outlook+water: Drought doubled, La Niña delayed, improvements expected

SUMMARY:

• Drought conditions increased to 36% of the state (D1–D4); statewide reservoir storage declined slightly to 74.9% full, about five percentage points below normal for this time of year.
• La Nada is here with a 70% chance of La Niña arriving in September-October-November.
• Drought is expected to improve or dissolve in much of the state over the next three months except for the El Paso area and parts of the Panhandle.

I wrote this article on September 8, 2024.

Given the local rains (and some flooding) in the Austin region and springflows back in Las Moras Springs, I figured things had improved across the state. I was wrong. Although intense, those rains were widely dispersed, and drought conditions have more than doubled from last month. The northern half of the state has re-entered drought while low flows in the lower Rio Grande continue to be an international water-supply challenge.

And what’s become of our hurricane season? The Climate Prediction Center updated its outlook last month, and it’s pretty much the same as the one released in May (Figure 1a). The Gulf is nice and toasty in a record-setting kinda way, but we haven’t seen much tropical action since early July with Beryl (which favored water-rich East Texas). As far as seasons go, September is the busiest month for tropical systems, with August and October tied for second, so there’s still a lot of time left on the clock.

Figure 1a: August hurricane season outlook compared to the May outlook (graph from NOAA).

Figure 1b: Tropical storm and hurricane frequency in the Atlantic (from Wikipedia with data from NOAA).

August was on the dry side this year, as it usually is, with most of the state seeing less than an inch except for the coastal area of Houston and in the Panhandle northwest of Amarillo (Figure 2a). August is usually rain-poor, but this August was unusual, with much of the state receiving less than half of normal (Figure 2b). However, spotty thunderstorms did provide some relief and even flooding across some parts of the Hill Country and Edwards Plateau (Figure 2b).

The amount of the state under drought conditions (D1–D4) increased from 20% four weeks ago to 36% today (Figure 3a), with drought development across the northern half of the state (Figure 3b). Extreme drought or worse increased from 3.4% of the state four weeks ago to 6.3% today, with exceptional drought increasing from 0.3% to 1.7% (Figure 3a). In all, 70% of the state is abnormally dry or worse (D0–D4; Figure 3a), up from 42% 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 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 11.8% four weeks ago to 7.4% more recently (Figure 4c), about six percentage points below median storage.

Combined conservation storage in the Amistad and Falcon reservoirs increased to 20.0% full from 19.7% four weeks ago, 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 74.9% full, down about 660,000 acre-feet from 77.0% four weeks ago and about six percentage points below normal for this time of year since 1990 (Figure 5b). Recent rains have not topped up most of the reservoirs in the central part of the state, and Lake Amistad is only 26.5% full (Figure 5c). The reservoir marked in orange in the Dallas-Fort Worth area (the lemon in a sea of blueberries) is New Terrell City Lake and has dropped from full in early March (Figure 5c). I couldn’t discern the cause, but I suspect it’s for maintenance on the reservoir and/or dam.

My (drumroll please) “Hydrograph of the Month” is Lake Amistad due to continuing low levels and the appearance of water problems in the lower Rio Grande, making the press more and more (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, and sea-surface temps continue to drop (Figure 6a). Projections suggest a transition to La Niña conditions in mid-to-late summer (Figure 6a, b). This month’s projection is similar to last month’s except for a cooler La Niña, a monthly trend we’ve seen most of the year. If there’s good news here, it’s that the coming La Niña looks to be a mild one.

The official probabilities, a month newer than the projections in the graph, give La Nada conditions a ~50% chance for La Niña conditions arriving August-September-October (Figure 6b). The chances for La Niña increase to 70% for September-October-November (Figure 6b). Accordingly, we remain under a La Niña Watch. The chances of La Niña remain strong, but the probabilities have dropped from last month.

The U.S. Seasonal Drought Outlook through August projects drought improvement and removal for almost all of the state except for the El Paso area and parts of 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).