Weather

Weather is the measure of short-term atmospheric conditions such as temperature, wind speed, and humidity. The Northwest Central is an active location for atmospheric events such as thunderstorms and tornados. It also experiences a variety of other weather hazards, including high temperatures and drought.

Storms, Tornados, and Derechos

Several types of severe storms present challenges to people living in the Northwest Central. Summer brings severe thunderstorms associated with cold fronts. Fall and spring can bring ice storms, while winter brings snow and, in some cases, blizzard conditions. In October 2013, for example, a major blizzard affected the Northwest Central and much of the Midwest, dumping up to 1.5 meters (5 feet) of snow across the Great Plains. The snow affected 5000 ranches in South Dakota, scattering and killing herds of cattle and sheep, as well as disabling power for more than 20,000 homes and trapping people inside their cars. The storm system’s winds blew up to 112 kilometers per hour (kph) (70 miles per hour [mph]), generating 22 separate tornados as well as severe thunderstorms and ice storms.

Rainstorms arise where colder air from higher latitudes abruptly meets warmer air. Severe thunderstorms are a common occurrence for people living in the Northwest Central because the conditions over the Great Plains are perfect for the development of severe weather. The flat, open fields are warmed by the summer sun, which sits high in the sky during this time of year. This results in large temperature differences when cold air masses move across the country. At the boundary between warmer and cooler air, buoyant warm air rises, and then cools because air pressure decreases with increasing height in the atmosphere. As the air cools, it becomes saturated with water vapor, condensation occurs, and clouds begin to form. Because liquid water droplets in the clouds must be very small to remain suspended in the air, a significant amount of condensation causes small water droplets to come together, eventually becoming too large to remain suspended. Sufficient moisture and energy can lead to dramatic rainstorms. Because warm air has a lower pressure relative to cold air, and the movement of air from areas of high pressure to areas of low pressure generates wind, the significant difference in air pressure associated with these boundaries and rainstorms also generates strong winds. Flat regions, such as the Great Plains, allow winds to move unimpeded by topography, and are often subject to severe thunderstorms.

While severe thunderstorms are common in some parts of the Northwest Central, two less common storm hazards have the potential to cause serious property damage and endanger lives: derechos and tornados. Both of these storm events are associated with wind shear, which occurs when the wind’s speed or direction changes with increasing height in the atmosphere. Wind shear can happen when a cold front moves rapidly into an area with very warm air. There, the condensing water droplets mix with the cooler, drier air in the upper atmosphere to cause a downdraft. When these downdrafts are very powerful, they can cause a derecho, or a set of powerful straight-line winds that exceed 94 kph (58 mph) and can often approach 160 kph (100 mph). These powerful windstorms can travel over 400 kilometers (250 miles) and cause substantial wind damage, knocking down trees and causing widespread power outages. The lightning associated with these intense storms can cause both forest fires and house fires. Approximately one derecho every two years or so will occur in easternmost South Dakota and Nebraska, and they appear with decreasing frequency as one travels westward (Figure 10.28).

Figure 10.28: Derecho frequency in the continental US.

The differences between tornados and derechos are indicated in their names: derecho is the Spanish word for “straight ahead,” while the word tornado has its roots in the Spanish word tonar, which means “to turn.” Both types of storm events can be associated with the same major cold front boundary because they require similar conditions to get started. However, tornado formation is more complicated. At the frontal boundary, warm, moist air rapidly rises as cooler, dry air descends; in the meantime, the pressure differences between the warm and cold air masses cause strong winds. Clouds with a visible horizontal rotation can appear, appearing to roll like waves crashing on the shore of a beach. This horizontal motion can tilt, lifting the rotating cloud vertically, and the rolling cloud will form a tornado. Most tornados will last a few seconds to several minutes. During that time, many tornado-prone areas will use tornado sirens to alert residents of the danger. A smaller tornado might generate flying debris that can cause injury or damage to buildings, while larger tornados can cause buildings and houses to be completely broken apart. Tornados are classified by their ranking on the Enhanced Fujita scale, or EF scale. These classifications are estimates of wind speeds based on the type of damage that is observed following the storm.

Measuring Tornado Intensity

Tornado intensity is measured on the Fujita scale, or simply F-scale, based on the amount of damage that a tornado can cause. The scale ranges from F0 to F5. The scale was modified recently to more accurately reflect specific wind speeds; this newer scale is known as the “Enhanced Fujita scale” and ranges from EF0 to EF5.

“Tornado Alley” is the nickname for an area, extending from Texas to Minnesota, that experiences a high number of exceptionally strong tornados due to its flatter topography and high incidence of severe thunderstorms. Both Nebraska and South Dakota reside within Tornado Alley, leading to more tornados in this part of the Northwest Central (Figure 10.29). From 1991 to 2010, for example, an annual average of 57 and 36 tornados occurred in Nebraska and South Dakota, respectively (Figure 10.30). To the west and north of Tornado Alley, fewer tornado strikes occur, with an annual average of 32, 12, 10, and 5 striking North Dakota, Wyoming, Montana, and Idaho, respectively. The boundaries of Tornado Alley vary in application, depending on whether the frequency, intensity, or number of events per location are used to determine the area’s borders.

