Present Climate

Due to their diverse topographical features, the Northwest Central States encompass a broad range of climates, including subarid steppe in the Great Plains, warm temperate highlands in the Cordilleran, and humid continental plains in the eastern Central Lowland. Even individual states can have tremendous diversity—depending on which of the many Köppen system maps you refer to, the state of Idaho alone contains as many as eight different climate types. The main drivers of climate in the Northwest Central US are exposure to Arctic air from Canada in the winter, the lack of large bodies of water nearby (except for Idaho, whose climate is influenced by the Pacific Ocean), and the presence of the Rocky Mountain chain in the west. These mountains block moist Pacific Ocean air from the interior of the continent and create a cold, high altitude zone.

Temperatures in the Northwest Central are characterized by seasonal extremes. South Dakota’s temperature, for example, varies between an average low of -14°C (6°F) in January and an average high of 86°F (30°C) in July. Record lows and highs are astonishing: -57°C (-70°F) in Montana in 1954 and 49°C (121°F) in North Dakota in 1936. Average temperatures in the Northwest Central tend to decrease northward, which is in part influenced by latitude: lower latitudes receive more heat from the sun over the course of a year. The overall warmest temperatures are found in Nebraska, and the coolest are found in North Dakota and parts of Wyoming (Figure 9.8). The Northwest Central States’ overall average high temperature of 14°C (57°F) and average low of 0.7°C (33°F) are indicative of a generally cool climate. By comparison, the average high and low temperatures for the entire United States are 17ºC (63ºF) and 5ºC (41ºF), respectively.

The Köppen Climate Map

Wladimir Köppen developed a commonly used system of climate categorization based on the kinds of vegetation areas sustain. He defined 12 climate types, many of which are familiar: rainforest, monsoon, tropical savanna, humid subtropical, humid continental, oceanic, Mediterranean, steppe, subarctic, tundra, polar ice cap, and desert. Updated by Rudolf Geiger, it has been refined to five groups, each with two to four subgroups.

Figure 9.8: Mean annual temperature for the Northwest Central States.

The Northwest Central US is dry compared with many other parts of the United States, so dry that all the states within it except Nebraska rank within the top 10 driest states based on annual precipitation. Precipitation generally tends to decrease to the west across the Rocky Mountains, with an average annual precipitation of 65 - 90 centimeters (25 - 35 inches) in the Central Lowland region of the eastern Dakotas and Nebraska, about 25 - 50 centimeters (10 - 20 inches) in the Great Plains, and less than 25 centimeters (10 inches) in parts of Wyoming and Idaho (Figure 9.9). By comparison, the average amount of precipitation for the United States is 85.6 centimeters (33.7 inches). The decrease in precipitation is due in large part to rain shadow effects from mountain ranges located west of as well as within the Northwest Central. Rain shadows occur when moist air moves eastward with the prevailing winds, and is pushed upward and cools when it encounters a mountain chain. Water vapor condenses from this cool air and falls as rain or snow on the western side of the mountain. The air that continues to move east over the mountains is now much drier, and as it moves down the eastern side of the mountain range it warms, promoting evaporation (Figure 9.10) .The mountainous Continental Divide, which runs through western Montana, creates a rain shadow effect that contributes to the aridity of the plains and badlands in the eastern part of the state. Nebraska’s semi-arid west and fairly uniform average temperatures are moderated by dry, warm rain shadow winds blowing eastward from the Rocky Mountains.

Figure 9.9: Mean annual precipitation for the Northwest Central States.

Figure 9.10: The key characteristics of a rain shadow.

Exceptions to the westward drying trend are found in the mountainous parts of northwestern Wyoming and Montana, and in northern Idaho, where average annual precipitation is typically 101 to 127 centimeters (40 to 50 inches), demonstrating the impact of moisture carried inland from the Pacific Ocean. Idaho’s climate is strongly moderated by the Pacific Ocean, even though the state lies nearly 560 kilometers (350 miles) from the coast. In the winter, humidity from the ocean creates heavy cloud cover and precipitation that helps to moderate temperature.

Harsh winter storms are a fact of life in the Northwest Central US, carried in by the polar jet stream, which typically falls near or over the area, especially in the winter. Blizzards with high winds, large amounts of snowfall, and low visibility are common and are brought on by cold air masses known as the Alberta Low from the north and the Colorado Low from the south. Since the Rocky Mountain region is dry, some residents use fences to capture snow for later use as a water source (Figure 9.11). Spring storms are also common, and heavy downpours can lead to flash flooding. Rain coupled with rapid snowmelt in the spring is another common source of flooding in the Rocky Mountain region’s river basins.

Figure 9.11: A snow fence near the Grand Tetons in Wyoming. Fences like these are used to force windblown snow to accumulate in a desired place, keeping it off roadways or collecting it for later use as a water supply.

The Northwest Central US is sparsely populated, with less than seven million people. Weather hazards are a concern for communities and for agriculture. When the area experiences severe drought, as Wyoming did from 1999 to 2004, residents experience costly losses in food and water supply, grazing land for livestock, soil erosion, wildfire damage, and air quality. The Red River in North Dakota is highly susceptible to flooding, and since it runs through Fargo and Grand Forks, the populations and infrastructure of those cities are put at risk during floods. In the winter, cold waves brought on by Arctic air masses entering the area can damage livestock and crops. Nebraska, located in a corridor known as Tornado Alley, commonly experiences violent thunderstorms and tornados in spring and summer.

What is a jet stream?

Jet streams—there are more than one—are narrow bands of fast moving air high above a planet’s surface. (Jupiter and Saturn have jet streams too.) The Earth’s rotation drives these rivers of air and causes them to blow from west to east. On Earth, they are typically found between 6 and 13 kilometers (4 and 8 miles) above the surface and can move at speeds tens to hundreds of kilometers (miles) per hour. Jet streams separate warm and cold air masses, and thus their movements can greatly influence the weather. Polar jet streams are typically found between 50° and 60° North or South latitude, and subtropical jet streams are typically found around 30° North or South latitude. As the boundaries between hot and cold air are sharpest in the winter months, jet streams are stronger in the winter. In the Northwest Central States, the polar jet stream strongly influences the area’s weather.

The polar vortex is a pattern of winds around the North Pole, including the polar jet stream. In the winters of 2013 - 2014 and 2014 - 2015, the polar vortex shifted southward, bringing unusual weather patterns to much of North America. Weaker polar vortices can occur when weather near the pole is warmer than usual, and a weak polar vortex allows for a wandering jet stream. Some climate scientists believe the unusual winters of recent years are explained by natural variations, while others suggest that they could be driven due to decreases in sea ice and faster increases in arctic temperatures when compared to areas at lower latitudes.

The polar jet stream over North America (shown in red). Warmer colors indicate regions of faster airflow.