Upper level (500 millibar) flow pattern and associated weather across the U.S., early May, 2009
Heat Wave Sears RGV in Early May 2009
Temperatures 7-9° Above Average; Heat Index 105°+ for Many

Yes, it was hot. Even by Rio Grande Valley standards, where heat and humidity are a common denominator from June to September, the combination of heat, humidity, and full sunshine wilted both flora and fauna alike. Gusty southeast winds on most of the first 14 days of May helped little, and added to the ‘blast furnace’ feel to the air, particularly away from the coast. On days when the heat index, a measure of how it hot it actually feels on human skin, relaxed, the combination of wind, lowering humidity, and increasingly drought and sun stressed rangeland maintained an elevated threat for erratic wildfire behavior. Relief from the heat was found on the coast, where the relatively cooler Gulf of Mexico tempered the hot sunshine, even as water temperatures warmed toward late May levels.

Unfortunately, the unusual weather conditions may have been a factor in at least two deaths during the period. Searing heat may have contributed to one fatality of a ranch hand on the King Ranch around the end of April/start of May. A young man, seeking relief from the heat in the cooler Gulf along South Padre Island, reportedly drowned when a strong current, the result of days of elevated seas and surf driven by the persistent southeast winds, pulled him away from the shore.

Climatological data told the tale for the first fourteen days of May. Typical afternoon temperatures ranged from 90 to 95 from Port Isabel to Port Mansfield west to Arroyo City and Brownsville, and 95 to 100 from Kenedy County through Raymondville, Harlingen, west to Pharr, McAllen, and Edinburg. Hottest temperatures were felt in a swath from Brooks County southwest through western Hidalgo County, extending out to the Rio Grande Plains of Zapata and Jim Hogg County. In these areas, afternoon temperatures repeately broke 100°F, with heat index values along the Rio Grande nearing or briefly exceeding 110°F for a few hours between May 3rd and May 6th. At primary climate locations, the following temperatures (day and night) and departures from average were observed:

Mean Temperature and Departure from Long Term Average, May 1-14, 2009
Average Temp.
Port Isabel/Bayview

The differences in temperature are shown graphically for McAllen Miller Internatinal Aiport, below. Graphical temperature departures for Brownsville and Harlingen were also produced.

Why Such Heat, and Why So Long?
One only needs to look at the general circulation, or steering flow pattern, across the Lower 48 United States to describe how the heat built across the far southwestern region of the country, stretching east and southeast along and north of the Rio Grande from New Mexico across Southwest Texas, the Texas Big Bend, and South Texas and the Valley. A flat ridge of high pressure at up at around 19,000 feet, was bordered by fast moving west to west-northwest flow extending from the Pacific coast of northern California and Oregon southeast across the Rockies, central Great Plains, and into the Southest U.S. (lines on the chart, above).

This pattern allowed repeated development of surface low pressure troughs moving rapidly out of the lee of the Rockies. While the primary surface lows tracked across the nation's heartland, drawing Gulf of Mexico moisture northward resulting in a mix of occasional heavy rains and severe thunderstorms which extended through the Tennessee Valley into the Carolinas, occasionally stretching north into the Ohio Valley and south into the Deep South. At the same time, a persistent area of low pressure hung back across the Texas Big Bend into the lee of the Sierra Madre Oriental in northern Mexico. The surface low pressure, combined with the upper level high pressure ridge, increased the thermal depth (also known as the ‘thickness’ within the aforementioned 19,000 foot deep layer, translating to the intense late spring heat.

The pattern began in earnest at the end of April and continued right into the second week of May, before finally loosening its grip by the middle of the month.

McAllen Miller Airport average temperatures and departure from average, May 1 - 10, 2009 (click to enlarge)
Graph of observed maximum temperatures (blue bars) compared with average maximum temperatures (red bars) for McAllen Miller Airport, May 1st through 10th, 2009
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