The Sandhills are impacted by diverse phenomena broadly encompassed by the term “storm.” While important distinctions exist between snowstorms, windstorms, and thunderstorms, in general, all storms are manifestations of the atmosphere’s efforts to remove imbalances in principal atmospheric forces. Disequilibria are ubiquitous in the atmosphere and are ultimately a consequence of the unequal heating of the Earth by the sun. Each imbalance necessitates an atmospheric response to reestablish balance, which usually involves only small corrections to the atmospheric state. A shallow cumulus cloud, for example, is the visual manifestation of a small correction: within it, energy is redistributed vertically to correct an unbalanced vertical stratification of temperature. However, the complex interplay of land, water, and air combine to yield significant and sustained imbalances that cannot be removed via small corrections. It is only through highly energetic responses that these high-amplitude disequilibria can be eliminated. Storms are the vigorous atmospheric responses to very large imbalances.
The geography of the Sandhills makes them particularly favorable for storm formation. First, they are located in midlatitudes (~30° to ~60°) on a continent that extends from the tropics to poles. Given the primary role played by the unequal heating by the sun in driving imbalances, it stands to reason that storms should be more prevalent where lateral differences in temperature are largest. A net loss of energy from the poles and a net gain in energy in the tropics places the midlatitudes in the globally preferred location for the largest lateral gradient in temperature. However, differences in the thermal properties of land and water mean midlatitude continents and midlatitude oceans will not be characterized by the same latitudinal temperature gradients that support storm formation. Owing to the large heat capacity of water, oceans mute the impact of solar radiation on temperature. Thus, all else equal, midlatitude continents tend to be a preferential breeding ground for storms.
The second geographic characteristic of the Sandhills that favors storm formation is its position east of a north-south mountain range. Large-scale (1000s of km in diameter) surface low pressure centers that enter the United States from the west exhibit a unique behavior as they traverse the Intermountain West: they seem to disappear. The atmospheric anomaly associated with a “disappearing” system still exists, however, and when the anomaly crosses the Front Range of the Rocky Mountains into the Great Plains, the surface low pressure center reappears. Conceptually, the process can be viewed as the vertical compression and spin-down of the system as it moves over the higher terrain of the western U.S. and an expansion and spin-up as it emerges into the Great Plains. The rapid reemergence of the low can result in an imbalance that necessitates violent equilibration. This is particularly true when polar-Arctic air, dammed up against the Rocky Mountains, sags south into the Great Plains, creating a large horizontal temperature gradient that provides energy for nascent storms.
In spring and summer, the north-south oriented Rocky Mountains serve another critical role in storm formation. Across the U.S. Great Plains, the land rises gradually from east to west toward the base of the Front Range. The slope is generally only one to two meters per kilometer but, under the influence of solar heating, even this gentle slope can create a significant pressure gradient that accelerates air westward, up slope. After a northward deflection from the Coriolis force, the heated sloping terrain ultimately yields a southeasterly wind near the ground over much of the Great Plains that transports tropical heat and moisture northward, providing energy for growing storms.
The final geographic distinction of the Sandhills that favors storms is its relative proximity to the Gulf of Mexico. Nebraskans who are longing to visit an ocean beach will surely consider the Sandhills to be anything but close to the Gulf of Mexico. However, tropical air originating over the gulf requires only about a day to reach the Sandhills, as it rides winds pulled north by rapidly amplifying surface low-pressure centers in the lee of the Rockies and accelerated by heated terrain.
Whether large (e.g., surface low-pressure centers thousands of kilometers in diameter) or small (e.g., thunderstorms tens of kilometers in diameter), each storm alters, even if just briefly, the personality of the Sandhills. Winter storms can transform the landscape for weeks, if not months, with snow that drifts in blizzards to depths that cover houses. This snow, with origins in water vapor wicked off the Gulf of Mexico, can incapacitate the Sandhills’ residents. However, it can also quench the seemingly unquenchable thirst of the land and its flora and fauna, as winds, warmed through adiabatic compression, flow down from the Rockies into the plains and melt the blanket of ice. These Chinook winds also bring a respite from winter, if only briefly—a taste of spring that those farther east rarely enjoy.
The impacts from storms can be devastating. Rapidly amplifying low-pressure centers pull tropical air north across the Sandhills. Several kilometers above the surface, jet stream winds blow west to east, driven by strengthening north-south temperature gradients. With warm and moist tropical air in the low levels of the atmosphere (typically below 1–2 km in height) and seasonably cool air above this, an unbalanced (unstable) vertical stratification is created. Triggered to rise by a cold front or dryline (a frontal boundary between dry air to the west and moist air to the east), for example, this low-level air will accelerate upward as a thunderstorm, in a violent equilibration of the imbalance. If the change in wind speed and/or direction with height (known as vertical wind shear) is sufficiently strong, a storm can convert the kinetic energy of environmental winds into a storm-scale vortex (a mesocyclone) that spans the entire depth of the thunderstorm. This type of thunderstorm, called a supercell, is maintained through the complex interplay between the mesocyclone and environment within which it is embedded. This interplay promotes anomalously long storm lifetimes, creates anomalously strong upward motion, and produces the disproportionately severe impact of supercells on the people, places, and things they encounter because, on average, supercells produce the largest hail and the strongest tornadoes of any thunderstorm type.
Individual thunderstorms (tens of kilometers across), whether supercells or not, can aggregate into a self-organizing system several hundred kilometers across. Such mesoscale convective systems (MCSs) are multiple thunderstorms organized into a nearly continuous line or arc, pushed forward by rain-cooled air behind the line/arc, and sustained through constant regeneration of new thunderstorms where the cold air and warm/moist air ahead of the system meet. MCSs can produce all severe convective hazards: hail, tornadoes, severe straight-line winds. It is the latter hazard that is the most definitive aspect of MCSs. The term straight-line winds distinguishes these winds from those associated with tornadoes or other small-scale (<10 km) vortices. The practical differences are (1) on average, the damage area covered by straight-line winds (often in the thousands of square kilometers) is much larger than the damage area tied to vortices like tornadoes, and (2) the severity of damage at any one point within the damage swath of straight-line winds tends to be less than for vortices like tornadoes.
Due to both intrinsic and practical predictability, accurate storm forecasts are challenging. It unlikely that forecasts of small-scale phenomena (like tornadoes) will ever evade the intrinsic limits of predictability inherent in their size. However, it is possible to close the gap between intrinsic predictability and practical predictability; the latter can be addressed through improved understanding, improved numerical weather prediction models, and more precise observations. The remoteness of the Sandhills is arguably one its greatest appeals, but it also makes it difficult to improve the practical predictability of storms in this area because of the lack of observations. However, new methods of remotely sensed and targeted observing can contribute to improved predictions, even in the Sandhills where there are more dunes than people.