Climate in the Northwest-central United States

Present climate of the northwest-central U.S.

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 Rocky Mountains, 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 U.S. 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.

Temperature

Temperatures in the northwest-central U.S. 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 U.S. 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. 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.

Average Annual Temperatures in the Northwest-central United States

Precipitation

The northwest-central U.S. is dry compared with many other parts of the United States. In fact, it is 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. 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 region. 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.

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.

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.

Severe weather

The northwest-central U.S. is sparsely populated, with fewer 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, it is affected by costly losses to food and water supplies and grazing land for livestock, as well as soil erosion, wildfire damage, and poor 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 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.

Polar jet stream and winter storms

Harsh winter storms are a fact of life in the northwest-central U.S. These storms are carried in by the polar jet stream, which typically falls near or over the area, especially in the winter. Jet streams—more than one exists—are narrow bands of fast moving air high above Earth's surface. The Earth’s rotation drives these rivers of air and causes them to blow from west to east. Jet streams 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.

Because jet streams separate warm and cold air masses, their movements influence the weather. Polar jet streams are typically found between 50° and 60° North and South latitude, and subtropical jet streams are typically found around 30° North and 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 impacts the area’s weather. Blizzards with high winds, high snowfalls, and low visibility are common. They are brought on by cold air masses known as the Alberta Low from the north and the Colorado Low from the south. Snow fences are used to keep snow from drifting over roadways (snow fences can also be used to capture snow as a water source). Spring storms are common, and heavy downpours can lead to flash flooding. Rain coupled with rapid snowmelt in the spring is a common source of flooding in the Rocky Mountain region’s river basins.

The polar vortex

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.

Past climate of the northwest-central U.S.

In this Guide we divide the northwest-central states into five regions, but it is possible to more generally recognize two broad areas of strikingly different geology: the Cordilleran (Idaho, western Montana, and western Wyoming; Regions 3–5) and the Great Plains (North and South Dakota, the rest of Wyoming and Montana, and Nebraska; Regions 1 and 2). The main difference between the two areas is that the Cordilleran area has been subjected to mountain building, while the Plains area has remained tectonically quiet.

Paleozoic (541 to 252 million years ago)

Cambrian to Ordovician (541 to 444 million years ago)

Throughout most of the Paleozoic, the Northwest Central was part of a large passive margin that formed when Rodinia broke up, and major changes in deposition there were related to changes in climate and sea level. With the start of the Paleozoic era, climates across the world were warm, and North America was located in the low and warmer latitudes of the Southern Hemisphere. As the Cambrian progressed, North America moved northward; by about 480 million years ago, what would become the northwest-central region was located just above the equator. Cambrian fossils reveal that most of the area was covered by warm, shallow seas during this time.

The Earth went through another ice age from 460 to 430 million years ago, and although sea level dropped during this event, North America’s position near the equator kept its climate relatively warm. The change in sea level meant that the environment of the Plains area fluctuated from shallow marine, to brackish, to freshwater, and back. Farther west, in what is now the northern Rocky Mountains, the environment mostly alternated between shallow and deeper marine.

Ordovician rocks in Idaho contain abundant fossils of graptolites, which are thought to have floated in the open ocean, and thus indicate deeper waters than those implied by bottom-dwelling brachiopods, corals, cephalopods, and other fossils common in other Paleozoic rocks. One of the characteristics of these warm, shallow sea deposits is that they often alternated between limestone, sandstone, and mudstone; reefs were not common at this time.

Silurian to Devonian (444 to 359 million years ago)

A major interruption in this overall picture occurred during the Devonian period, when the huge Bakken oil formation that underlies parts of Montana and North Dakota formed. This oil-rich rock is part of a larger complex of such deposits that covered not just this area but large areas farther east as well. The richness of the organic matter indicates a sea that was highly productive, with such abundant planktonic life that the organic matter from the dead organisms took up all the oxygen in the water, allowing the rest to remain undecayed and preserved in the sediments.

