Rio Grande Valley’s wettest month isn’t living up to billing

HARLINGEN — As our wet window of September closes, peak rain doesn’t look like it’s in the cards — or clouds — for the Rio Grande Valley.

The emergence of the annual persistent Bermuda high and the diminishing impact from the summer high pressure ridge over West Texas called La Canicula usually create a path for “peak tropical moisture” to flow from the Gulf of Mexico into South Texas.

In the Valley, September rains account for about one-fifth of our annual precipitation total, with Brownsville averaging 5.91 inches, Harlingen 5.28 inches and McAllen 4.49 inches.

Brownsville is close to average when it comes to rainfall for the year, down just 1.56 inches, with 17.17 inches recorded. But Harlingen is now down 4.78 inches for the year, and McAllen is down 9.52 inches.

But so far, September is not adding the usual inches to our precipitation numbers for the year and the forecast for the rest of the month contains little rainfall.

That’s important because there isn’t much chance, historically speaking, to make up a rainfall shortfall over the last three months of the year.

>> Brownsville averages 3.74 inches of rain each October, 1.81 inches in November and 1.14 inches in December.

>> Harlingen averages 3.11 inches next month, then 1.34 inches in November and 1.50 inches in December.

>> For McAllen, the final three months of the year average 2.09 inches, 0.91 inches and 1.18 inches of rainfall.

Looking at these averages, the problem becomes clear. Once the window allowing gulf moisture to flow into the Rio Grande Valley ends Sept. 30, it pretty much shuts down the likelihood of above-average rainfall for the remainder of the year.

“Kind of in a nutshell, if we don’t get a lot of rain here the remainder of the summer into the fall, it looks like once we get into the winter months on average our rainfall amounts are not that high, and in combination with development of La Nina conditions over the Pacific, that probably means we’re going to have a fairly dry winter for our area,” said Mike Castillo, a meteorologist with the National Weather Service in Brownsville.

The drought monitor

The McAllen area and southern Hidalgo County have been listed for months as being in “moderate drought,” while the western half of Cameron County has been listed as “abnormally dry” by the U.S. Drought Monitor.

But on Tuesday, the drought monitor updated its map to expand the “abnormally dry” listing to all of Cameron, Hidalgo, Willacy, Starr, Kenedy, Jim Hogg, Brooks and Zapata counties.

Southern Hidalgo County remains in “moderate drought” conditions.

So far, the dry summer days have not had much impact in the Valley, officials say.

Brad Cowan, AgriLife extension agent in Hidalgo County and an expert on Valley agriculture, said growers have not been bothered by the dry conditions.

“Now that cotton is harvested, everyone wants rain — and now,” he said via email. “We are behind but the drought monitor shows we are not that bad off yet.”

In McAllen, which has been the driest big city in the Valley for the past several months, city officials say everything is within normal parameters.

“No effects at all,” Mark Vega, general manager of McAllen Public Utility, said via email. “The reservoirs that serve the RGV (Lake Amistad and Falcon Lake) are still at over 50 percent capacity (which determines our Drought Conservation Plan) with MPU maintaining ample water supply to take us through the 2017 calendar year.”

As far as other water-intensive municipal users in the city, Parks and Recreation Director Mike Hernandez said no problems have arisen.

“We have not been affected,” Hernandez said.

Weather crystal ball

Castillo said longer-range forecasts indicate that through Thursday anywhere from a quarter-inch to a half-inch of precipitation is possible for the Valley, although other parts of Texas can expect more rain as a cold front descends from the north.

“The heavier amounts are going to be out in West Texas, out in the Panhandle, where they can see anywhere from five to seven inches over the next seven days,” he said. “Heavy rain will be well to our north and northwest through the next seven days, so we’ll have to wait and see how much rain we’ll get by the end of next week when that front moves down.”

It isn’t much, but Castillo says it’s the best chance of rain we’re going to have this September, barring any tropical disturbance which may come ashore from the gulf.

“As we go into the winter our rainfall is usually a lot lower than it is during the summer months and spring and fall, and so we tend to be dry during the winter months,” Castillo said.

Using a new and experimental forecasting model, Castillo said the Valley looks dry through the middle of October, and the three-month forecast through December “also has us in the below-normal average for precipitation.”

La Niña effect

A growing presence of a La Nina pool of water in the Pacific which is three to five degrees cooler than normal could mean our spell of dry weather will continue through the end of the year and eventually through the winter, Castillo said.

“With El Nino, we typically see wetter and cooler conditions during the winter months because of the way the weather patterns are impacted,” he said of La Nina’s alter ego with its warmer pool of Pacific water. “We tend to see more systems coming out of the Pacific during the winter that tend to provide more cloud cover and also more rainfall.”

A dry fall and winter, while unpleasant for home gardeners and those trying to keep a lawn happy, should not be a major factor economically.

But that could change over the winter, Castillo said.

Once we get into the growing season in spring, the Valley’s agriculture industry could be affected by a run of dry months, and the amount of water which can be siphoned from aquifers and surface water could be limited.

“If we go into the winter with a rainfall deficit and we don’t see a whole lot during winter months, come spring that’s going to possibly impact irrigation for the planting season,” Castillo said.