National Weather Service United States Department of Commerce


Photos from urban (Weslaco) and rural (Willacy County) flooding after two bouts of torrential rains dropped more than a foot of rain in each location. Credits: Willacy and Weslaco/Donna/Mercedes Emergency Management, McAllen Monitor, and NWS Brownsville/Rio Grande Valley, KRGV Channel 5, and KGBT Channel 4 Facebook Pages.
They Needed Noah’s Ark
Tropical Moisture, Hurricane Patricia’s Remains Create "Lakes" Willacy and Weslaco
More than a Foot of Rainfall Inundates Communities, October 22 through 24, 2015

We thought this might happen.

In October 1997, when the combination of a rapidly strengthening El Niño and an equally strong, positive Pacific Decadal Oscillation was present, the first dozen days of October featured abundant tropical moisture across the Rio Grande Valley, fed by strong and deep (atmospherically) southerly flow that tapped not only the Bay of Campeche but the very warm eastern tropical Pacific Ocean and the remnants of Hurricane Pauline. A strong cold front would arrive on the 13th, which provided the "knockout blow" of additional rains to produce significant water inundation in eastern Cameron County. Ten to fifteen inches of rain fell in Cameron and Willacy County; a brief report on the event can be found here.

Fast forward to 2015. While the specific setup was a little different, the outcome was quite similar. After spending much of September of Texas, and keeping tropical moisture (and cyclones) at bay, atmospheric high pressure shifted into the central Gulf and amplified into Mississippi and Ohio Valley during the week of October, as the season’s first strong atmospheric trough slid through the Canadian Maritimes and brought the first freezes and frost to the Mid Atlantic and Northeast U.S. during the weekend of October 17–19. The sprawling, strong surface high that stretched from eastern Canada into the Gulf pushed persistent east/northeast swell across the Gulf, and while fair weather ruled the Rio Grande Valley, the beaches of South Padre Island filled with water during each high tide and began to slowly erode with pounding surf that began the same weekend and continued through October 24th.

Perhaps the harbinger of rough weather to come occurred during Wednesday evening, October 21, when a small scale low pressure system just east of Tamaulipas helped ramp up easterly winds for several hours on South Padre Island, gusting to Tropical Storm force at times. The winds, on top of already high seas and surf, created a "mini surge" of tidal flooding into the dunes at South Padre Island, and pushed water along the bayside at Port Isabel to more than 3 feet above Mean Lower Low Water, or nearly 2 feet above ground at the immediate shoreline.

The very next afternoon (October 22), as tropical moisture continued to stream north between the amplifying ridge and a slow moving but broad disturbance moving through the southwest U.S., scattered to numerous afternoon thunderstorms mainly along and east of U.S. 77/IH–69E coalesced into a system of storms that flooded rural Willacy County and turned much of the agricultural–rich farmland into a lake. Moisture continued to pool across the Rio Grande Valley on October 23rd; heavy rains formed well north of the region, closer to the better forcing provided by the aforementioned upper level disturbance. However, on Saturday, October 24th, atmospheric puzzle pieces aligned to create some of the highest single event rain totals since Hurricane Dolly in 2008: The upper level disturbance moved into southwest and west Texas, lifting moisture into organized rain bands from Tamaulipas and Nuevo Leon into the Upper Valley and ranchlands by daybreak; the remnants of Hurricane Patricia raced northeast toward the Lower Valley and hooked up with the disturbance as each reached the Lower and Mid Valley, ultimately dumping 5 to 12 inches of rain and putting parts of Weslaco/Donna, Santa Rosa, and much of Willacy County under several feet of water.

In the end, thanks to excellent community preparedness and perhaps a little bit of luck, there were no known injuries and zero deaths from the rising water. The National Weather Service salutes our public safety officials and first responders for a job well done before, during, and after perhaps the most impacting weather event, at the time, of the year. The October Flood of 2015 will not soon be forgotten by residents and drivers affected by it.

Radar estimated rainfall from late afternoon through early evening October 22, 2015, in Cameron and Willacy County
"Lake" Willacy? Radar estimated rainfall, showing 6 to 9 inches in a small core near San Perlita, TX, after more than five hours of rain October 22.

Round 1, Oct. 22: Willacy Farm Country Goes Under Water
A plume of tropical moisture through the mid levels of the atmosphere combined with a weak upper level disturbance (and perhaps other atmospheric forcing to be determined) created a slow–moving intense thunderstorm in northern Cameron County just after 3 PM. The storm eased into rural, agricultural Willacy County after 4 PM but not before dropping 2 to 4 inches between the north side of Harlingen and Rio Hondo. While over Willacy, mainly along and east of Farm–to–Market Road (FM) 1420 and west of the Laguna Madre, the cell stopped on a dime, began a broad rotation (known as a Mesoscale Convective Vortex) and dumped more than three hours of torrential rain in and near the communities of San Perlita, Willamar, and Santa Monica. As the rain totals increased, the water piled up; FM 1420 was inundated by several inches to more than a foot of water (some flowing), and nearby farms turned into one big lake, with water "as far as the eye can see" according to a reporting team from KRGV Channel 5 News. Photos and anecdotal evidence suggested two to three feet in the fields during the peak of the storm and probably well into the overnight hours. In Raymondville and Lyford, heavy rains estimated at 2 to 4 inches fell, with urban flooding that would later reach into a few businesses in town due to additional drainage.

