The jet stream is the single biggest driver of day-to-day weather changes across the midlatitudes. It’s a river of fast-moving air high in the atmosphere, blowing from west to east at speeds that can reach a couple of hundred miles per hour. Where it bends, where it speeds up, and where it stalls directly determines whether your week will be sunny and warm, cold and rainy, or stuck in an unusual pattern that won’t budge.
What the Jet Stream Actually Is
The polar jet stream sits in the troposphere, the lowest layer of the atmosphere where all weather occurs. It flows at roughly 30,000 to 35,000 feet, driven by the temperature contrast between cold polar air to the north and warmer tropical air to the south. The sharper that contrast, the faster and straighter the jet blows. When the contrast weakens, the jet slows down and starts to meander in large waves.
Those waves are called Rossby waves, and they’re the key to understanding your local forecast. Picture the jet stream not as a straight line but as a series of dips and bulges snaking across the continent. Each dip (called a trough) pulls cold air southward. Each bulge (called a ridge) pushes warm air northward. The position of these features over your area at any given time largely determines your temperature, cloud cover, and chance of precipitation.
Ridges Bring Warmth, Troughs Bring Cold
The relationship is straightforward. Air inside a ridge is sinking, which compresses and warms it while clearing out clouds. If a ridge parks over your region, you get sunnier skies and temperatures warmer than normal. This effect is especially strong at higher latitudes, where a ridge can push temperatures well above seasonal averages.
Under a trough, the opposite happens. The jet stream dips southward, pulling colder air down from higher latitudes. Clouds are more likely to form, and northerly winds reinforce the cooling. When a trough builds over a region, the combination of cloudier weather and cold-air transport can keep temperatures stubbornly below normal for days.
How the Jet Stream Steers Storms
Most major storm systems in the midlatitudes don’t wander randomly. They follow the jet stream like a conveyor belt. The steering-level winds (the winds at jet stream altitude) push low-pressure systems from west to east, which is why weather fronts in the United States generally move across the country in that direction. Tropical storms entering the midlatitudes are typically swept northeastward and out to sea by these same westerly winds.
But the jet stream doesn’t just move storms along. It also helps create them. Faster-moving segments within the jet, called jet streaks, generate pockets of rising air beneath them. When air at high altitude spreads apart (diverges), it essentially pulls air upward from below to fill the gap. That upward motion cools the rising air, encourages cloud formation, and can trigger or intensify storm systems at the surface. Forecasters watch jet streak positions closely because the zones of divergence beneath them are prime territory for storm development.
Hurricane Sandy in 2012 illustrated what happens when the jet stream behaves unusually. Instead of steering the hurricane safely out to sea, a southward-shifted Atlantic jet stream combined with a blocking high-pressure system over the western Atlantic to push Sandy directly into the New Jersey coast. The storm’s unprecedented left turn was a product of jet stream positioning.
Fueling Severe Thunderstorms and Tornadoes
The jet stream’s role in severe weather goes beyond just steering storms. Upper-level divergence from jet streaks actively primes the atmosphere for thunderstorm development. When divergence at high altitude creates upward motion, it cools the upper atmosphere while warmer, moist air sits near the surface. That growing temperature difference between the ground and the upper atmosphere makes the air increasingly unstable, which is exactly the setup needed for deep, powerful thunderstorms.
During major tornado outbreaks, you’ll often find a strong jet streak positioned so that its zone of maximum upward motion sits directly over a region where warm, humid surface air is already in place. The jet stream provides the “lift” from above while surface conditions provide the fuel from below. When those ingredients overlap with wind shear (winds changing speed or direction at different altitudes, also a product of the jet stream), the result can be rotating supercell thunderstorms capable of producing tornadoes.
Atmospheric Rivers and Moisture Transport
The jet stream also acts as a guide rail for atmospheric rivers: long, narrow corridors of water vapor that transport enormous amounts of moisture through the sky. An average atmospheric river carries roughly the same volume of water vapor as the average flow at the mouth of the Mississippi River. Exceptionally strong ones can carry up to 15 times that amount.
These moisture plumes travel along and near the jet stream, and when they make landfall, they can dump extraordinary amounts of rain or snow. The U.S. West Coast is particularly familiar with this pattern. When the jet stream aims an atmospheric river at California or the Pacific Northwest, the result can be days of heavy precipitation, flooding, and mudslides. When the jet stream shifts and atmospheric rivers miss the coast, drought conditions can develop.
When the Jet Stream Gets Stuck
Some of the most extreme local weather happens not when the jet stream is moving normally but when it stalls. Blocking events occur when a large ridge becomes nearly stationary, disrupting the usual west-to-east flow for a week or longer. The high-pressure block sits in place like a boulder in a stream, forcing storm systems to detour around it.
The consequences depend on which side of the block you’re on. Under the blocking ridge itself, sinking air produces clear skies and dry conditions. If this persists, the result is a heat wave in summer or an unusual warm spell in winter, and prolonged blocking is a major contributor to drought. Regions on the periphery of the block, meanwhile, may get hammered by storms that are forced to take unusual paths. The weather on both sides of a blocking pattern tends to be abnormal, and the longer the block persists, the more extreme conditions become.
Seasonal Shifts Change Your Baseline
The jet stream doesn’t stay in the same place year-round. It migrates with the seasons, following the zone of greatest temperature contrast between polar and tropical air. Over eastern North America, the shift is dramatic: the core of the strongest westerly winds sits over the southeastern United States during January through March, then migrates all the way to southern Canada by July and August. This northward retreat is a major reason summer weather patterns feel so different from winter ones. Storms track farther north, and the southern and central states fall under the influence of subtropical high pressure rather than active jet stream weather.
In winter, the jet drops south again, bringing the storm track with it. This is why winter storms routinely affect areas that see little severe weather in summer. The jet stream’s seasonal latitude determines which regions sit in the active storm track and which sit in the quieter zones to the north or south of it.
A Warming Arctic and Waviness
The Arctic is warming faster than the rest of the planet, a phenomenon called Arctic amplification. Because the jet stream is powered by the temperature difference between polar and tropical regions, a warming Arctic narrows that gap. The proposed result: a weaker, wavier jet stream that meanders more and moves more slowly, leading to more persistent weather extremes at the surface.
This idea, first formally proposed in 2012, generated intense scientific interest. Researchers did observe an increase in jet stream waviness between 1990 and 2010. However, a clear consensus hasn’t emerged. Recent analysis shows that the waviness observed in recent decades falls well within the range of variability seen in the early to mid-twentieth century, before Arctic amplification was a factor. The connection between a warming Arctic and a wavier jet stream remains an active and genuinely unresolved question, with the answer depending partly on how waviness is measured.
What is clear is that the jet stream’s behavior, whether it’s straight and fast or slow and wavy, has an outsized influence on the weather you experience. A slight shift in its position of just a few hundred miles can mean the difference between a record heat wave and a cool, rainy week.