Flash Flooding and Flood in Texas, June 19 to 22, 2018

Measured and estimated rainfall of between 7 and more than 12 inches in north central Brooks County from the pre-dawn hours through around noon on June 19th created high standing water in and around Falfurrias. More than a dozen roads in town were flooded, some with 2 to 3 feet of water depth, and a number of properties saw floodwater inside of them. Farther north, along US 281 but mainly in Jim Wells County, two to three feet of flowing water closed the highway for a time during the morning and early afternoon. Highest rainfall totals for the event included 12.1 inches at the NWS cooperative observing station 4 miles west of Falfurrias and 8 inches near the center of town. Much of the flooding was the result of a combination of creeks, including Palo Blanco and Cibolo, overspilling their banks in nearby Jim Wells and Duval County, along with sheet flow from the same areas. An unknown number of water rescues were conducted during the afternoon and evening of the 19th.

Heavy rains of 8 to more than 14 inches (estimated) across northern Brooks, northeastern Jim Hogg, eastern Duval, and much of Jim Wells County allowed substantial water flows into the riverine system of creeks, including Los Olmos, Palo Blanco, and Cibolo. Each creek flowed outside of its banks during and well after the heaviest rains fell, for much of June 19th and June 20th before waters receded below bank full. At Los Olmos Creek along the Brooks/Jim Wells County line, the gage reached flood stage (11 feet) around 4 PM CST on the 19th, and continued to moderate levels (12 feet) shortly after 6 PM. The creek would remain in moderate flood for a few hours, before dipping below 12 feet at around 9 PM CST. Slower recession brought the stage below 11 feet by early evening on the 20th.

The peak stage at 12.32 feet ended up being the third highest on record, with only values associated with locally heavy rainfall in late August 1967 and the remnants of Hurricane Fern in September 1971 at higher levels.

Impacts included lowland flooding between 300 and 700 feet into the flood plain, as well as high enough flow to close several farm to market roads on the east side of Falfurrias. Flooding of the Cibolo/Palo Blanco system aggravated high water from the rainfall alone that created initial flash flooding in the area earlier on June 19th.

A band of thunderstorms, moving slowly from west to east, intersected the sea breeze and stalled between Brownsville and Los Fresnos, dumping 3 to 5 or more inches of rainfall in a 2 to 3 hour period, causing significant street flooding and some road closures. Flood prone roads, including Morrison Blvd. in north Brownsville was closed due to 3 feet of water; Dennet road nearby was also covered in 2 to 3 feet of water. Other roads between North Brownsville and Los Fresnos were flooded with an unknown amount of water depth. Several to dozens of vehicles were flooded, with damage unknown as of this writing.

Heavy rainfall measured and estimated between 7 and 18 inches in a 3 to 5 hour period produced locally devastating flooding in Weslaco and Mercedes, just north of the expressway. Measured rainfall at the Weslaco Cooperative station was 15.57 inches for the event, with 11.36 of it falling in a three hour period generally between 3 AM CST and 6 AM CST. Radar estimates suggested higher amounts at the Las Brisas and other neighborhoods in northeast Weslaco, where nearly all homes had 18 inches of water depth or higher inside.

The rain flooded (at least 18 inches) 2500 homes in Weslaco, with an unknown number (but likely in the 1,000 range) in next-door Mercedes. Video, photograph, and first response/law enforcement evidence showed widespread 2 to 4 feet of water depth, with a few poorly draining areas in east Weslaco and Mercedes of at least 5 feet of water depth. An unknown number of vehicles were flooded throughout the area, and the Interstate 2 frontage road was closed through most of the morning. Eighty-six of the eighty-nine school buses in the Weslaco Independent School District sustained significant flood damage.

Publicly reported damage was estimated to be at least $12.5 million; standard doubling estimated the damage at $25 million though in reality, values could be much higher due to poor measurement due to the volume of uninsured properties that were impacted. The rainfall in the east/north side of Weslaco was the highest on record (since 1910) for a single event, besting the Labor Day 1933 Hurricane (15 inches).

