outlook+water: Drought declines, La Niña delayed again, more drought expected

SUMMARY:

• Drought conditions decreased to 34% of the state (D1–D4); statewide reservoir storage declined slightly to 73.1% full, about 5 percentage points below normal for this time of year.
• La Nada is here with an 80% chance of a short La Niña arriving in September-October-November.
• Drought is expected to remain and increase in much of the state over the next three months.

I wrote this article on October 5, 2024.

I watched with concern as Hurricane Helene’s remnants pounded North Carolina with epic rainfall and flooding. Some in the press stated that it was unprecedented for a storm to move so deep into the continent, but we’ve had our share in Texas.

One gob smacker was Hurricane Alice in June 1954 when she drifted over the Edwards Plateau and dumped upwards of 28 inches over two days (oddly, there is another Hurricane Alice that same year that spun from December 1954 into 1955). The resulting runoff sent an 82-foot wall of water down the Pecos River, followed by a 97.8-foot wall of water, wiping out the US 90 bridge at Langtry and killing one person. Rainfall from Alice also sent a 30-foot wall of water through Ozona, killing 22. Over 250 miles of the Rio Grande were devastated, destroying water and wastewater treatment plans and leaving downtowns under water. In all, Alice killed between 55 and 153 due to the probability of unreported deaths in rural Mexico and an unknown amount of financial damage. The storm caused $2 million of damage ($23 million in today’s dollars) in Ozona.

Figure 1. Bird’s eye view of the flooding Rio Grande River over the International Bridge in Laredo (source).

Figure 2a: Inches of precipitation that fell in Texas in September 2024 (modified from source). Note that cooler colors indicate lower values, and warmer indicate higher values.

Figure 2b: Rainfall as a percent of normal for September 2024 (modified from source).

The amount of the state under drought conditions (D1–D4) decreased from 36% four weeks ago to 34% today (Figure 3a), with dry conditions developing the state (Figure 3b). Extreme Drought or worse increased from 6.3% of the state four weeks ago to 8.9% today, with exceptional drought increasing from 1.7% to 3.4% (Figure 3a). In all, 74% of the state is abnormally dry or worse (D0–D4; Figure 3a), up from 70% four weeks ago.

The North American Drought Monitor, which runs a month behind current conditions, shows drought in New Mexico, Oklahoma, and Mexico, among other places across the continent (Figure 4a). The Rio Grande Basin north of Elephant Butte Reservoir is in short- and long-term drought (Figure 4a). The Rio Conchos basin in Mexico, which confluences with the Rio Grande just above Presidio and hosts the largest tributary to the Lower Rio Grande, remains in extreme to exceptional drought (Figure 4a).

Conservation storage in Elephant Butte Reservoir—an important source of water for the El Paso area—decreased from 7.4% four weeks ago to 5.7% more recently (Figure 4c), about 15 percentage points below a rather pitiful median storage.

Combined conservation storage in the Amistad and Falcon reservoirs increased to 20.8% full from 20.0% four weeks ago and is almost 40 percentage points below normal for this time of year (since 1990) and the lowest in at least the past 33 years for this time of year (Figure 4d).

Basins across the state continue to host flows over the past week below historical 25^th, 10^th, and 5^th percentiles (Figure 5a). Statewide reservoir storage is at 73.1% full, down about 570,000 acre-feet from 74.9% four weeks ago and about 6 percentage points below normal for this time of year since 1990 (Figure 5b). The reservoir marked in orange in the Dallas-Fort Worth area (the persimmon in a sea of blueberries) is New Terrell City Lake and has dropped from full in early March (Figure 5c). I wasn’t able to discern the cause, but I suspect it’s for maintenance on the reservoir and/or dam.

My (drumroll please) “Hydrograph of the Month” is Choke Canyon Reservoir due to record low lake levels (Figure 5d).

Sea-surface temperatures in the Central Pacific that, in part, define the status of the weather phenomenon known as the El Niño Southern Oscillation are under neutral conditions, but sea-surface temps continue to drop (Figure 6a). Projections suggest a transition to La Niña conditions in September-November (81% chance, Figure 6a, b). If there’s good news here, it’s that the coming La Niña looks to be a weak and short one. For some reason (a takeover by the computer machines?), the expert projection has dropped from the projections, so we only have the dynamical and statistical models.

The U.S. Seasonal Drought Outlook through August projects drought now shows drought development over the eastern third of the state and the Panhandle (Figure 7a).

The three-month temperature outlook projects (surprise, surprise) above-average temperatures for the state (Figure 7b), while the three-month precipitation outlook favors rainfall deficits for much of the state (Figure 7c). Both of these maps look worse than last month.