Valley expected to see solid rainfall due to frontal system

Even though the National Weather Service’s recently released December-January outlook for South Texas predicts warmer and drier than normal conditions, it doesn’t mean rain is out of the picture entirely.

Tuesday’s heavy rainfall across much of the Lower Rio Grande Valley, and a good chance of more rain later in the week, is a case in point.

Mike Buchanan, who as of Nov. 30 is the former meteorologist-in-charge with the NWS Brownsville/RGV station, was nevertheless willing to put on his South Texas meteorologist’s hat one more time when reached by phone Tuesday morning in Slidell, Louisiana.

Buchanan, who headed the Brownsville station five years and now joins the NWS New Orleans/Baton Rouge office, said it’s shaping up to be a “pretty active week weather-wise,” especially for the Lower Valley.

Rainy conditions were expected to continue through Tuesday night, hit a bit of a lull Wednesday, and then pick back up starting Thursday, he said.

“There’s a 60-70% chance Thursday, and then Friday, 40-50%, and then a slight chance of rain on the weekend,” Buchanan said. “Kind of a fairly active pattern.”

The main reason for Tuesday’s downpours was a frontal system stretching across the area, while rain later in the week will mostly be due to several upper-level disturbances moving in — fairly decent moisture for this time of year, he said.

“And we have what’s called a coastal trough,” Buchanan said. “What it is, it’s an area of convergence along a coast. The shape of the coast kind dictates that. It’s not uncommon this time of year. During the cool season we get a coastal trough, and that’ll enhance the cloudiness a lot of times. If it’s strong enough you’ll get precipitation.”

That same troughing was responsible for last week’s clouds and light drizzle, he said.

“Anytime you get a convergence near the surface, the air has to go somewhere,” Buchanan said. “It can’t go into the ground. It has to go up. When it goes up, that’s when you get condensation occurring, you get clouds developing and eventually you can get precipitation if it’s strong enough.”

Motorists drive through the rain along Business 77 Sunshine Strip on Tuesday, Dec. 3, 2024, in Harlingen. (Miguel Roberts/The Brownsville Herald)

With the persistent drought and low reservoir levels afflicting deep South Texas, the Valley needs every drop of rain it can get, he noted. The Valley’s warmer-and-drier winter outlook, meanwhile, is being driven by La Nina, characterized by a cooling of the eastern Pacific (El Nino is the opposite). When La Nina forms, it strongly correlates with a warmer/drier cool season for South Texas, Buchanan said.

“The caveat to that is it’s not going to be warm and dry every single day,” he said. “Obviously, this week is an exception to that. But you look at the overall three to five months, it’ll be on average below normal precipitation and above normal temperatures.”

A La Nina winter also doesn’t necessarily rule out a freeze, Buchanan said.

“We still can get an Arctic outbreak, and we’ve had one pretty much the last couple of years,” he said. “We’ve had at least one freezing event, sometimes multiple freezing events.”

Buchanan added that he’ll remember his time in Texas fondly.

“Me and my wife, we enjoyed our time there in the Valley,” he said. “The people were just awesome.”

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