outlook+water: Climate trends, the return of drought and a good chance for La Niña

outlook+water: Climate trends, the return of drought and a good chance for La Niña

SUMMARY:​

  • Drought has flashed into the High Plains
  • La Niña conditions look more likely for this fall and winter
  • An historical trends analysis indicates that, indeed, it has been getting warmer and floodier

I wrote this article on June 20, 2020.

I haven’t met a Texan who doesn’t think that it’s been getting warmer and floodier around here. A new report published by the Office of the State Climatologist presents a study of historical trends to confirm and quantify those casual observations. Since 1975, Texas has been warming at a rate of 0.63° F per decade with increased temperatures in all seasons and all parts of the state. The number of days warmer than 100° F have tripled in urban areas and almost doubled in rural areas since 1950 with the number of 100 degree days about doubling for every two degree increase in average temperature.

Rainfall trends vary across the state with parts of the eastern half of the state seeing a 15 percent increase in rainfall over the past century and with parts of West Texas seeing flat to declining rainfall (Figure 1a). Extreme one-day rainfall has increased across our region by 0.2 to 0.5 inches since the 1950s with the median change in rainfall intensity in Texas increasing by 7 percent (but with a lot of variability across the state; Figure 1b). The flip side of the precipitation coin—drought—has too much variability and uncertainty to assess what’s happening (and might happen in the future).

Due to relative sea-level rise from land subsidence and rising sea levels, coastal areas may have twice the flood risk by 2050 compared to 2000. Hurricanes are expected to be stronger, but they’re not expected to increase in number (and may even decrease).

With this past May being the warmest month on record for the globe, we’re well on the path of warmer temperatures, wetter and more intense storms and greater flooding.  

Figure 1a: Precipitation trends in Texas (source).

Figure 1b: Precipitation trends in Texas (source).

Rainfalls gave most of the eastern half of the state at least two inches over the past month with quite a few areas receiving five inches or more while the western half received less than two inches with much of Far West Texas and the western fringes of West Texas getting less than a tenth of an inch (Figure 2a). Most of the state received less than normal rainfall over the past four weeks with much of West and Far West Texas getting less than 10 to 25 percent of normal and South Texas getting 50 to 100 percent more than normal (Figure 2b). Looking back 90 days, most of the state is in deficit with Far West Texas and much of the High Plains at less than 25 percent of normal (Figure 2c).

Figure 2a: Inches of precipitation that fell in Texas in the 30 days before June 20, 2020 (source). Note that cooler colors indicate lower values and warmer indicate higher values.

Figure 2b: Rainfall as a percent of normal for the past 30 days as of June 20, 2020 (source).

Figure 2c: Rainfall as a percent of normal for the past 90 days as of June 20, 2020 (source).

Not surprisingly, based on 90-day rainfall deficits, the amount of the state under drought conditions (D1-D4) about tripled from 10.7 percent four weeks ago to 27.2 percent today (Figure 3a). Rains over the past month continued to smother drought along the Gulf Coast and the Lower Rio Grande Valley while severe and extreme drought flashed into the High Plains (Figure 3b). About 11 percent of the state is now experiencing severe drought (Figure 3a). In all, about 54 percent of the state is abnormally dry or worse (D0-D4; Figure 3a), up from 38 percent four weeks ago.

Figure 3a: Drought conditions in Texas according to the U.S. Drought Monitor (as of June 16, 2020; source).

Figure 3b: Changes in the U.S. Drought Monitor for Texas between May 19, 2020 and June 16, 2020 (source).

The North American Drought Monitor for April shows the heart of the High Plains drought up in the Colorado-Kansas-New Mexico-Oklahoma area stretching west into the southern Colorado-northern New Mexico region (Figure 4a). Precipitation in the Rio Grande watershed in Colorado over the last 90 days is less than 10 to 25 percent of normal (Figure 4b). Conservation storage in Elephant Butte Reservoir decreased from 22.7 percent full on May 20 to 16.9 percent on June 20, some 15 percentage points below normal for this time of year (Figure 4c).

