Navajo Unit Coordination Meeting April 22, 2025
Day: April 11, 2025
Why some storms brew up to extreme dimensions in the middle of America ā and why itās happening moreĀ often
Shuang-Ye Wu, University of Dayton
A powerful storm system that stalled over states from Texas to Ohio for several days in early April 2025 wreaked havoc across the region, with deadly tornadoes, mudslides and flooding as rivers rose. More than a foot of rain fell in several areas.
As a climate scientist who studies the water cycle, I often get questions about how extreme storms like these form and what climate change has to do with it. Thereās a recipe for extreme storms, with two key ingredients.
Recipe for a storm
The essential conditions for storms to form with heavy downpours are moisture and atmospheric instability.
First, in order for a storm to develop, the air needs to contain enough moisture. That moisture comes from water evaporating off oceans, lakes and land, and from trees and other plants.
The amount of moisture the air can hold depends on its temperature. The higher the temperature, the more moisture air can hold, and the greater potential for heavy downpours. This is because at higher temperatures water molecules have more kinetic energy and therefore are more likely to exist in the vapor phase. The maximum amount of moisture possible in the air increases at about 7% per degree Celsius.
Warm air also supplies storm systems with more energy. When that vapor starts to condense into water or ice as it cools, it releases large amount of energy, known as latent heat. This additional energy fuels the storm system, leading to stronger winds and greater atmospheric instability.
That leads us to the second necessary condition for a storm: atmospheric instability.
Atmospheric instability has two components: rising air and wind shear, which is created as wind speed changes with height. The rising air, or updraft, is essential because air cools as it moves up, and as a result, water vapor condenses to form precipitation.
As the air cools at high altitudes, it starts to sink, forming a downdraft of cool and dry air on the edge of a storm system.
When there is little wind shear, the downdraft can suppress the updraft, and the storm system quickly dissipates as it exhausts the local moisture in the air. However, strong wind shear can tilt the storm system, so that the downdraft occurs at a different location, and the updraft of warm moist air can continue, supplying the storm with moisture and energy. This often leads to strong storm systems that can spawn tornadoes.
Extreme downpours hit the US
It is precisely a combination of these conditions that caused the prolonged, extensive precipitation that the Midwest and Southern states saw in early April.
The Midwest is prone to extreme storms, particularly during spring. Spring is a transition time when the cold and dry air mass from the Arctic, which dominates the region in winter, is gradually being pushed away by warm and moist air from the Gulf that dominates the region in summer.
This clash of air masses creates atmosphere instability at the boundary, where the warm and less dense air is pushed upward above the cold and denser air, creating precipitation.
A cold front forms when a cold air mass pushes away a warm air mass. A warm front forms when the warm air mass pushes to replace the cold air mass. A cold front usually moves faster than a warm front, but the speed is related to the temperature difference between the two air masses.
The warm conditions before the April storm system reduced the temperature difference between these cold and warm air masses, greatly reducing the speed of the frontal movement and allowing it to stall over states from Texas to Ohio.
The result was prolonged precipitation and repeated storms. The warm temperatures also led to high moisture content in the air masses, leading to more precipitation. In addition, strong wind shear led to a continuous supply of moisture into the storm systems, causing strong thunderstorms and dozens of tornadoes to form.
What global warming has to do with storms
As global temperatures rise, the warming air creates conditions that are more conducive to extreme precipitation.
The warmer air can mean more moisture, leading to wetter and stronger storms. And since most significant warming occurs near the surface, while the upper atmosphere is cooling, this can increase wind shear and the atmospheric instability that sets the stage for strong storms.
Polar regions are also warming two to three times as fast as the global average, reducing the temperature gradient between the poles and equator. That can weaken the global winds. Most of the weather systems in the continental U.S. are modulated by the polar jet stream, so a weaker jet stream can slow the movement of storms, creating conditions for prolonged precipitation events.
All of these create conditions that make extreme storms and flooding much more likely in the future.
Shuang-Ye Wu, Professor of Geology and Environmental Geosciences, University of Dayton
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Hereās why 3 decent winters in a row still isnāt enough to fill Lake Powell: This week is when snowpack typically peaks across the #ColoradoRiver Basin — The Salt Lake Tribune
Click the link to read the article on The Salt Lake Tribune website (Anastasia Hufham). Here’s an excerpt:
April 9, 2025
Over the last three years, the Colorado River Basin has experienced three relatively healthy winters. But that decent snowpack, after melting, hasnāt filled reservoirs like Lake Mead and Lake Powell as much as water users across the West might like, due to years of drought and overuse. Recent forecasts show Lake Mead and Lake Powell will remain roughly one-third full after snow melts down from the mountains across the West into the Colorado River and its tributaries this year…
Winter of 2023 brought substantial snow to the parched region,Ā with snowpack levels reaching over 150% of the 30-year median. The banner snowpack led toĀ 10.6 million acre-feet of waterĀ flowing into Lake Powell over the spring and summer ā a whopping 166% of the average runoff seen between 1991 and 2020. That helped bring Lake Powell up to 38% in July 2023 after the reservoir hit a record low of 22% in February of that year. That was followed by another good snow year for the basin basin, with levels hittingĀ 113% of the medianĀ in 2024. But that above-average snowpack translated to a below-average runoff of 83%, or about 5.3 million acre-feet of water reaching Lake Powell. That wasĀ lower than what forecasters had predictedĀ earlier in 2024, and the reservoir saw a smaller bump ā reaching 42% full in July.
