Rainy Relief, Just in Time: March 8-10

After a winter when the El Niño wet and cool forecast failed to materialize, spring, albeit briefly, had other ideas. A single atmospheric cyclone, with origins in the East–Central Pacific Ocean north of the more typical subtropical zone where El Niño–borne cyclones form, took a sharp dive into Mexico – Central Mexico on March 9th and 10th, before curling across the Valley on March 11th and dissipating into the strong westerly flow in northeast Texas on the 12th. The deep, non–tropical cyclone, which for some became known as the "Mayan Express" due to its peak intensity in Mexico, produced widespread and in some cases record flooding rains across Louisiana and east Texas and snow as far south as Zacatecas State in central Mexico at 7 thousand foot elevation! There were at least six related fatalities in east Texas and Louisiana directly or indirectly from the floods as of mid March. For the Rio Grande Valley, the system produce drought–relieving rains, but did create a few headaches along the way.

Rainfall
The rains came in fits and starts, not too surprising given the type of system – "closed off" non–tropical cyclone prior to arrival in the Valley. [Note that open systems tend to maintain energy release on the east and poleward side in the northern hemisphere, which can produce copious rainfall in more organized fashion. Such was the case from Houston through Shreveport and Lake Charles, Louisiana.] Initial, welcome rains dropped an inch or more across Jim Hogg, Starr, and Zapata County; the cold temperatures above the warm surface helped with hail formation in stronger cells between 8 PM and 2 AM on the 8th/9th. Across the Valley, the atmosphere was effectively "capped" – which meant surface warmth and humidity could not penetrate even warmer air above the inversion. Rain was walled off from developing across the populated Rio Grande Valley while the atmosphere was at its most unstable above the cap. This would change rapidly just before daybreak, as the "cap" weakened. A band of thunderstorms developed rapidly across central and western Hidalgo County, and would ultimately drop up to an estimated 4 to 8 inches of rain across the rural King Ranch of Kenedy County and along the Coastal Bend from Kingsville to Corpus Christi by mid morning. In Hidalgo County, the band generally dropped between 1 and 3 inches of rain (highest north), which was mixed with hail at times (below).

Rainfall, Continued
As the disturbance slid toward central Mexico on the 9th, a spoke of energy rotated on its east side and regenerated bands of thunderstorms with locally heavy rainfall, this time across the Lower Valley, during the afternoon and evening of the 9th. Additional light to moderate rains built back across the mid Valley; in all, for the period from sunrise March 9th through sunrise March 10th, 2 to 3 inches fell east of Brownsville, with 1 to 2 inches elswhere along U.S. 77/IH 69E. On March 10th, with the core of the disturbance edging northward toward Zapata County, afternoon sunshine destabilized the atmosphere substantially across the Upper Valley and along the lee of the Sierra Madre through Tamaulipas and Nuevo Leon. Bands of thunderstorms broke out prior to sunset and would continue a few hours after, peaking along the river east of Rio Grande City in the colonias of La Casita, Alto Bonito, and La Victoria, where flash flooding shut down U.S. 83 and reached into a few homes. In total, the entire region received one to four times the March monthly average in less than 48 hours. The core of the cyclone passed early on the 11th, taking the rain with it – but not before dumping enough in all areas to eliminate, for the time being, dry to moderate drought conditions.

Damaging Wind and Hail
As the "cap" eroded just before daybreak on March 9th across the Valley, southerly winds at 2 thousand feet were as high as 71 mph (Brownsville) at just after 5 AM CST. When thunderstorms developed rapidly between 5 and 530 AM, they were able to tap into some of these winds and pull them toward the surface. In addition, very cold air above the weakening cap supported hail formation. Fortunately, storm motion – also driven by those speedy low level winds – was fast enough to keep rotating supercell thunderstorms from developing; hence the more linear look to the cells as they cruised to the north–northeast.

Still, some cells along the line were able to sustain enough updraft to produce hail, which fell briefly in spots across the McAllen/Edinburg area, with a few reports of half inch to quarter sized hail just before 6 AM. Sometime between 630 and 7 AM, one cell was able to drop an estimated 65 mph wind gust on the small town of Linn, where a power pole fell and several mesquite trees were uprooted along FM 1017 just west of the U.S. 281/IH 69C expressway. The cells would further consolidate into a line west of Linn and extend toward La Joya before lifting out by 9 AM.