A strong La Niña could mean more dry weather for Austin this fall

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For the third year in a row, La Niña, the cooling of the tropical waters in the eastern Pacific Ocean, will strongly influence Central Texas weather this fall, which could mean persistent dryness for an area stalled in drought, weather experts say.

The phenomenon affects the jet stream in the northern hemisphere and tends to keep cold air masses pent up north, leading to warmer and drier seasons in Central Texas. Last year’s La Niña autumn was Austin’s sixth-warmest fall season. This year, La Niña has already contributed to Austin’s second-hottest spring on record and the city’s second-hottest summer ever.

“We’ve been more or less clobbered with this,” longtime Austin meteorologist and University of Texas senior lecturer Troy Kimmel said of La Niña. “Our problem is that we’re going into Year Three of this.”

What is La Niña?

During La Niña, surface winds across the tropical Pacific Ocean are stronger than usual, resulting in sea temperatures that are cooler than average. Its counterpart, El Niño, involves the warming of the Pacific waters, resulting in wetter conditions for Central Texas.

La Niña is not a storm or a weather system, meteorologists like to point out, adding that the cooler waters cause changes to atmospheric circulation around the world.

“The typical influence of La Niña in south-central Texas occurs in the late fall through the early to mid-spring,” National Weather Service meteorologist Keith White said. “The cooler waters in the eastern tropical Pacific Ocean usually lead to the wintertime storm track over the US remaining further to the north than average, with conditions in Texas usually ending up warmer and drier than normal as a result.”

But La Niña years aren’t all the same, White warned.

“La Nina conditions usually begin to subside in the spring months, but this year, La Nina actually strengthened some in April,” he said. “This assisted a stagnant pattern of subtropical ridging (high atmospheric pressure) keeping our skies clear and rain at bay — at least until mid-August — and helped allow us to see our hottest summer since 2011.”

La Niña winters can be similarly unpredictable. In February 2021, only days before snow and the record-breaking freeze, the Austin area was coming off of a hot autumn and a mostly rain-free fall and winter.

How has La Niña affected weather this year?

La Niña has had a strong influence on our region’s weather since last fall, said Bob Rose, meteorologist for the Lower Colorado River Authority. As the agency charged with managing the water supplied by the Highland Lakes along the Colorado River, the LCRA keeps a steady eye on Texas weather patterns, including the effects of La Niña.

“This weather feature has been one of the main reasons rainfall has been below normal across our region since last November,” Rose said. “Although La Niña’s influence on Texas weather weakens during the summer months, the dry winter and spring across Texas caused by La Niña allowed the summer heat dome to build early and persist all summer and into this fall.”

Rose and other forecasters pointed fingers at La Niña for worsening our current drought cycle.

“The heat dome kept most of the rain away, drying out the ground further,” Rose said. “This in turn allowed the ridge and the dry pattern to grow stronger.”

White explained further: “Once our soils dry out, more of the sun’s energy is able to go directly into heating us up rather than being used to evaporate that moisture from the soils, which further exacerbates the drought.”

According to the latest data released Thursday by the US Drought Monitor, a joint effort of the National Drought Mitigation Center, the US Agriculture Department and the National Oceanic and Atmospheric Administration, about 78.8% of the state is experiencing drought. Three months ago, that percentage was about 93%.

But some of the most severe levels of drought remain centered in the Hill Country southwest of Austin and the San Antonio metro area. Drought monitor data indicate that at least 9.6 million Texans live in drought-stricken areas and that 2022 to date is the state’s 11th-driest year in the past 128 years.

What would a strong La Niña mean for us?

La Niña is likely to persist into winter, with a 91% chance of lasting to November, before decreasing to a 54% chance from January to March 2023, according to the latest forecast from the National Weather Service’s Climate Prediction Center,

Rose said he is concerned about such a high probability for a persistent La Niña.

“In most years, we will see a second peak of rain in the fall months, which often recharges our lakes and aquifers, and reduces the fire danger,” he said. “This fall, with limited rain forecast, we may not see any recharge to our lakes and aquifers. Although drought conditions improved some in early September, they are expected to grow worse across most of Texas in the coming months.”

The volume of water stored in the reservoir lakes, Travis and Buchanan, would be full at about 2 million acre-feet, according to the LCRA. As of Friday, that number was down to about 1.11 million acre-feet. An acre-foot is the amount of water it takes to cover an acre a foot deep.

Lake Travis, the popular aquatic playground and regional water source, was considered filled to only 50% of capacity, LCRA data show. The water elevation at Mansfield Dam, which forms the lake, was about 643.8 feet above mean sea level, which is not only about 19.3 feet below the historical average for September but also almost a foot lower than a week ago.

The prospect of a continued La Niña “should be very concerning to Texans,” Kimmel said.

“I think we’ve got to be very worried,” he said. “The biggest issue is water – and when we go into drought like this, water, it’s not an infinite quantity.”

We “lucked out” with a dry spring season, Kimmel said with irony, alluding to the absence of major widespread flash flooding this year.

“As far as I’m concerned, until we see a good rain and a shift to an El Niño, we should be worried,” he added. “But this is not a ‘flip the switch’ kind of thing.”

White said drought will likely begin to expand and worsen again in some areas later this fall and winter, based on observed patterns associated with La Niña.

“However, not all La Niñas are created equal,” he said. “In fact, the 2011 drought began to abate in winter 2011-2012 despite that winter also being a La Niña. You may also remember that last spring, particularly late April through early June, was also much wetter than normal despite an ongoing (but waning) La Niña.”

How will La Niña affect hurricane season?

La Niña seasons tend to favor more development of tropical storm systems in the Atlantic because of less wind shear, according to Rose.

“However, La Niña itself has little to do with where tropical systems develop and move,” he explained. “That has more to do with the general circulation pattern across the Atlantic, and, so far this summer, this pattern has tended to guide most of the developing storms into the eastern Caribbean Sea and across the western Atlantic, rather than towards the western Gulf of Mexico” and Texas.

Rose also pointed out that “the majority of the storms we’ve seen so far this season have developed since early September, when the steering currents have been focused on taking these storms well to our east.”

White echoed Rose’s assessment, saying that “this year’s Atlantic Basin hurricane season did get off to a slower start than expected, with only seven named storms so far and two of those are ongoing at the moment.”

“The season has really begun to pick up in the last week or so, and the National Hurricane Center is watching three additional disturbances for potential development,” White said. The Atlantic hurricane season officially lasts until Nov. 30, but storms rarely approach Texas beyond the first half of October, he added.

“There is a system moving into the Caribbean … that has the potential to pose a risk to the Gulf Coast anywhere from Texas to Florida, but it’s too early to say much else about this system as it’s currently not well-defined,” White said. “We’ll be keeping a close eye on it over the coming days.”

Tropical Depression Nine, currently moving west in the central Caribbean Sea, is expected to produce heavy rainfall and flash flooding in Aruba and Curacao, then soaking Jamaica, the Cayman Islands, and Cuba in the coming days, according to the National Hurricane Center.

These late-developing tropical disturbances, though, could be undercut by the seasonal arrival of cooler air.

Texans still need to watch for tropical storms through October, Rose said. But “the threat for storms will begin to decrease along the Texas coast in late October and November as we see more frequent cold fronts moving through Texas.” The fronts, he said, “will help to steer any potential storms more to the eastern Gulf rather than towards Texas.”

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