The discrepancy between snowpack and runoff was largely because soil across the Colorado River Basin was so dry, [Jack] Schmidt said. Dry soil absorbs melting snow, so less water ends up in reservoirs. And this year, soil across most of the Colorado River Basin is drier than normal ā and even drier than it was this time last year.
Insects are growing threat to #Colorado forests, Colorado State Forest Service report finds
Click the link to read the release on the Colorado State University website:
Bark beetles and other insects are spreading through Coloradoās forests, leaving dead and dying trees in their wake ā a cycle that could fuel future wildfires, according to an annual Colorado State Forest Service report released March 26.
Following a wet and cool year in 2023, the shift back to near-record temperatures and below-average precipitation in Colorado last year exacerbated the proliferation of forest pests and weakened treesā defenses against them.
āTrees in Colorado canāt catch a break as our climate becomes warmer and dryer,ā said Matt McCombs, state forester and director of the Colorado State Forest Service. āThis ongoing trend toward persistent drought and higher temperatures not only makes trees easier prey for insects but increases the risk of large and severe wildfires.
āCouple that with more people living in areas prone to burn, and the state faces enormous challenges,ā McCombs added. āThe good news is we know Colorado is on the right path to address these challenges and foster forests and communities that are resilient to wildfire and forest pests.ā
The 2024 forest health assessment details which insects and diseases are the biggest threats to the stateās forests and where outbreaks are expanding. The report also describes the state forest serviceās science-based management practices that are promoting wildfire-resilient forests and healthy watersheds.
The CSFS updates the Colorado General Assembly and residents annually on the health of the stateās forests based on an annual aerial survey, field inspections and information from forest landowners.
Read the 2024 Report on the Health of Coloradoās Forests and view a story map with additional data here.
Adapted from a CSFS press release.
Research Article — Thirstwaves: Prolonged Periods of Agricultural Exposure to Extreme Atmospheric Evaporative Demand for Water — M. S. Kukal,Ā M. Hobbins (AGU publication)
Click the link to read the research article on the AGU website (M. S. Kukal,Ā M. Hobbins):
Abstract
Global atmospheric evaporative demand has increased, impacting agricultural productivity and water use. Traditionally, trend assessments have been limited to total evaporative demand, overlooking shifts in daily extremes, which are meaningful for agrohydrological outcomes yet largely unknown. Here, using a fully physical metric of evaporative demand, that is, standardized short crop reference evapotranspiration, we introduce the concept of thirstwavesāprolonged periods of extremely high evaporative demandāand analyze their characteristics during 1981ā2021 growing seasons for the conterminous US. Findings show that long-term mean spatial patterns demonstrated by thirstwave characteristics do not follow that of total or mean evaporative demand. Weighted for cropland area harvested, thirstwave intensity, duration, and frequency have increased by 0.06 mm dā1 decadeā1, 0.10 days decadeā1, and 0.39 events decadeā1, respectively during 1981ā2021. Statistically significant trends appear across 17%, 7%, and 23% of cropland area for intensity, frequency, and duration. Not only have thirstwaves increased in severity, but the likelihood of no thirstwaves occurring during the growing season has significantly decreased. Our work proposes a novel metric to describe periods of extremely elevated evaporative demand and presents a systematic analysis of such conditions historically for US croplands.
Key Points
- Regional hotspots of thirstwaves do not necessarily align with areas of high overall evaporative demand
- Intensity, duration, and frequency of thirstwaves have increased significantly (pĀ <Ā 0.05) over 17%, 7%, and 23% of US cropland area, respectively
- The likelihood of no thirstwaves occurring during the growing season has significantly decreased
Plain Language Summary
The atmosphere is getting more demanding for water around the world, and this affects water use and farming outcomes. Previously, studies mainly looked at the overall atmospheric demand for water, but little is known about changes in occurrence of very high atmospheric demand for water over consecutive days. In this study, we use introduce the idea of āthirstwaves,ā which are long periods of very high atmospheric demand for water. We looked at these thirstwaves that have occurred during 1981ā2021 in the US and analyzed them for how intense and how frequent they were and how many days they lasted. We found that the worst thirstwaves happened in places that do not see the highest demand. Over time, all aspects of these thirstwaves have gotten worse. It has also become much less likely that a growing season will pass without any thirstwaves. These findings suggest that in addition to monitoring overall atmospheric demand for water, it’s important to track, measure, and report thirstwaves to those managing agriculture and water resources.