After a relatively quiet period for several days the situation changed dramatically on July 25^th and continued through the end of July.  The conduit for this change was a surge of Gulf of California moisture caused by Tropical Storm Emilia then located a couple of hundred miles west of the southern tip of Baja California.  The Yuma, AZ VAD wind profile around 12z (Figure 1) showed low level southerly winds approaching 40kts indicating a rather strong surge. With this surge of moisture, surface dewpoints dramatically rose during the early morning hours from the 40’s into the 60’s across Southern Arizona.   This moisture did indeed have good vertical continuity aloft as shown by the 12Z KTUS sounding (Figure 2) with a precipitable water value of 1.46”.  The 00z KTUS sounding (Figure 3), released before any convective activity impacted the immediate Tucson area gave a great indication of the pre convective environment these storms had.  The 00Z KTUS sounding had 30kts of 0-6km shear in addition to surface based CAPE of about 1800 J/kg with a “fat” positive area indicating good updrafts and potential for large hail.  The mid level flow near 500mb was also near 10kts indicated storm motion would be relatively slow causing the potential for flash flooding. 

The first thunderstorms of the day developed between 19 and 20Z over the Chiricahua mountains near Rucker Canyon and the Huachuca mountains near Parker Canyon Lake.  As expected these storms exhibited slow westward motion.  While the thunderstorm complex over the Chiricahua Mountains that moved west through Cochise County had relatively few large hail reports and no reports of flash flooding, this was not the case for the thunderstorm that developed near Parker Canyon Lake.  This storm became severe as it slowly moved westward over Santa Cruz county in the afternoon and merged with a southward moving cell that developed over extreme northwest Santa Cruz.  The merged storm over Santa Cruz county had echo tops eclipsing 50,000 feet with 70+dBZ returns at 30,000 feet.  This led to golf ball (1.75”) hail being reported north of Rio Rico and 1” hail in Patagonia.  The slow movement caused several inches of rain in a couple of hours over Northern Santa Cruz county leading to flash flooding on Interstate 19 at the Chavez Siding Road exit.  Eight inches of water was reported flowing across the Interstate 19 from an unnamed wash which led to its closure for a time during the afternoon.  There was so much hail from this storm that it could be seen flowing through the washes well after the storm had passed.  The storm continued its slow march west into the Tohono O’odham nation where nickel sized hail was reported early in the evening.  It should be noted that this storm eventually merged with another cluster of storms over Maricopa county which evolved into an MCS overnight. 

Thunderstorms also developed over the Mogollon rim in the early evening hours and moved southwest through Graham county toward Pinal and Pima counties.  These thunderstorms produced a strong wind gust of 64 mph with a downburst at the Safford airport.  These thunderstorms also produced some locally heavy rainfall over Northeast Pima and Pinal county late in the evening.