The Rio Conchos basin in Mexico, which confluences into the Rio Grande just above Presidio and is an important source of water to the lower part of the Rio Grande in Texas, has developed moderate drought conditions with short-term impacts (Figure 4a). Combined conservation storage in Amistad and Falcon reservoirs has remained level over the past month at about 46 percent, about 15 percentage points below normal for this time of year (Figure 4d).

Figure 4a: The North American Drought Monitor for May 31, 2020 (source).

Figure 4b: Percent of normal precipitation for the past 90 days for Colorado and New Mexico as of June 20, 2020 (source). The red line is the Rio Grande Basin. I use this map to see check precipitation trends in the headwaters of the Rio Grande in southern Colorado, the main source of water to Elephant Butte Reservoir downstream.

Figure 4c: Reservoir storage in Elephant Butte Reservoir since 2018 with the median, min and max for measurements since 1990 (source).

Figure 4d: Reservoir storage in Amistad and Falcon reservoirs since 2018 with the median, min and max for measurements since 1990 (source).

A number of river/stream basins in the state have flows less than 25 percent of normal with a half dozen or so catchments with flows less than 10 percent of normal, one less than 5 percent of normal, and one in extreme hydrologic drought (Figure 5a). Statewide reservoir storage is at 85.7 percent full as of June 20th, down from 86.8 percent a month ago and is about normal for this time of year (Figure 5b). Storage in individual reservoirs remained relatively stable over the past month (Figure 5c).

Figure 5a: Parts of the state with below-normal seven-day average streamflow as of June 20, 2020 (source).

Figure 5b: Statewide reservoir storage since 2018 compared to statistics (median, min and max) for statewide storage since 1990 (source).

Figure 5c: Reservoir storage as June 20, 2020, in the major reservoirs of the state (source).

Sea-surface temperatures in the central Pacific remain in El Niño territory (Figure 6a), but we remain in La Nada (neutral) conditions because there is no coupling between the ocean and the atmosphere, the second of two requirements to be an El Niño. The Climate Prediction Center decreased the chance of neutral conditions through the summer to about 60 percent with about a 40 to 50 percent chance of neutral conditions or La Niña for the fall and winter (Figure 6b). I’m beginning to think that the consistent rise in the consensus projection for the last two seasons reflects the belief that we can’t accurately project that far out and the projectors are defaulting to an equal chance between El Niño, La Niña and La Nada. The probabilistic forecasts hover near the La Nada-La Niña threshold for those seasons (Figure 6b).

Figure 6a. Forecasts of sea-surface temperature anomalies for the Niño 3.4 Region as of June 11, 2020 (modified from source).

Figure 6b. Probabilistic forecasts of El Niño, La Niña and La Nada conditions (source).

The U.S. Seasonal Drought Outlook through September 30, 2020, projects improving drought conditions along much in much of West and Far West Texas with drought remaining in the Panhandle (Figure 7a). The Outlook also shows the drought consolidating across the western US. The three-month temperature outlook projects warmer-than-normal conditions statewide with greater warming to the southwest (Figure 7b) while the three-month precipitation slightly favors wetter-than-normal conditions for the eastern part of the state (Figure 7c).

Figure 7a: The U.S. Seasonal Drought Outlook for June 18, 2020, through September 30, 2020 (source).

Figure 7b: Three-month temperature outlook from June 18, 2020 (source).

Figure 7c: Three-month precipitation outlook from June 18, 2020 (source).

Author

Robert Mace

Robert Mace

Executive Director & Chief Water Policy Officer at The Meadows Center for Water and the Environment

Robert Mace is the Executive Director and the Chief Water Policy Officer at The Meadows Center. He is also Professor of Practice in the Department of Geography at Texas State University. Robert has over 30 years of experience in hydrology, hydrogeology, stakeholder processes, and water policy, mostly in Texas.

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