Preceded by more than a week of heavy rain, a slow-moving storm system dropped tremendous precipitation across much of Texas and Oklahoma during the nights of May 24–26, 2015, triggering record-breaking floods. Additionally, many areas reported tornado activity and lightning. Particularly hard hit were areas along the Blanco River in Hays County, Texas, where entire blocks of homes were leveled. On the morning of May 26, the National Weather Service issued a flash flood emergency for southwest Harris County (which includes the city of Houston) and northeast Fort Bend County. The system also produced deadly tornadoes in parts of Mexico, Texas, and Oklahoma. This flood significantly contributed to the wettest month ever for Texas and Oklahoma.
According to Bob Henson from Wunderground, inflow of low-level moisture from the Gulf of Mexico and the combination of very slow moving large-scale storm systems set the stage for the floods. He added,"Upper-level winds were largely aligned with the low-level frontal zone, an ideal setup for “thunderstorm training” (successive downpours over the same area). In addition, rains were concentrated by several mesoscale convective vortices, small-scale centers of low-pressure that developed along the frontal bands, in some cases resembling mini-tropical cyclones." The stationary trough during May is linked to both the El Niño and amplified midlatitude short-wave train.
The system also produced numerous tornadoes across the Great Plains on May 23 and 24. Most of these tornadoes were weak, though EF2 tornadoes caused considerable damage near New Boston, Illinois, and Ensign, Kansas. In Texas, EF1 tornadoes impacted mainly residential areas of Dallas, Houston, and Corpus Christi as well. In the early morning hours of May 25, a destructive tornado struck the Mexican border city of Ciudad Acuña, Coahuila; it was the first twister to strike the city in its history. Numerous cars and buses were thrown and mangled, and 750 properties were damaged or completely destroyed. Several masonry construction homes along the path were completely leveled. 14 fatalities occurred as a result, and an additional 229 residents were injured. The tornado was officially rated F3, though some sources list it as an F4. Additional tornadoes, a few of which were strong, touched down in Texas later throughout the day. This included an EF2 that killed one person near Cameron, Texas. Another EF2 tornado struck Henderson, causing considerable damage in town. In Oklahoma, an EF2 tornado caused heavy damage near the towns of Wister and Panama, while an EF3 tornado killed one person near Blue. Overall, the outbreak produced 75 tornadoes and killed 16 people, making it the deadliest North American tornado outbreak of 2015. This is the largest tornado outbreak in Central Texas history.
Note: One tornado in Mexico was rated F3 but is listed as EF3 in the above table for convenience.
On May 14, 2015, prior to extensive flooding beginning around May 24, flash flood warnings were issued for counties in South East Texas. At least 14 people were killed, including 10 in Texas and 4 in Oklahoma, and another 11 remain missing.
Flooding along the Wichita River prompted evacuations of 390 homes in Wichita Falls. Projections from the National Weather Service indicated that the river would reach a record crest of 25.5 ft (7.8 m) on May 26. Up to 100,000 customers were without power.
Wichita Falls and Corpus Christi broke their previous records for all-time wettest month. Many parts in northern Texas recorded over 20 in (510 mm) of rain.
A 25-foot-by-25-foot sinkhole near a runway at the Dallas/Fort Worth International Airport caused the closure of the runway for a few hours.
Also on May 25, historic flooding along Shoal Creek inundated House Park in Austin, Texas damaging the turf among other things and forcing the Austin Aztex to search for a temporary home.
More than 500 water rescues were carried out by firefighters, which involved for the most part stranded motorists. At least 2,500 vehicles were abandoned, when drivers had to seek higher grounds.
During the overnight of May 25–26, nearly 11 in (280 mm) of rain fell near Houston, setting off further flash floods. Two people drowned after being trapped in their cars while a third was later found in bayou. A fourth person lost their life when an evacuation boat capsized during the morning of May 26. On May 26, Houston Mayor Annise Parker, estimated up to 4,000 properties with significant damage.
During the overnight of May 24–25, more than 12 in (300 mm) fell along the watershed of the Blanco River. In just a few short hours, the river rose from roughly 5 ft (1.5 m) to a crest just over 41 ft (12 m), well over the 13 ft (4.0 m) flood-stage, near Wimberley. This broke the river's all-time peak of 32 ft (9.8 m) set in 1929. Tremendous flash flooding ensued as a result, with areas of Hays County being particularly hard hit.
Entire blocks of homes in Wimberley were swept away by a "tidal wave of water" with officials reporting 350–400 residences destroyed in the town alone. For the first time, the river covered portions of Interstate 35, shutting down traffic for hours. Large trees and debris were deposited across the roadway.
One person in San Marcos, Texas was confirmed dead. Thirteen people were carried away when the home they were in was swept downstream. Residents reported seeing it being destroyed after striking a bridge. One occupant was recovered with significant injuries while the other twelve remain missing. At least 1,200 people have been left homeless along the river.
On May 23, flash flooding along Highway 20 near Claremore claimed the life of a firefighter attempting to rescue people in an apartment complex. He was pulled into a storm drain obscured by the rising waters and carried hundreds of yards away. A fellow firefighter attempting to save him was pulled in as well but managed to escape. The day also saw some roads submerged in as much as 9 inches (230 mm) of water, and thousands of OG&E customers experienced power outages as a result of the flooding in Oklahoma City and Del City.
A station Oklahoma City recorded 19.48 inches (495 mm) of rainfall in May 2015, which made it the all-time wettest month on record. Records date back to 1890.
From May 1 to May 25, 122 tornado warnings were reported by the National Weather Service in Oklahoma, more than in the same period
The Oklahoma Department of Transportation reported on May 26 that a number of highways were closed because of high water or damage from previous flooding.