After a hot, dry summer and little respite during typically wet fall months, San Antonio is deep in drought and likely to see little rainfall through the rest of winter.
According to local weather data, San Antonio is on track to end the year with 12 inches less precipitation than normal, with the period of record being 1981 through 2010. The city has gotten 20 inches of rainfall this year, compared with an average of 32 inches.
Drought conditions have crept across most of Texas over the past few months, according to the most recent U.S. Drought Monitor report for Dec. 22. Northern Bexar County is now in “extreme drought,” along with much of Central Texas. Droughts of this severity can affect crop yields, hurt wildlife habitat, increase wildfire risk, and stress drinking water supplies.
While droughts are typical in Texas, climate change influences their severity, said Texas State Climatologist John Nielsen-Gammon, a professor of atmospheric sciences at Texas A&M University. Over the past six months, temperatures across Texas have averaged 1 to 2 degrees higher than normal, which is “about what you’d expect, given climate change,” he said.
“In that sense, climate change is having its normal effect on dry conditions,” Nielsen-Gammon said. “We’re a bit drier than we would have been if temperatures were near normal rather than a few degrees above.”
Rivers and reservoirs in central and western Texas are running lower than they have over the past few years. Near San Antonio, Medina Lake is 42 percent full, compared with 79 percent a year ago. Canyon Lake is 89 percent full, compared with 93 percent this time last year.
San Antonio gets most of its drinking water from underground aquifers. Its main source, the Edwards Aquifer, is “a little below historical average for this time of year, mostly because we’re well below rainfall for the fall and winter months,” Paul Bertetti, director of aquifer science for the Edwards Aquifer Authority, which manages pumping of the water source, said in a briefing at the EAA’s Dec. 8 board meeting.
As of Saturday, the Edwards Aquifer below San Antonio stands at 663 feet above mean sea level. Outdoor watering restrictions that limit sprinkler use don’t kick in unless the aquifer’s 10-day average drops to 660 feet or below.
The aquifer could get a boost from scattered thunderstorms expected this Tuesday and Wednesday. Forecasters are predicting showers as a weak cold front moves into the region this week.
But in the longer term, national forecasters anticipate mild and dry conditions across much of the southern U.S., including Texas. The warm, clear weather could last through the end of March, according to the National Oceanic and Atmospheric Administration.
“I don’t think we’re going to have a significant respite from these dry conditions until at least February through March,” Bertetti said.
A main factor in this winter’s dryness is La Niña, the cold phase of the El Niño Southern Oscillation, a cycle of ocean temperatures that influences weather around the globe. During La Niña, sea surface temperatures in the southern Pacific Ocean hover at a temperature lower than average.
In Texas, La Niña tends to cause warmer and drier conditions than normal because of its influence on the jet stream.
“La Niña makes the jet stream stay farther north than usual, so we don’t get as many cold fronts as we normally would,” Nielsen-Gammon said.
The presence of La Niña invites some comparisons to 2010-2011, when Texas entered its worst drought in recent history. That year, a dry winter influenced by La Niña conditions gave way to a spring and summer that provided no relief.
It’s too early to tell whether that drought will repeat itself or whether regular rainfall will return in spring. One concerning sign is that this year’s drought “had a three-month head start,” compared with the same period of 2011, Nielsen-Gammon said.
“Comparable dryness during the rest of this winter would make the overall drought conditions even worse than they were in 2011,” he said.
One change coming in 2021 is sure to affect how people across the U.S. perceive the weather. When comparing the day’s weather to “normal,” meteorologists use a 30-year timescale to calculate averages. The current period is 1981 through 2010.
Every decade, that time period shifts forward to incorporate 10 years’ worth of new weather data. In 2021, climate experts will begin using 1991 to 2020 as a reference point. Global average temperatures are now 2 degrees warmer than they were during the 20th century, a sign of the definition of “normal” shifting hotter.
“Any given date won’t be as far above ‘normal’ as it used to be,” Nielsen-Gammon said.