After a relatively
quiet period for several days the situation changed dramatically on July
25th 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.
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
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.
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.