High heat to lock down Valley this week

HARLINGEN — The South Texas summer isn’t finished with us just yet.

Weather forecasters have issued a heat alert for the entire Rio Grande Valley from Tuesday through Saturday.

Heat indices — the feels-like temperature — will range from 105 to 110 degrees for a few hours each afternoon.

Actual temperatures during the week will range from the mid-90s along the gulf to 105 and higher in the Upper Valley.

In fact, Tuesday may turn out to be the hottest day, with temperatures forecast at 98 in Brownsville and along the coast to as high as 108 degrees in the Upper Valley. The break line between the 90s and the 100s will be I-69E/U.S. 77.

“It’s going to be a hot Valley week ahead,” said Matthew Brady, meteorologist with the National Weather Service in Brownsville.

Forecasters say the worst of the potentially-dangerous heat and humidity levels could be Tuesday and Wednesday in Hidalgo and Starr counties, with heat indices soaring to between 111 and 113 degrees in the afternoon.

The blame for the searing temperatures is ridging in the mid-levels of the atmosphere which will continue to dominate the week’s weather pattern.

“The heat is due to a ridge of high pressure that is around the mid-levels of the atmosphere, let’s say 15,000 to 20,000 feet up,” Brady said. “It’s a very dry pattern with very high temperatures, and there’s not going to be much cloud cover.”

The continuation of the hot, dry weather pattern — disrupted briefly last week by the remnants of Hurricane Franklin — is also known as La Canicula, Spanish for the dog star Sirius, which in the second half of the summer rises in the evenings in the southeastern sky.

La Canicula is caused by a persistent and stable high-pressure system which parks over New Mexico, West Texas and northern Mexico and dominates Valley weather.

Historically it usually ends around Aug. 11, but forecasters concede the date is just an average over the years, and often the weather pattern endures much later into August and even September.

La Canicula delayed its departure until well past Aug. 11 both in 2015 and 2016, forecasters say.

On the plus side, at least for Valley residents, forecasters say La Canicula’s forceful presence is the key factor in driving tropical systems like Hurricane Franklin south of Texas to make landfalls in Mexico.

“In this position, tropical waves or cyclones are ’steered’ well south of the Rio Grande around the periphery of the ridge — most often into Veracruz,” Brownsville forecasters said in a blog post Saturday.

Overnight lows will remain warm, ranging from the upper 70s into the low 80s.