Atmosphere just needed a trigger

HARLINGEN — Long before Monday’s dawn, an innocuous thunderstorm system shouldered through the heat and humidity of southeast Texas, spilling its thunder, lightning and rain from Dallas to Houston and on to Galveston.

This system was not particularly powerful, and not at all unusual for Texas in summer, and it reached the coast about dawn before slowly slewing off to the south, drifting down the shores of the Gulf of Mexico.

This was the humble genesis of what will be remembered as the Great Flood of 2019 in the Rio Grande Valley.

Primed for an explosion

The catalyst for what was about to befall the Valley can be traced back two to three weeks,

Unseasonably hot and humid weather had been locked in day after day, with the normal mid-90 high temperatures for early June popping up to hit 100 and sometimes approaching 108 and even 110 farther up the Valley.

Heat index alerts and warnings over “feels-like” temperatures as high as 115 to 118 were being issued nearly daily by the National Weather Service in Brownsville.

As the cooler air of the storm front began to shift back over land by mid-afternoon Monday, still trending to the south, a volatile, charged atmosphere was primed for an explosion of pent-up energy.

“First of all, we’d been very hot, warm, muggy with very moist air near the surface, and that contributed to a lot of instability in the atmosphere,” said Joshua Schroeder, a meteorologist at the Brownsville weather office. “Part of that was the time of the day. Everything kind of started coming together in mid to late afternoon when temperatures are at their highest and the atmosphere is most unstable.”

Such conditions, as Schroeder noted, are not uncommon here in the Rio Grande Valley. What was different was the emerging edge of the slow-moving storm front into South Texas.

“Really the kicker was an outflow boundary, kind of like a mini-cold front, which pushed down from the north from storms that were ongoing the previous night from the Dallas area, Austin, up toward Houston,” he said “That boundary kept pushing on south and it never really lost a lot of steam and it plowed into that hot, unstable air.”

The storm hits

By late afternoon, meteorologists at NWS Brownsville were becoming alarmed, and just before 5 p.m. issued a tornado warning and a severe thunderstorm warning for Kenedy County and northern Willacy County.

Although no tornado is believed to have touched down, the weather watchers saw tornado-like signatures showing up in Doppler radar of the storm system as it continued dropping to the south.

“There also was a little bit of component of the sea breeze that pushed in that afternoon as well, and when that sea breeze collided with the outflow boundary over Kenedy County, that’s when things really kicked off with what we call the high-precipitation supercell storm,” Schroeder said. “It kind of coalesced from there.”

Officials at the Brownsville station issued a storm alert with what they thought a reasonable prediction: Up to four inches of rain in some areas of the Valley.

Records shatter

As the evening progressed, the rain began to fall, increasing to shuddering waves of sideways water which churned across the Valley until around midnight. In many areas the heavy rain stopped and a gentle to moderate precipitation took over for several more hours.

Although the damage was still to come, by then the plot was written. As rain-engorged drainage ditches and highways continued to fill, incoming storm water was finding nowhere to go.

In Raymondville, 9.7 inches of rain was recorded officially, although city officials believe some areas received much more. The previous record rainfall for the date was 1.83 inches set in 1922.

In Harlingen, 6.29 inches was the official recorded rainfall total, breaking the previous record for one day, 2.23 inches, set in 1926.

For Weslaco, with 6.95 inches having fallen, it was more of the same. The previous record for the date was 3.72 inches set in 1951.

McAllen had 3.30 inches, breaking a 1973 record of 1.83 inches, and Edinburg had 2.49 inches, besting a record of 1.36 inches set in 1973.

Even near the coast where rainfall was lightest, Brownsville recorded 2.06 inches, breaking a record 2-inch rainfall for the date set in 1920.

In many Valley cities, officials insist certain areas received much more rain — perhaps twice as much — as the official totals indicate, and meteorologists at the National Weather Service generally agree.

Storms such as this frequently dump precipitation haphazardly even as close as a block or two away, but meteorologists can only go by the amounts collected in the official rain gauges.

The heaviest rainfall during the storm estimated at more than 12 inches in some places is believed to have occurred in a roughly oval shape with western Willacy County the boundary to the north, western Harlingen in the east and eastern Hidalgo County in the west.

In most Valley cities, not only did the rainfall break daily records, in many cases the single-day totals broke monthly records for June.

“In the long-term historical record, late June is normally not a real wet time,” Schroeder said. “But in the three or so years I’ve been living here in the Valley, it seems like we’ve had pretty decent rains every year, whether it was from sea breeze shower clusters or of course the great flood last year.

“And then again, there’s what happened Monday night.”