Figure 10.29: Annual tornado reports per 29,500 square kilometers (10,000 square miles) in the continental US, between 1950 and 1995.

Figure 10.30: Two tornados touch down simultaneously in a South Dakota field between the towns of Enning and White Owl.

Although specific tornado paths are not predictable, the conditions that produce them are used to alert people so that they can seek shelter. The National Weather Service issues a watch, if the conditions are right for a type of storm event, or a warning, if the conditions are occurring or imminent for the storm event. The National Weather Service is part of the National Oceanographic and Atmospheric Administration, which maintains a US map of all current watches and warnings. Since the atmospheric conditions can change very quickly, an important factor in preventing loss of human life is getting the public to act upon the severe weather alerts. One recent attempt to improve public response to warnings is through a tornado alert index that helps people evaluate the risk of a local tornado. The Tor:Con index used by the Weather Channel provides a number from 1 to 10 that represents the probability of a tornado occurring. Meteorologists evaluate the atmospheric conditions associated with a storm and assign a score. For example, a 4 on the Tor:Con index would indicate a 40%, or moderate, chance of a tornado forming in a particular area.

Extreme Temperature and Drought

Extreme temperatures can create dangerous conditions for people and may lead to property damage. Heat waves are periods of excessively hot weather that may also accompany high humidity. Temperatures of just 3°C (6°F) to 6°C (11°F) above normal are enough to reclassify a warm period as a heat wave. Under these conditions, the mechanism of sweating does little to cool people down because the humidity prevents sweat from evaporating and cooling off the skin. Heat waves have different impacts on rural and urban settings. In rural settings, agriculture and livestock can be greatly affected. Heat stress recommendations are issued to help farmers protect their animals, particularly pigs and poultry, which, unlike cattle, do not have sweat glands.

The impacts of heat waves on urban settings include a combination of the natural conditions of excessive heat and the social conditions of living in a densely populated space. Cities contain a considerable amount of pavement, which absorbs and gives off more heat than vegetation-covered land does. Air conditioning units that cool down the inside of buildings produce heat that is released outside. Pollution from cars and industry also serve to elevate the outdoor temperatures in cities. This phenomenon, in which cities experience higher temperatures than surrounding rural communities do, is known as the heat island effect. Other social conditions can increase the hazards associated with heat waves in urban areas. People who are in poor health, live in apartment buildings with no air conditioning, or are unable to leave their houses are at greatest risk of death during heat waves.

During the first half of 2012, North America experienced a heat wave that set thousands of temperature records, particularly in the Midwest and Northwest Central and parts of central Canada (Figure 10.31). Within the Northwest Central, the Great Plains region experienced some of the most anomalous temperatures in the country. The event was attributed to persistent low-level winds blowing warm air from the Gulf of Mexico toward Canada. Like other climate events, the heat wave could not be directly attributed to global warming, but climate change is thought to have increased the event’s severity by 5 to 10%. The heat wave was also associated with the start of a serious drought in the central United States.

Figure 10.31: Land surface temperature anomalies in March 2011. Red areas represent above average temperatures and blue areas represent below average temperatures.

While high temperatures can be directly dangerous, a larger scale hazard arises when these temperatures are coupled with a lack of precipitation in an extended drought period. Most famously, high temperature and drought in the 1930s, combined with deep plowing that removed moisture-trapping grasses, led to the Dust Bowl—dust storms that carried vast clouds of black dust across the Midwest and central US, greatly damaging both the ecology and agriculture across that portion of the country (Figure 10.32). Although the Dust Bowl was most intense in the panhandles of Texas and Oklahoma, the event impacted agriculture throughout the Great Plains, including Nebraska and South Dakota. Dust storms destroyed topsoil, buried equipment and houses, and contributed to the incidence of lung disease.

Figure 10.32: A car and other farm equipment lies buried following a dust storm near Dallas, South Dakota in 1936.

Recently, a different extreme temperature phenomenon has made the news: the polar vortex. As the name implies, a polar vortex is a regularly occurring area of low pressure that circulates in the highest levels of the upper atmosphere. Typically, the polar vortex hovers above Canada. However, a pocket of the counterclockwise rotating, low-pressure center can break off and shift southward at a lower altitude, covering the northern United States with frigid air. The jet stream then shifts to a more southward flow than usual, and its chill can even reach the southern states. A polar vortex can lock the jet stream in this new pattern for several days to more than a week. In early January 2014, the polar vortex dipped low over the upper United States, bringing with it some of the coldest temperatures seen in over 20 years. Temperatures in North Dakota plummeted to -30°C (-23°F), with wind chills of up to -51°C (-60°F). The lowest temperature in the US— -34°C (-30°F)—was recorded near Poplar, Montana. Although the cold temperatures of a polar vortex can be uncomfortable and make traveling dangerous in the winter, the Northwest Central has not yet experienced any major economic or health-related impacts from this type of extreme weather event.