thermodynamic indices. The image in the top right is a hodograph showing the change in speed and direction of the
atmospheric winds with height.

TABLE 1
ID
3306
15006
15306
35206
82506
82706
84206
84706
87506

Name
Durango Complex
Lake Pleasant
Lake Pleasant North
EMF @ AZ Ave.
Horseshoe Lake
Bartlett Lake
Estrella Fan
Mobile
Camelback L303

Location
Durango St. / 27th Ave., Phoenix
South end of the Lake, Peoria
Deer Island near the north end of the Lake, Peoria
2.5 mi. S of Hunt Hwy. on Arizona Ave., Pinal Co.
South end of the Lake, Tonto NF
West side of the Lake, Tonto NF
Alignments Queen Creek & El Mirage Roads, Avondale
83rd Ave. at Nahalia Rd., Goodyear
Camelback Rd. at Loop 303 Fwy., Glendale

Peak Wind Gust (mph)
48
37
36
44
63
35
35
49
50

Table 1: Peak wind gusts measured at FCDMC Weather Stations at or above 35 mph, Aug. 12-14, 2021

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PRECIPITATION
FIGURE 4

Figure 4 above presents a plot of rainfall amounts for all FCDMC ALERT rain gages for the period 8/12/2021 0:00 through
8/14/2021 06:00. One gage recorded no rain (Narrows Damsite) and one gage failed to operate (Buckeye FRS #1). Tabular
hourly data for all gages for the 54-hour period is available here.

FIGURE 5

Figure 5 above presents an image of rainfall amounts for all FCDMC ALERT rain gages for the period 8/12/2021 0:00 through
8/14/2021 06:00. Click the image for a full-size view in your browser. Use CTRL +/- to zoom in/out.

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FIGURE 6

Figure 6 above presents an image of rainfall amounts from gage-adjusted radar rainfall estimates for the period
8/14/2021 0:00 through 06:00. Recorded rainfall amounts at FCD automated rain gages are show in white numbers.
The image gives a visual representation of where heavy rain fell in relation to Sand Tank and Bender Washes. The
average rainfall over the entire Sand Tank Wash watershed was 1.20 inches.

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Rainfall from 8/11/2021 at Noon through 8/12/2021 at Noon

FIGURE 7

Figure 7 above presents an image of rainfall amounts from gage-adjusted radar rainfall estimates for the period
8/11/2021 12:00 through 8/12/2021 12:00.

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Rainfall from 8/12/2021 at Noon through 8/13/2021 at Noon

FIGURE 8

Figure 8 above presents an image of rainfall amounts from gage-adjusted radar rainfall estimates for the period
8/12/2021 12:00 through 8/13/2021 12:00.

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Rainfall from 8/13/2021 at Noon through 8/14/2021 at Noon

FIGURE 9

Figure 9 above presents an image of rainfall amounts from gage-adjusted radar rainfall estimates for the period
8/13/2021 12:00 through 8/14/2021 12:00.

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TABLE 2

Table 2: Return period calculations were performed for all FCD rain gages for the durations shown in the
Table above (15 min. – 3 days). Gages with at least two 10-year return periods across all durations are
listed in this Table. The Table is sorted by the column labeled “In or Near”, then by “Name”. Rain gage
elevations are conditionally coded from yellow (850’) to green (3,485’), while return periods are coded
from white (1 year) to red (500 years). Click on the Table to download a PNG file for detailed viewing. Data
from six individual gages are presented on the following pages - the figures include a location map, daily
rain totals, selected return periods and an hourly rainfall plot.

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FIGURE 10

FIGURE 11

FIGURE 12

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FIGURE 13

FIGURE 14

FIGURE 15

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Runoff and Impoundment Summary
Another wide-scale rainfall and runoff event occurred during the period of August 12–14, 2021.
Runoff was recorded by many streamflow stations during this period, and several locations had
record flows and impoundments. Gila Bend and specifically Sand Tank Wash had a very large
flow, and Fountain Hills saw their six dams record high or impoundments-of-record.
Additionally, there were other scattered runoff and impoundments of note. This section will
focus on Gila Bend and Fountain Hills with a brief mention of the other locations.
Gila Bend
The Sand Tank mountains south of I-8 and the town of Gila Bend had copious rainfall in a range
of about three to four inches. Much of that rain fell in a one to three-hour period. A very large
quantity of water flowed into Sand Tank Wash that then passed the I-8 bridge and streamgage
(#40007). The flow was so significant that the channel at the bridge and downstream of the
bridge was greatly altered.
FIGURES 16 a-d

Sand Tank Wash at I-8, April 12, 2021

Sand Tank Wash at I-8, August 16, 2021
The photos above are intended to depict the magnitude of the channel changes that occurred
due to the runoff event on August 14, 2021.

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From a post-event inspection on August 16, 2021, it was evident that the runoff at Sand Tank
Wash was of a greater magnitude than that recorded by the ALERT gage, which showed a peak of
7.69 feet and 5,000 cfs, which was the highest rated value.
FIGURE 17

In addition to the evidence at the gage, inspection of the high watermarks downstream of the I-8
bridge indicated that the flow was very wide and may have consumed much of the floodway as
defined by the current floodplain map. Inspection of the I-8 bridge showed water marks
approximately 3 feet below the low chord.

FIGURE 18
Figure 18: Current Floodplain Map for the area around Sand Tank Wash

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A peak discharge estimate was conducted from previous cross sections extended to the
floodplain limits. That analysis indicated a peak discharge of about 14,000 cfs at 8.7 feet gage
height. Using the USGS StreamStats program to estimate return periods for this event, 14,000
cfs is approximately a 60-year event. According to the current Flood Insurance Study, the 100year event has a peak discharge of 23,700 cfs.

