This early monsoon season day turned quite interesting for the Tucson metropolitan area. Moisture was abundant from the surface right through the upper levels as the 12Z Tucson sounding (Figure 1) indicated a precipitable water value of 1.64". At this time an upper level low was situated across the northern Gulf of California moving slowly north-northeast toward southern Arizona. This upper level low had convection near its core through the overnight hours which kicked off an outflow boundary that moved east during the morning. Mid level wind flow between 500mb (Figure 4) and 700mb (Figure 5) was generally 10kts or less which kept storm motion rather slow. The question of the day based on the 12Z sounding was whether we get much convection given the subsidence induced warming just above the 500mb level. The special 18Z sounding (Figure 2) indicated the subsidence had passed with several degrees of cooling near 500mb. In addition, convective parameters were quite unstable with an impressive surface based CAPE of 2056 J/kg and lifted index of -8ºC. Meanwhile, 300mb (Figure 3) winds were considerably stronger from the south at 30-40kts with diffluence on the leading edge of an approaching speed max likely aiding in the development of convection. This in combination with a dry slot above 500mb (also seen on the 18Z Tucson sounding) aided in keeping the skies mostly clear through the morning was a clear indication there would be an outbreak of strong to severe thunderstorms with flash flooding likely.

Thunderstorms did indeed develop rapidly by 20Z (Figure 6a-d) across the Tucson area with the outflow boundary from the overnight convection assisting in the generation of storms. Interestingly, while most of the city of Tucson had very heavy rain with widespread totals greater than one half of an inch, the complex of thunderstorms that hit the city developed just north of the official observation site, Tucson International Airport which only received 0.02" of rain. Just a few north of the airport near Interstate 10 and Country Club road, 0.75" fell. As is often the case with monsoonal thunderstorms, there was a very strong gradient between no rain and extremely heavy rains. These storms quickly merged over the city of Tucson dumping over 1 inch of rain in less than an hour across a good portion of the city. This led to flash flooding throughout the Tucson area with some underpasses and streets full of water. At the Stone Avenue underpass at 6th Street in Tucson there were cars trapped as high water rapidly filled in along the underpass. This is just another reminder that when heavy rain falls and streets get flooded, motorists must remember to turn around and take a safe alternate route. There were no reports of major river/wash flooding through this event, however most of the major washes and rivers through the Tucson area were flowing.