Round 2, Oct. 24: From Weslaco to Willacy, Water Water Everywhere
The Willacy County flood on the 22nd was the warm–up act to the main event that followed on Saturday. As the puzzle pieces of upper level energy and the remains of northeast–racing Hurricane Patricia fell into place, the expected bands of persistent, efficient, torrential rains began just before sunrise. First up was a thin band across the Upper Valley and ranchlands between sunrise and noon, from north of Falcon Dam (Zapata/Starr line) to southwest of Hebbronville (Jim Hogg). With the initial arrival of some of Patricia’s remains, total rainfall was estimated between 4 and 8 inches. Fortunately, with much falling over open rangeland, flooding was likely minimal; arroyo headwaters in northwest Starr County may have filled toward bankfull, but there were no reports of anything more than nuisance flooding around Rio Grande City and Roma.

Atmospheric profile for Brownsville from 7 AM October 24, 2015
Atmospheric profile at Brownsville, TX, 7 AM October 24, 2015. In addition to the strong southerly winds and shear favorable for "mini–supercells", the moisture filled column value of 2.57 inches was nearly one half inch higher than the prior record for the date (2.10 inches), back to 1948.
250 mb flow pattern across the U.S. at 7 AM October 24, 2015
250 mb flow (~33,000 feet or so) at 7 AM October 24, 2015. The combination of a favorable location for moisture lift east of the southwest Texas/northern Mexico upper disturbance (maroon) and a subtle enhancement by the jet streak associated with the remains of Patricia (orange) fed the deepest moisture into several bands of torrential rain across the Lower Valley during the afternoon.

Round 2, Oct. 24: From Weslaco to Willacy, Water Water Everywhere (continued)
As the Upper Valley/Ranchlands band moved east, it linked up with the bulk of the remaining moisture from Patricia and a second, even more intense band began dumping buckets of rain in southeastern Hidalgo County soon after noon. The rain persisted at a 1 to 2 inch rate per hour for four hours, with nuisance flooding before 1 PM soon becoming a full–on flash flood before 2 PM with numerous water rescues from flooded neighborhoods beginning shortly thereafter and continuing through the rest of the afternoon. Several inches to at least a foot of water rose into homes in the Las Brisas subdivision on the east side of town, with three to five feet of water on neighborhood streets. At least three feet of water submerged vehicles along the frontage roads between the Westgate and Airport Road exits during the peak of the flash flooding between 3 and 6 PM. Additional high water closed portions of FM 1015 between Weslaco and Progresso (which also had flash flooding of unknown depth)and FM 493 near Donna. The estimated and measured 10 to 12+ inches of rain in the Weslaco/Progresso area was far to much for drainage systems to handle; flooding worsened in some areas even well after the rain ended, and continued through at least Tuesday, October 27th, affecting dozens of homes and businesses.

Insult to Injury: "Lake Willacy" Expands and Deepens
While Weslaco was beginning to see rain taper off by mid afternoon, Cameron and Willacy County were just getting started. Moisture from Patricia, additional lift from the slowly east–moving band, and the approach of a front/wind shift from the ranchlands combined to produce a two hour period of blinding rains across most of Willacy, with the highest totals once again in the San Perlita–Willamar–Santa Monica area. Radar estimates of 3 to 6 additional inches in about two hours deepened already high water; water rescues were underway prior to the heaviest rains, which arrived just before 3 PM and continued through 5 pm before exiting into the Gulf. Though homes are few and far between (mainly farm houses or workers homes), the majority likely had water inside them by late Saturday. Hazardous, high water across nearly all of the County east of the expressway (and the larger communities) continued through this writing (October 28). Two additional bands of heavy rain, sparked by the front and developing coastal gale, dumped 2 to 5 inches from the Port of Brownsville to just north of Port Isabel, including South Padre Island, before the torrents shifted into the Gulf around sunset.

Both Hidalgo and Willacy County were readying paperwork for Federal States of Emergency due to the widespread flooding and dozens, if not hundreds, of affected properties. Damage will easily run into the tens of millions of dollars when assessments are complete; final values will be available at a later date.

Composite reflectivity loop for northern Cameron and Willacy County (round 1 flood), October 22 2015
Composite Reflectivity for October 22, 2015, from 358 through 659 PM, the time of the heaviest rainfall. Click for a larger image.
Composite reflectivity loop for the Lower and Mid Rio Grande Valley, October 24, 2015
Composite Reflectivity for October 24, 2015, centered on the Lower and Mid Rio Grande Valley from 1201 PM through 6 PM, the time of the heaviest rainfall. Click for a larger image.
Water Vapor satellite for October 23, 2015 7 PM.  Click for a full animation for the 24 hour period that follows
Click for an animation of the 7 PM, October 23, 2015 through 7 PM, October 24, 2015 water vapor (satellite) images that led up to and included the flooding rainfall across the Lower and Mid Valley.

Rainfall, October 19th through 26th, dominated by events on October 22 and 24, 2015
Note: The vast majority of the rainfall shown occurred during the afternoon of October 22nd and from after midnight through just after sunset on October 24th, 2015.

Rainfall Percentage of 1981-2010 Average, October 1 through 26, 2015
Comparative data based on 1981 to 2010 averages.