This was the culmination of a week long deluge of heavy rainfall as one final night of widespread heavy rainfall across Southeast Arizona. The heavy rain combined with the very wet soils to lead to the widespread July 31st flooding throughout the area. The July 31st flooding caused at least $4 million in damage and was the biggest flooding event in Tucson since the floods of 1983 and 1993. There were dozens of debris flows along many of the south facing canyons on the Santa Catalina range, and dozens more in Coronado National Monument and Fort Bowie causing extensive damage to areas including the recreationally popular Sabino Canyon. On the Rillito River, which is downstream from the hard hit Catalina Mountains, the United States Geological Survey recorded a record flow of 30,000 cubic feet per second at Dodge Blvd in Tucson. This flow was stronger than the flow from the devastating Floods of 1983. Record flow also occurred along the Sabino Creek, Tanque Verde Wash, Pantano Wash and Rincon creek. Flood waters along the Rillito Creek briefly came out of the banks near Dodge Blvd and onto the river walk paths and sweeping several joggers into the creek. Farther downstream, the Santa Cruz River exceeded flood stage with flooding along the river in Marana including the Avra Valley Road Bridge, Trico-Marana Road Bridge, the Barry Estates subdivision, and a cement plant along the river.

This was quite a remarkable event meteorologically speaking as well. The early morning hours of July 31st marked the fifth and final consecutive overnight and early morning period with heavy rainfall across portions of Southeast Arizona. There was a pesky upper level low located across west-central New Mexico which was about to kicked eastward by an approaching upper level trough. Arizona was located in a deformation zone between these two features for this event beginning the afternoon the 30th when an MCS developed across the Mogollon Rim. This MCS generated a mesoscale outflow boundary which could be seen on infrared satellite imagery on the evening of the 30th pushing southwest away from the Mogollon Rim. This was the likely triggering mechanism for a cluster of thunderstorms which developed across Pinal County after 10 PM on July 30th. The 00Z and 12Z July 31st KTUS sounding showed an abundance of moisture through the atmospheric column with 1.81" of precipitable water from the 00Z sounding (Figure 1) and 2.00" from the 12Z sounding. Both soundings also showed quite a bit of shear, with low level south to southwesterly flow, and north to northwesterly flow in the mid and upper levels. The cluster of thunderstorms initially over Pinal County blossomed after midnight and through the early morning hours as it moved southeast through Eastern Pima County and the Tucson metro area. Infrared imagery showed cloud tops with this cluster of thunderstorms as low as -74°C across the Santa Catalina Mountains at about 3am. (Figure 2) As the cluster of thunderstorms moved across the Tucson area and the Catalina range, the low level south-southwesterly flow along the steep front range of the Catalinas forced quite a bit of additional orographic lift. As the early morning progressed, the pulses of heavy rainfall continued over the Tucson metro area including the Catalinas. However, the convection transitioned to that of warm top thunderstorms with cloud tops bottoming out in the -55°C to -60°C range. (Figure 3) Rainfall amounts were quite impressive (see table below) with nearly 5 inches along Molino Canyon on the 31st.

This MCS continued to push south to southeast into Santa Cruz and Cochise County, which caused additional heavy rainfall and flash flooding across these areas. It should be noted that about 10 inches of rainfall fell in a 7 day period causing extensive flooding and damage to Coronado National Monument and Aravaipa Canyon.

9-hour rainfall totals across Tucson area ending 900 AM MST on July 31st are below:

Molino Canyon 4.65 inches
El Camino Rinconado 3.66 inches
Tanque Verde Guest Ranch 2.68 inches
Agua Caliente Wash 2.40 inches
Agua Caliente Park 2.32 inches
Sabino Canyon Dam 1.97 inches
Tucson Int'l Airport 1.90 inches
Manning Camp 1.85 inches
University of Arizona 1.17 inches

7 day totals ending 12 PM on July 31st:

Mt. Lemmon 11.10 inches
Catalina Hwy. near MP3 9.21 inches
Sabino Canyon Dam 8.19 inches
Samaniego Peak 7.95 inches
Oracle Ridge 7.48 inches
Manning Camp 6.77 inches
Sunrise Rd/Ventna Canyon Wash 6.69 inches
Davidson Canyon/I-10 6.69 inches
Rosewood St/Harrison Rd 6.22 inches
Tucson Int'l Airport 3.83 inches
Pima Air Museum 3.03 inches