FIGURE 19

Figure 19 – Sand Tank Wash @ I-8 hydrograph adjusted to the estimated peak of 14,000 cfs

As of this writing, the USGS is conducting an indirect measurement of the peak from this event
and that peak estimate is still pending. This report will be updated with their conclusions when
that estimate is received.
FIGURE 20

Another area where
occupied structures
suffered flood damage
was along Scott Ave.
Wash (left side of
Figure 20). Line B to B’
is a raised area or ridge
line between Scott
Ave. and Sand Tank
Washes. It is likely that
water from Sand Tank
Wash overtopped this
ridge line and was
intercepted by Scott
Ave. Wash, aggravating
the flood condition for
structures north of the
railroad underpass.

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FIGURE 21

This large flood continued northward to the Gila River. While passing through the Town, it
caused significant damage to the Paloma Canal, Union Pacific Railroad bridge, Indian Road and
Watermelon Road. Significant flooding of low-lying residential and commercial areas was
experienced.
A second major wash affecting the town of Gila Bend in this area is Bender Wash (#40507).
Though the precipitation gage at the Bender Wash streamgage had an impressive 3.94 inches of
precipitation, the verified flow was not overly significant at 395 cfs. The automated streamgage
failed to register any flow during the event but the on-site crest-stage gage provided an
estimate of the peak flow.
Fountain Hills
On the opposite side of Maricopa County is the town of Fountain Hills which has six dams
constructed for flood control in the steep hilly terrain of that community. Table 3 below
summarizes the impoundments from August 13, 2021.

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Location

ID

Aspen Dam
Golden Eagle Park Dam
Hesperus Dam
North Heights Dam
Stone Ridge Dam
Sun Ridge Canyon Dam

78707
78007
79007
78207
77307
77507

Peak GH
(feet)
9.03
13.00
11.28
18.47
10.48
7.50

Percent
Full
12.2
17.4
4.1
22.0
13.6
0.3

Time of
Peak
22:39
22:28
22:23
22:24
22:34
22:12

Record
Event
Yes
No
Yes
Yes
Yes
No
TABLE 3

Hydrographs for each dam are provided below.
FIGURE 22

FIGURE 23

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FIGURE 24

FIGURE 25

FIGURE 26

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FIGURE 27

Other Runoff
As previously mentioned, the storm coverage extended to much of the county and many
streamgages had runoff and many impoundment stations had stored water. The most significant
was the station at East Fork Cave Creek near 7th Avenue (#8507.)
It had a peak discharge of 3,610 cfs at 7.08 feet gage height on 08/13/2021 23:26:00. Though
not a record, this flow was near the record from 2005. A hydrograph is provided below.
FIGURE 28

Beyond this noted event, Skunk Creek, Cave Creek, New River, Indian Bend Wash, Arizona Canal
Diversion Channel, East Maricopa Floodway, Pecos Basin, and Phoenix Dams had significant flows
and impoundments. Additional information about these events can be gained by querying the
FCDMC water-level web page for Daily Surface Water Reports and Data Plots.

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Public Outreach Summary
The District has a total of 410 ALERT stations located throughout Maricopa and adjoining
Counties. All of the information for these stations is available in real-time on our webpage.
During this event (Aug 12-14) we used social media to help inform the news and public of the
current weather and rainfall information.
Our best platform continues to be Twitter: @FCDFloodInfo. We are up to 2,709 followers which
consist of different agencies, news media and the public. We had a total of 72,302 impressions
and 14,198 engagements over this 3-day period. Our top post had over 29,192 impressions. Our
second highest post with 10,331 impressions was the MSP Weather Outlook posted at 1:45pm on
August 13.

FIGURE 29

FIGURE 30
Our Facebook page Flood Control District of Maricopa County had 1,289 impressions and 98
engagements over this 3-day period. Our top post had a total of 300 impressions.
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FIGURE 31
Our ALERT Interactive Data Display Map is the primary tool for access to our ALERT data. During this event there
were a total of 34,205 sessions. The highest impact day was August 14 th with 20,277 pageviews. This ranks as the
third highest daily pageviews count since we started tracking them in June 2013.

FIGURE 32

Figure 32, ALERT Interactive Data Display map pageviews: June 2013 – Aug. 2021

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DATA SOURCES
1.

NOAA National Severe Storms Laboratory; Multi-Radar Multi-Sensor System, Norman, OK
http://nmq.ou.edu/

2.

Iowa State University, Iowa Environmental Mesonet, Local Storm Report App, Ames, IA:
https://mesonet.agron.iastate.edu/lsr/

3.

Flood Control District of Maricopa County, Phoenix, AZ
https://www.maricopa.gov/floodcontrol and https://www.reportaflood.org

4.

National Weather Service, National Hurricane Center, Miami, FL:
http://www.nhc.noaa.gov/?epac

5.

NOAA Atlas 14 Point Precipitation Frequency Data Server,
https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html

6.

National Weather Service, Weather Prediction Center, College Park, MD:
http://www.wpc.ncep.noaa.gov/

7.

National Weather Service, Tucson WFO, Monsoon Tracker, Tucson, AZ:
https://www.wrh.noaa.gov/twc/monsoon/monsoon_patterns.php

8.

National Weather Service, Weather Prediction Center, College Park, MY:
http://www.wpc.ncep.noaa.gov/

9.

National Weather Service, Phoenix WFO, Storm Report for August 13-14, 2021:
https://www.weather.gov/psr/StormReportfor1314August2021

10.

WeatherTAP RadarLab®: https://www.weathertap.com

11.

USGS StreamStats: https://streamstats.usgs.gov/ss/

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