Boo! Brownsville Takes It's October Flood Turn: 4 to 7 Inches Inundates Southmost, Airport, East Side October 30, 2015

This time, Brownsville took the main hit.

While the remnants of a tropical storm were not a factor, the atmosphere "exploded" into action on the day prior to Halloween, dumping buckets of rain on the City of Brownsville, mainly from downtown east toward the Port, and including all of the neighborhoods in Southmost, which took the brunt of the torrents. City–wide, 4 to 7 inches fell, with highest totals in the more densely populated sections of town where poor drainage is more of an issue as well. The heaviest rainfall, four to six inches, fell in a two to three hour window between 11 AM and 2 PM. Flooding rapidly ensued, transitioning from nuisance to more life–threatening in less than an hour; by noon, feet of water were inundating some roads and by 2 PM up to 5 feet covered one of the most prone areas of town – the "Four Corners" area where State Road 4 (International Blvd.) intersects State Road 48 (Boca Chica Blvd./Padre Island Highway). Neighborhood roads in Southmost were inundated with two to three feet of water, and dozens of homes and businesses flooded due to a combination of high water that filtered into homes and vehicles that created waves of water which surged into them. At Brownsville/South Padre Island International Airport, the 6.5 inches that fell with the storms was the most rapid accumulation since 6.48 inches fell with a tropical feed of moisture on September 19, 2010. For the day, the 6.55 inches fell just behind Hurricane Dolly’s 6.68 inches, and the value ranked 3rd highest daily total all–time in October, dating back to 1878, but was the highest total to occur without the aid of a tropical cyclone (depression, storm, or hurricane). For some, particularly downtown, the total on October 30th nearly mirrored those from August 31, 2015.

Why the Atmosphere Went "Boom"!
Shown above is the upper atmospheric pattern that set up prior to the flooding rainfall, and the small tornadoes, across the Lower Valley on October 30th. Similar to October 24th, a high amplitude (north to south) upper level trough of low pressure slid across the southwestern U.S. and northern Mexico. The upper level trough induced an intensifying surface and low level low pressure trough; increasing southerly flow brought summer–like swelter to Deep South Texas beginning on the 29th, especially at night when temperatures prior to the arrival of rainfall held close to 80°F, more than 15° above late October averages. The arrival of deep tropical moisture (left, atmospheric profile) was the powder keg for initial, isolated but potent rotating thunderstorms from northern Cameron through Kenedy County just before daybreak. Such storms often form ahead of a developing squall line, which was the case here. The squall line, perhaps a minimal version of a quasi–linear convective system, soon intensified as the upper level energy (above description) reached the Lower Valley. The heart of the system formed southwest of Matamoros and soon would ooze into Brownsville and once again turn into a mesoscale convective system, which held the rain in place long enough to create the second flood of this type in Brownsville in sixty days.

If flooding rain weren’t enough, excessive, nearly continuous cloud to ground lightning featured in the core; the statuesque NWS Brownsville/Rio Grande Valley Doppler Radar dome was in the way of one such strike, captured here, courtesy of Electronic Technician Derek Urch. By stroke of luck, and skill, backup circuit boards were in stock and were installed later during the afternoon when conditions improved, and the radar was back in service by early evening. Several thousand Lower Valley residents did lose power for a time, however.

A final damage estimate from the flooding and lightning will be available at a later date.

By the afternoon of October 30th, rural, agricultural central Willacy County had to wonder what they had done to deserve such a cruel fate. After a three–day slam (October 22 to 24) that left up to 14 inches of rain, two early morning small tornado producing storms (with their own locally heavy rain) were followed by the northern end of the Brownsville deluge by mid to late morning on the 30th. Radar and observations indicated another 2 to 4+ inches fell over the same areas, raising water levels once again. Pity San Perlita (above). Following several days of dry weather between the 25th and 28th finally drained the majority of water, the still–saturated soil and full drainage ditches and pipes quickly overflowed that afternoon; high water of 1 to 3 feet covered many streets and surrounded more than a dozen buildings. The American Red Cross, along with the Salvation Army, had already opened a shelter after the initial events a week earlier.

In total, an estimated, preliminary monthly rain total of 14 to 18 inches, perhaps 20 inches in some locations, gave Willacy County an October to remember; combined with the pockets of 10+ inches from southeast Hidalgo through northwest Cameron, as well as around Brownsville, rainfall for the Lower and Mid Valley likely exceeded benchmark recent Octobers in 1997 and 2002.