Carboniferous to Permian (359 to 252 million years ago)

During the late Paleozoic, the sea gradually began to withdraw, and the Plains area became terrestrial. The sea still flooded parts of the northern Cordilleran area—this time farther west, in southeastern Idaho—and became exceptionally productive during the Permian. This is evidenced by the large deposits of phosphorite, a rock mined for fertilizer in Idaho, Wyoming, and Montana. By the late Carboniferous, North America had collided with Gondwana, leading to the formation of Pangaea. Pangaea was a supercontinent composed of nearly all the landmass on Earth.

Mesozoic (252 to 66 million years ago)

Triassic (252 to 201 million years ago)

Around 220 million years ago the northwest-central states moved north from the equator. By this time, the sea had withdrawn completely from the area. Sediments suggest that the Triassic climate in the northwest-central U.S. was warm. Initially arid, it gradually shifted to a humid climate with abundant, seasonal rainfall. The climate resembled that of modern India, where monsoons soak the land in the summer and completely dry out in the winter. At the very end of the Triassic, climate once again became arid.

Some Triassic rocks now found in the northwest-central were not, however, part of the North American continent at that time. Triassic rocks in western Idaho include tropical reefs, and the fossils found in them (such as corals and brachiopods) are not similar to fossils found in the rest of North America. Many parts of the western U.S. originated as microcontinents (also called terranes) that drifted in during the process of subduction at the continent’s active plate margin and accreted to North America as they collided with it. The Triassic reef deposits in Idaho rode in on one such microcontinent.

Jurassic (201 to 145 million years ago)

Pangaea began to break up during the Jurassic, rifting apart into continents that would drift toward their modern-day positions. The supercontinent was split by spreading along the mid-Atlantic ridge, initiating the formation of the Atlantic Ocean. The Jurassic remained warm, but gradually became wetter, this time without the strong seasonality of the Triassic. The region was ruled by dinosaurs, and some of the most famous Jurassic dinosaur localities in North America are found in the northwest-central states, such as Como Bluff in Wyoming.

By this time, mountain building (the Laramide Orogeny) was underway. The Black Hills were uplifted and sediment was deposited from both west and east. Ancient metamorphic rocks of the continental core were uplifted and eventually exposed in the Black Hills and even farther west.

Cretaceous (145 to 66 million years ago)

As a result of displacement due to continental rifting and seafloor spreading, sea level throughout the Cretaceous was much higher than it is today. Global temperatures during the Cretaceous were very warm, as much as 10°C (18°F) above those at present. There was likely little or no glacial ice anywhere on Earth, and temperatures were highest in lower latitudes.

Shallow seaways spread over many of the continents, including South America, Africa, Eurasia, and North America. In the middle Cretaceous, oceans covered most of the Southwest, with the exception of parts of Arizona and New Mexico. By the start of the Late Cretaceous, this inland sea, called the Western Interior Seaway, divided North America in two. Cretaceous fossils from the northwest-central states, especially the Great Plains region, show that the seaway supported sharks, rays, mosasaurs (large marine reptiles), and giant turtles, while crocodiles and dinosaurs were abundant on land. This seaway was also productive, although most of its organic-rich rocks lie just south of the northwest-central states.

In the latest Cretaceous, sea level dropped again and the western northwest-central became a broad coastal plain that hosted lush forests, abundant dinosaurs, and large swamps. By the end of the Cretaceous, uplift to the west was great enough that the resulting hills shed large amounts of sand and gravel in an easterly direction, pushing the shoreline eastward until sediment (combined with a worldwide drop in sea level) filled the area formerly occupied by the Western Interior Seaway. At the close of the Mesozoic, global climate—although warmer than today—was cooler than at the start of the era.

End-Cretaceous extinction (66 million years ago)

At the very end of the Cretaceous, the Gulf Coast experienced an enormous disruption when a large asteroid or bolide collided with Earth in what is now the northern Yucatán Peninsula in Mexico. The impact vaporized both water and rock, blocking out sunlight for weeks to years, which led to a collapse of photosynthesis and food webs on land and in the oceans. The event devastated the northwest-central U.S.