Heavy rainfall measured and estimated between 4 and 7 inches in a general 3 to 4 hour period produced flash flooding, mainly in the east half of McAllen stretching east to the Donna area, mostly during the pre-dawn hours through a bit after daybreak on June 20th. Between 2 and 4 feet of inundation was reported by law enforcement where the heaviest rains fell and combined with known poor drainage. The frontage road between McAllen and Donna was closed in most spots during and just after the peak of the rainfall and during the early morning commute. Several hundred homes and hundreds of vehicles received some level of flooding.

Multiple cells and clusters of thunderstorms during the pre-dawn hours into the morning commute on June 20th dropped measured and estimated rainfall between 5 and more than 8 inches in western Cameron County, which caused widespread flooding between La Feria, Harlingen, San Benito, and other locations. The flooding produced locally 2 to 4 feet of water depth, with Santa Rosa through La Feria particularly hard hit. Interstate 2 frontage road was closed from Harlingen to McAllen due to the intense rainfall, which fell at an estimated rate of 2 to 3 inches per hour in a few spots for 2 to 3 hours - and on top of prior rainfall earlier in the week.

Detailed damage for this particular event was not known, though total damage through multiple agencies along with standard doubling for uninsured properties, as well as significant damage to drainage canals, suggested event totals in Cameron County were over $60 million. For this particular event, which included some of the highest rainfall rates, damage was estimated at around $35 million. There were an unknown number of properties and vehicles damaged in this particular flood, though the numbers are likely above 1,000 for each case.

Full details will be available within 12 to 18 months of the event.

The same clusters of thunderstorms that dropped between 12 and an estimated 18 inches in the Weslaco/Mercedes area early on June 20th dropped an estimated 5 to 10 inches on central through northeastern Willacy County (primary population areas), including a number of farm fields (agricultural damage unknown). Final damage totals from initial emergency management surveys of public damage were nearly three-quarter of a million dollars, with standard doubling estimating the value at just over $1.5 million.

One day after 3 to more than 5 inches fell between Brownsville and Los Fresnos during the afternoon and early evening hours, a repeat event occurred, fed by deep tropical moisture and multiple boundaries left behind from devastating morning flooding across western Cameron and eastern Hidalgo County. A persistent line of torrential rainfall dominated through the early evening commute, with another 3 to 6 inches having fallen for a two-day total of 6 to 11+ inches. The second day rainfall produced more widespread flooding across locations in and east of Brownsville and Los Fresnos, with farm and subdivisions under 2 to 3+ feet of water when all was said and done.

The additional flooding on saturated soils (pump priming from June 19) likely produced more damage this second go-round, including more erosion to drainage canals/ditches and mainly rural roads. Doubling of the damage left a total of around $20 million.

The Roma Fire Department reported that Arroyo Roma exceeded its banks, flooding parts of town with 1 to 3 feet of standing and flowing water during the pre-dawn hours of June 21st, 2018. The flooding was associated with the initiation of a thunderstorm system that would later cause significant flooding in western Hidalgo County, including the McAllen/Mission metropolitan region during the daylight hours later that morning. Measured and estimated rainfall was between 3 and 7 inches across southern and central Starr County. Estimated public damage, along with a standard doubling, left nearly $3 million in property and other damage (estimated).

State and local emergency management officials, as well as ground truth rainfall totals, reported widespread flooding in south-central Hidalgo County, focused on the Mission/Alton/West McAllen area as a firehose of thunderstorms persisted for more than five hours through the morning commute until around noon before finally dissipating. Estimated and measured rainfall for the event ranged from 6 to 11 inches, with periods of 2 to 3 inches per hour embedded during the heart of the morning commute. Numerous roads, thousands of homes and vehicles (specific number unknown) were under a range of water depth, ranging from 18 inches to 3 to 4 feet in either poorly draining neighborhoods from the west half of McAllen through Mission.

A peak total rainfall of 18.31 inches fell in Mission for the combined events of June 20-22. Estimated damage, using a combination of county-reported damage and standard doubling, was an estimated $22 million for this particular event. Actual values may be much higher as data continues to come in throughout 2019.

An additional 1 to 2.5 inches of rain fell over already flooded neighborhoods in the McAllen/Mission area early on June 22, exacerbating already dangerous conditions from prior flooding on June 20th and 21st. Neighborhoods in the south and west side of McAllen that had a foot or more of water depth temporarily saw levels rise higher during the peak of the heavy rainfall, which fell for an hour or two during the pre-dawn hours. Additional damage was unknown from the prior estimates.