General weather pattern across the U.S. during summer 2012
Critical weather events across the eastern two–thirds of the U.S. overlaid onto map of average 500 mb height contours from June 1 through August 31, 2012.
Another Summer, Another Smoker
Hot, Dry Weather Dominates RGV for Second Year in Row

If you sweltered through the heat this past summer (June through August, 2012), you weren’t alone. For the second year in a row, and third of the most recent four (2009), many locations across the Rio Grande Valley recorded a top ten hottest summer, with records dating back more than 100 years in several locations. For the vast majority of the Rio Grande Valley population, temperatures were among the top five hottest all time (see table below). By early September, McAllen/Miller Airport had exceeded 60 days with high temperatures at or above 100°F; this ranked 2012 with the third highest number of 100°F days behind 2009 and 1998 (tied at 78 days for the calendar year). A map of 100°F days for summer 2012 is shown at right, bottom. While seasonal temperatures were similar to 2011, rainfall varied when compared with the withering drought (through September 2011). Periodic weak upper level disturbances assisted the sea breeze in Cameron County, leading to nearly 10 inches of rain in Brownsville; some of these disturbances combined with uplift provided by thunderstorm outflow boundaries across the Rio Grande Plains ranchlands to leave similar totals across portions of Jim Hogg, Starr, and Zapata County (right). June 30th was one such day. Unfortunately, no such rainy luck found the mid Valley, as rainfall across much of Hidalgo County fell below 25 percent of average.

Why so Hot...Again?
The map above tells the tale of summer 2012 best. For a second year in a row, robust high pressure extending well into the atmosphere across west Texas and New Mexico, often stretching north and east to cover much of the central United States and leading to harsh drought conditions throughout the corn, soy, and wheat belt. The ridge, sometimes referred to as "La Canícula" by Mexicans along the border region, compressed air into hotter than average levels in these areas. Early in the summer, the ridge occasionally poked eastward into the Deep South and Mid–Atlantic region; a record heat wave of more than 10 days plagued Washington, DC, with temperatures more typical of McAllen, Texas, from the end of June into early July. Frequent expansion north and east of the ridge did help quench bits and pieces of Texas’ Drought; disturbances pulled Gulf and Atlantic moisture along decaying "fronts" from southeast to east Texas and provided ample rainfall to alleviate much of the drought in those areas. Much of the moisture and energy faded before reaching the Valley, and Severe to Extreme Drought dominated as summer moved toward autumn.

There may be several interlocking reasons that led to the pattern of hot, dry weather across South Texas and the Rio Grande Valley. One key player may have been the negative phase of the North Atlantic Oscillation, which can favor atmospheric low pressure troughs across eastern North America and broadening high pressure ridges across the central and south central U.S. El Niño Southern Oscillation, which transitioned into the positive phase (El Niño) by summer’s end, shows some correlation between an ongoing or developing El Niño and the North Atlantic Oscillation.

Estimated rainfall totals for June through August 2012
Rainfall Estimates for the Rio Grande Valley for June 1 through August 31, 2012.

Estimated rainfall totals for June through August 2012, detailed view
Detailed view of Rainfall Estimates for the Rio Grande Valley for June 1 through August 31, 2012, using GIS.

Estimated rainfall departures from average for June through August 2012
Rainfall Departure Estimates for the Rio Grande Valley for June 1 through August 31, 2012, in percent, based on the most recent 30 year average from 1981 to 2010.

100 degree days across the Rio Grande Valley, June 1 through August 31, 2012
Preliminary Temperature and Rainfall, Summer 2012
Location
Average Temp.
Hot Ranking
Rainfall (in.)
Dry Ranking
Harlingen/Cooperative (since 1911)
86.5
3
1.16
1
McAllen/Miller Airport (since 1961)
88.8
3
1.17
3
McAllen/Cooperative (since 1941)
88.4
3
0.46
3
La Joya/Mission (since 1910)
88.9
3
1.16
8
Hebbronville (since 1904)*
87.2
3
5.71
53
Brownsville (since 1878)
86.1
4
9.87
101 (33rd wettest)
Mercedes 6 South-Southeast (since 1913)*
85.6
4
3.03
13
Rio Grande City (since 1897)*
88.9
6
2.80
21
Port Isabel (since 1928)
85.1
7
6.69
53 (28th wettest)
Port Mansfield (since 1957)
83.8
8
2.34
15
Raymondville (since 1913)
85.4
16
3.14
14
Falcon Dam (since 1962)
87.1
20
7.31
31 (18th wettest)
Sarita 7 East (since 1898 rain only)
85.0
N/A
3.01
21
*Missing more than 25 percent of data.
Please note that some sites do not have a complete record back to the listed starting year.

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