Boom!

With a fury unseen since September 19, 2010, much of Greater Brownsville, particularly along and west of Paredes Line Road (Farm to Market 1847), was quickly inundated by one to four feet of water after nearly three hours of torrents dropped four to more than six inches of rain (map, above) during the mid afternoon of August 31. The situation began innocuously enough shortly after noontime – moisture deep in the atmosphere was activated by late morning/early afternoon heating which triggered the sea breeze and boundaries set off by early morning storms. Between 2 and 4 PM, however, the thunderstorm cluster organized into a "system" of storms with broad rotation. The broad rotation maintained a core of very heavy rain from Rancho Viejo through downtown Brownsville, including all neighborhoods along and west of Paredes Line Road.

Numerous vehicles were flooded, a school bus was trapped near Price Road and southbound U.S. (Expressway) 77 frontage, and dozens of properties had some water inundation, particularly west of the expressway. The City of Brownsville and the American Red Cross (South Texas) combined to open a shelter near downtown to accommodate impacted families, particularly in the Galaxia Colonia in the city's west side, where flooding was most severe. Though damage estimated were not available at the time of this writing, values are likely to rise into the millions of dollars.

As the system drifted south, the rotation continued to tighten up. By the time the heaviest rains reached downtown Brownsville, a little before 4 PM, the system had morphed into a classic High Precipitation (HP) Supercell Thunderstorm. As downpours continued to flood much of Brownsville, on the cities south side, and into parts of Matamoros, Tamaulipas, the rotating updraft portion of the storm was clearly visible (image, bottom center). Doppler–derived velocity signatures (top right column) suggested a possible tornado in Tamaulipas at around 415 PM CDT; as of this writing, there had been no reports of damage though some videos were made available that suggest a funnel cloud may have formed well south of the Rio Grande.

For some neighborhoods, the 6+ inches of rain was nearly three times the monthly average of around 2.5 inches, in less than three hours!. At Brownsville/South Padre Island International Airport, where official observations are kept, the 1.61 inches on the day punched the monthly totals over 3 inches (3.04), more than half an inch above average (2.44 inches). The last day rains ensured that nearly all of the Valley surpassed August normal rainfall, despite rain on just 2 to 4 days and the month most remembered for hot, dry weather.

Some Reasons Why an HP Supercell Formed...Here
Over the decades, visual evidence of "classic" supercell thunderstorms over/near the Rio Grande Valley has been very hard to find. While HP Supercells are few and far between, the southern tip of Texas is not immune to them. Often, they form in rural ranch areas where cattle outnumber people by the hundreds to thousands. Sometimes, they develop after sunset; other times, they may be rain–wrapped. The classic "corkscrew" updraft seen by storm chasers across the southern and central Great Plains, from north Texas through the Dakotas is a diamond in the rough here. During the late afternoon on August 31, 2015, the diamond appeared, majestic and scary all at once.

The typical season for such supercells across the Rio Grande Valley favors spring and early summer, not the deeper tropical moisture time of late August. In hindsight, the atmospheric puzzle pieces came together to suggest the possibility of such a rare event for somewhere in the Rio Grande Valley. The morning atmospheric profile (first image below; click to enlarge) indicated sufficient moisture (very common) but something else: cooling well above the surface, more common to May and June than late August, and light but turning winds in the lower and mid levels of the atmosphere. Each parameter provided the opportunity for deep, rotating updrafts. That opportunity was enhanced further by a developing jet streak (higher winds) at ~35,000 feet above the ground (second image), which rotated around the base of a trough (upper level disturbance) of lower values centered near Brownsville. That streak may have contributed to allowing the updraft to deepen further, which would sustain a large slow moving rotating thunderstorms. Finally, in the mid levels of the atmosphere (~10,000 feet), ample moisture (also shown in the profile) was present, ensuring any rapidly developing cells would be able to be sustained high into the atmosphere for several hours. More research needs to be conducted to try to unlock all the reasons why the perfect updraft formed near Brownsville, but some signposts were evident earlier in the day.