Beat the Heat at the Beach; no changes expected into early August 2009
Heat Wave Ad Infinitum
August To Continue Where July Leaves Off; Records Falling

July 2009: Not Seen Since 1998
Update, July 31: July ended fittingly for a record hot month, with a day of torrid heat and high humidity leaving peak heat index values above 110 across a large swath of the Rio Grande Valley, extending from inland Cameron County through southern Zapata County during the mid afternoon. Daily temperatures at or above the latest July averages ensured that a number of records, most set in 1998, will fall. Preliminary monthly tallies will be available on this website on August 1st.

Like a broken record, the searing heat continues in Deep South Texas. While the region is known for its hot and humid summers, the persistence of the heat, relatively gusty southeast winds, and lack of any widespread rain events for most areas since early June has been impressive. A quick comparison of the observed conditions on the ground and high into the atmosphere for July 2009 and July 1998, when temperatures in each year were 3 to 6°F above average. Through July 30th, average temperatures through the 22nd were 3°F above average at Brownsville, 3.3°F above average at Harlingen/Valley International Airport, and a whopping 6.9°F above average at McAllen/Miller Airport. Thus far, all but two days surpassed 100°F, with an average high of 104.1°F! As of this writing, no end was in sight to the heat and largely rain free weather. The continuing dry, breezy, and hot conditions will maintain extreme to exceptional drought across the region and keep a critical threat for rapid wild fire growth.

What About August?
In 1998, the summer heat was "front loaded" – that is, June was almost as oppressive as July, with nearly equal average temperatures, which also broke all time records. In 1998, June was 4 to 5°F above average, more than double the departure from June, 2009, when temperatures were 1.5 to 3°F above average. Now, it’s on to August. In 1998, with a similar atmospheric pattern well established, August began where July left off. For the first half of the month, daytime temperatures reached or exceeded the century mark in many locations, and sultry nights near 80°F were the rule, with temperatures continuing at 3 to 5 degrees above average. Readings slowly backed down for the last half of the month, but not enough to reduce what would be another statistically significant smoker. For the period of June through August, 1998 remains the benchmark. But for how much longer?

That depends on where. Barring a change in the overall pattern, which current model data suggest is unlikely through at least the first half of August, both climate stations in McAllen could see an all time record summer in 2009, barely edging out 1998. Harlingen has an outside chance at nearing the 1998 marker; Brownsville is currently more than a degree shy of its all time summer mark in 1998, but 2009 does rank in or near the top 10.

The Relentless Pattern
"La Canícula", a persistent ridge of atmospheric high pressure stretching across northern Mexico during the heart of summer, shows no sign of giving an inch anytime soon. A north to northwest flow around this ridge will continue to suppress the arrival of any welcome deep layer tropical moisture into South Texas. Through August 1st (Figure 1, below left), a broad flat trough extending from the east edge of the ridge, from Oklahoma through the upper Mississippi Valley and extending northeast into New England, will allow waves of energy to ride along its base, energizing the atmosphere into strong and severe weather near a stationary front. Farther north, near the core of the trough, fair but pleasantly cool weather will end July and welcome August across the Great Lakes and New England. From August 2nd to the 7th (Figure 2, below right), and probably beyond, the ridge will strengthen and spread east as it has several times before this summer, returning heat to the southern Plains, and likely returning lower surface humidity, along with breezy (rather than windy) conditions to Deep South Texas. One thing won’t change: Temperatures soaring over 100°F each afternoon away from the coast.

Records to Fall?
A quick glance at average temperatures for 2009 through July 30th, compared with historical averages since records have been kept, tells the tale. If you thought this was one of the hottest July’s ever, you’re not far off. For all stations referenced in the following table, 2009 ranks at or near the top of the heap. Or is that heat? We can stamp McAllen and Harlingen Airport in the record books. Other locations won’t be far behind.

Average Temperatures, July 2009 (through the 30th, preliminary data)
2009 Average
All-Time Record
McAllen/Miller Arpt. (since 1961)
McAllen/Coop (since 1941)
Weslaco (since 1914*)
Port Isabel (since 1928*)
Harlingen/Airport (since 1942*)
Hebbronville (since 1917*)
Harlingen/Coop (since 1911)
Falfurrias (since 1907)
Rio Grande City (since 1900*)
Falcon Dam (since 1962)
Brownsville (since 1871)
Raymondville (since 1913*)
*Data incomplete, especially early in 20th century.
**July 2009 missing 8 or more dates (incomplete)
Atmospheric pattern expected to continue through August 1, 2009 (click to enlarge)
Figure 1. U.S. Weather pattern through August 1st, 2009. Stationary front draped across the southern Plains will move northeast as a warm front before the end of the period. White curve with arrow denotes mean jet stream.
Atmospheric pattern evolution for August 2 - 7, 2009 (click to enlarge)
Figure 2. U.S. Weather pattern August 2nd through 7th 2009 and perhaps beyond. Atmospheric ridge will spread east and strengthen, which should reduce strong winds and humid conditions in the Rio Grande Valley. It won't reduce the heat, however. White curve with arrow denotes mean jet stream.
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