Rain, cooler temperatures arrive in the Rio Grande Valley to start September 2009

Much welcomed rain helps break heat wave and bring some drought relief to the Rio Grande Valley and Deep South Texas to end August and begin September

Rainfall estimates from https://water.weather.gov from the end of August through early September. Contours depict best estimates based on observed data and may not match the colored pixels in some areas. The Kenedy County Wind Farm, which was not removed from the rainfall estimate calculations, is contoured in black.

Rain, Lower Temperatures Break Heatwave
Some Drought Relief as August ends and September begins

Preliminary Rainfall, August 28 through September 2 2009
Location	County	Rainfall
2.2 SW Palm Valley	Cameron	3.10
Rancho Viejo	Cameron	3.08
Harlingen 4.7 WSW	Cameron	3.03
Santa Ana/Lower Rio Grande Valley	Hidalgo	3.00
Harlingen 4.3 WSW	Cameron	2.73
Hebbronville	Jim Hogg	2.64
Bayview/Cameron County Airport	Cameron	2.41
Harlingen/Cooperative	Cameron	2.22
Harlingen 2.6 ESE	Cameron	2.12
Harlingen/Valley Airport	Cameron	1.61
Falfurrias	Brooks	1.56*
South Padre Island	Cameron	1.55
Los Fresnos	Cameron	1.33
McAllen/Miller	Hidalgo	1.19
Brownsville 2.2 W	Cameron	1.18
McAllen/Cooperative	Hidalgo	1.14
Brownsville SPI International Airport	Cameron	1.14
Falcon Lake	Starr	1.04
Rio Grande City	Starr	1.04
Rio Hondo 9.4 NE	Cameron	1.04
Armstrong	Kenedy	0.81
Brownsville 4.5 NNW	Cameron	0.71
Falcon Lake	Starr	0.61
Port Mansfield	Willacy	0.46
Sarita	Kenedy	0.45

Overview
Those liquid drops from the sky are no lie. Indeed, like manna from heaven, some quenching and drenching rains have finally developed across Deep South Texas and the Lower Rio Grande Valley to end a record setting hot and dry summer (June through August) and ease the region into September. Between August 28th and September 2nd, 2009, thunderstorms dropped measured and estimated 1 to 3 inches across portions of the lower RGV, and a large pocket of 2 to 3 inches of rain in parts of Starr, Brooks, and Jim Hogg County. The rain, accomapanied by abundant cloudiness, has not only crushed the triple–digit heat wave, but held daytime temperatures in the 80s in many spots on August 31st. Temperatures rebounded to between 90 and 95 degrees on September 1st and 2nd, but were cut down by scattered to numerous afternoon thunderstorms or associated outflows. These lower temperatures brought an end to the prolonged period of above average readings, which in many cases lasted more than 65 days!

Sunshine and warmer temperatures returned to close the week before Labor Day. However, the combination of the earlier rains, light winds, and lenghtening nights helped return morning dew to the region, and kept afternoon temperatures fairly close to seasonal averages.

Pattern Matters
Subtle changes to the general summer pattern created large difference in the weather on the ground. The most significant difference was the development of a subtropical jet, located underneath an upper ridge that continued across the Four Corners region of the U.S. A weak longwave upper level trough, or disturbance, settled in near the Baja peninsula during the weekend of August 29th and 30th, with energy from the disturbance propagating eastward, across northern Mexico and eventually Deep South Texas. This energy combined with a weak low level trough to add tropical moisture from the lower levels to the higher level moisture supplied by the subtropical jet, seeding the atmosphere and setting the stage for increased rain potential. Smaller disturbances ejecting east from the larger system continued to produce abundant multi layered clouds, as well as times of showers and thunderstorms, through September 2nd. The strengthening four corners ridge eventually brought enough north to northwest flow into South Texas to push the deeper moisture southward into north central Mexico, bringing the aforementioned drier air back to Deep South Texas and the Rio Grande Valley.

Infrared satellite photo from August 31st, 1645 UTC (1145 AM CDT), showing persistent subtropical jet and deep moisture across Deep South Texas

Infrared satellite image showing deep moisture flowing along the subtropical jet (dark blue arrow). Some moisture from eastern Pacific Hurricane Jimena was feeding into the system.