After more than two weeks with no rain across Deep South Texas and the Rio Grande Valley, September briefly looked a little more like it should as weak sea breeze convergence combined with just enough moisture to trigger locally heavy rainfall near the coast on the 15th and 16th. The convergence combined with an upper level disturbance to kick off significant rains on the 17th across the ranchlands of Brooks, Starr, Jim Hogg, and Zapata County, with lesser coverage on the 18th across the central and eastern ranches before more isolated activity shifted back toward the coast on the 19th. Update: On the 19th, Brownsville recorded the largest single–day rainfall since late June from an isolated thunderstorm that dropped 1.4 inches of rain in less than two hours. Nearby locations reported little or no rain. For the ranchlands, the torrents of the 17th were the first notable rainfall since August 25th, and the most concentrated since the squall line of June 22nd, 2011.
Severe Weather Joins In
Some of the storms, particularly on the 17th, arrived with more than just rain; radar indications of hail cores and microbursts appeared across rural areas of the Brooks and Jim Hogg County ranchlands through the evening. By late evening, one cell strengthened while moving south into southwest Starr County. The intense cell dropped a microburst on the community of Fronton sometime between 1030 and 1045 CDT (west of Roma, below); damage to a number of roofs, trees, and power lines was reported by local media. Winds were suspected to be at least 60 mph based on the initial reports. More detailed information will be available in the September Storm Data report, which will be available in October, 2011.
Radar image showing 0.5° base reflectivity (left) and velocity (right). Note the "bow" shape to the storm cell approaching Fronton; microbursts can form within both the bow and the "head" (north of Fronton east of Falcon). Brighter reds in the velocity field indicate strongest winds.
While the ranchlands received generally 2 to 4 inches, locally more than 5 inches by rainfall estimate, other areas starved for downpours were largely missed by them. Aside from Arroyo City, which received nearly 3 1/2 inches of rain on the 17th, and a few pockets in Willacy County which received between 1 and 2 1/2 (estimated) rainfall on the sea breeze, most locations in the Valley, as well as those along State Highway 285 across extreme northern Jim Hogg, Brooks, and Kenedy County, received less than an inch for the entire period. In a month where daily rain frequencies reach their annual peak, these values are paltry unless strung together for days or even a week at a time. As of September 19th at 7 AM, Brownsville (0.05 observed, 3.52 average), Harlingen/VIA (0.26 observed, 3.25 average), and McAllen/Miller (Trace observed, 2.92 average) need a lot of rain in a hurry to make up the difference. Full month averages include Brownsville (5.92), Harlingen/Cooperative (5.26), and McAllen/Miller (4.47)
The map below tells the continuing tale of the 2010/2011 drought: Temporary relief among the persistence of the warm to hot, rain free pattern across Texas. Northerly flow deep into the atmosphere behind a developing low pressure system across the eastern U.S. will push dry air over the region for the final weekend of September, continuing into the last work week of the month. Such a forecast, should it pan out for the next ten days, will lock up all time driest water year levels for many locations in Deep South Texas, and ensure top 5 driest water years for the remainder. The table below shows how much rain would be needed to reach average as the 2010/11 water year draws to a close.