Cenozoic (66 million years ago to present)

Paleogene to Neogene (66 to 2.6 million years ago)

Following that event, the climate may have cooled briefly (as suggested, for example, by an abundance of ferns), but it soon rebounded to a warmer state, and continued to warm into the Eocene. Around 60 million years ago, much of the northwest-central U.S. had a milder climate than it does today, and it was even subtropical in some areas. Dinosaurs gave way to mammals, and forests with ferns, palms, and dawn redwoods provided food for browsers.

The climate was wet enough in the Eocene to support large lakes in Wyoming, although these lakes occasionally dried out. The lakes supported an abundant diversity of fish and other organisms that today are exquisitely preserved as the famous Green River Formation fossils.

By the early Cenozoic, the continents had approached their modern configuration, and India began to collide with Asia to form the Himalayas. The formation of the Himalayas had a significant impact on global climate, with the newly exposed rock serving as a sink to take up atmospheric CO₂. With the reduction of this greenhouse gas, global temperatures cooled. Antarctica moved south, and by 30 million years ago, temperatures were low enough that glaciers began to grow on its mountains. 

Studies of ancient soils show that parts of Montana, Nebraska, and Wyoming went through several periods of warm, wet climate between 35 and 4 million years ago, although overall the climate became drier. Grasses evolved during the Miocene as climate became drier. Miocene rocks in Nebraska support some of the most amazing sites for fossil mammals known anywhere.

Approximately 3.5 million years ago, glacial ice began to form over the Arctic Ocean and on the northern parts of North America and Eurasia. A major contributing factor to this event was a geological change that occurred far to the south. The Central American Isthmus, which today makes up most of Panama and Costa Rica, rose out of the ocean at approximately this time, formed by undersea volcanoes. The new dry-land isthmus blocked the warm ocean currents that had been flowing east-to-west from the Atlantic to the Pacific for more than 100 million years, diverting them into the Gulf of Mexico and ultimately into the western Atlantic Gulf Stream. The strengthened Gulf Stream carried more warm, moist air with it into the northern Atlantic, which caused increased snowfall in high latitudes, leading to accelerating cooling.

Quaternary (2.6 million years ago to present)

Eventually, a sheet of sea ice formed over the Arctic, and ice sheets spread over northern Asia, Europe, and North America, signaling the start of the most recent ice age. Since 800,000 years ago, an equilibrium has been reached between warming and cooling, with the ice caps growing and retreating primarily due to the influence of astronomical forces (i.e., the combined gravitational effects of the Earth, Sun, moon, and planets). During the ice sheet’s maximum extent, it reached into Montana, the Dakotas, and Nebraska. 

The portions of the northwest-central U.S. that were not covered by ice experienced a variety of cold climates and abundant lakes. These lakes were also related to two very large flooding events, among the largest floods on Earth.

The first flood event was the Bonneville megaflood: melting glaciers fed the waters of ancient Lake Bonneville (the remains of which are today the Great Salt Lake), which broke through a dam of loose sediment and rapidly drained northward through southern Idaho, along what is now the Snake River, all the way to northern Idaho.

The second flood event was a series of floods that occurred when the ice sheet alternately blocked and retreated from what is now the Clark Fork River in northwestern Montana and northern Idaho. When the river was blocked, an enormous lake built behind the ice dam, and when the ice dam failed, the water was released catastrophically. Although the floods mostly affected central Washington, large ripples from the intense flow are preserved both near Missoula, Montana, and just downstream from where the ice dammed the river in northern Idaho.

Between 13,000 and 8500 years ago, fossil evidence shows that spruce and aspen forests grew in areas of North Dakota that are now warmer, drier, and covered with prairie. Idaho became more humid and warmer than it was during the last glacial maximum.