The jet stream is a meteorological phenomenon our forecasters often mention when describing winter storms and snowfall. This article will discuss what the jet stream is, what causes it to form, and why it’s important for mountain weather and snow forecasting.
What is the Jet Stream?
The jet stream is defined as a high-speed current of air that travels around the world in roughly a west-to-east direction. You can almost think of it as an invisible fast-moving river high in the atmosphere that flows in one general direction but meanders and curves as it does so.
The jet stream exists at an altitude of roughly 30,000 feet above sea level, but this varies a bit by latitude and by season. Specifics aside, the jet stream is decidedly an upper atmosphere feature, but one that influences the weather at ground-level throughout the world.
Why Does the Jet Stream Exist?
The jet stream forms where sharp temperature gradients exist in the atmosphere, separating cooler airmasses from warmer airmasses. The jet stream forms along boundaries where temperature differences are significant, which in turn results in sharp gradients in air pressure.
The greater the difference in temperature and pressure along these boundaries, the stronger the winds in the upper atmosphere.
In the mid-latitudes of the atmosphere, the tilt and rotation of the earth result in winds associated with the jet stream moving in a general west to east direction. However, the jet stream also meanders to the north in south into ridges and troughs along its journey across the globe based on momentum and temperature and pressure gradients.
The Polar and Subtropical Jet Streams
While we often refer to the “jet stream” in singular terms, there are actually two dominant jet streams in each hemisphere – the polar jet stream, which exists between roughly 30º and 60º latitude, and the subtropical jet stream, which exists between roughly 15º and 30º latitude.
The polar jet stream is usually what we focus on in terms of mountain weather in North America, Europe, Japan, and the Southern Hemisphere. However, the subtropical jet can also influence weather in the mountain ranges of the Southern United States, especially in the winter and spring.
Sometimes, a trough in the polar jet will dip far enough south to interact with the subtropical jet over an area, resulting in what is known as a coupled jet.
How the Jet Stream Varies by Season
The jet stream fluctuates throughout the year in terms of strength and location. But in general, the average position of the polar jet stream shifts southward during the winter months in the Northern Hemisphere as colder air works its way further south, before retreating northward again during the summer (the opposite is true in the Southern Hemisphere).
The subtropical jet stream actually moves slightly northward during the winter months in the Northern Hemisphere. This is because temperatures further south over the tropics change little between winter and summer whereas areas closer to 30º latitude see a noticeable decrease in winter temperatures versus summer, creating more of a temperature gradient that strengthens the subtropical jet and favors a more northerly position.
Temperature gradients from north to south are also stronger during the winter months in the mid-latitudes, and this results in a stronger jet stream during the winter and early spring months versus the summer months when the temperature gradient is weaker.
Bottom line, as the seasons transition from summer to fall to winter, the polar jet stream will become progressively stronger and will dip further south.
Driving Force Behind Storms
The jet stream can be thought of as the driving force behind storm systems. As the jet stream ebbs and flows, it causes ridges and troughs to develop. When troughs develop in the jet stream, these favor areas of low pressure and surface fronts developing, which support the development of organized storm systems when moisture is present.
Speaking of moisture, winds associated with the jet stream can transport moisture from ocean sources great distances onto the continents and across mountain ranges. When moisture encounters mountain ranges, it is forced to rise and condense into precipitation – a phenomenon known as the orographic effect.
Major storm systems across the globe typically follow the jet stream with stronger wind speeds within the jet stream favoring stronger storm systems. Areas located near and just north of the jet stream are typically favored for the heaviest and most widespread precipitation.
The curvature of the jet stream and the location of the stronger winds also influence storm strength and behavior.
When stronger winds are located on the left side of a trough, this causes the trough to dig further south, indicating the storm system associated with the trough is still in its developing phase.
When stronger winds are located on the right side of a trough, this causes the trough to move toward the northeast or east. This also indicates the storm associated with the trough is in its mature phase, as the right side of the trough is where the most intense weather (precipitation and wind) occurs.
Sometimes, the jet stream will take on a relatively straight-line appearance over an area without much curvature, also known as a “zonal” flow. Zonal jet streams can result in heavy snowfall or rainfall rates as well.
What is a Jet Streak?
A jet streak is a fancy term for a local wind speed maximum within the jet stream. While the location of the jet stream is important as being the main driver of storms, jet streaks can result in locally heavy snow or rain.
This is due to diverging air streams caused by strong winds aloft, which in turn leads to rapid acceleration of rising air currents to "fill the void" left by the diverging air in the upper atmosphere.
Relatively narrow bands of heavy snow or rain can develop and remain nearly stationary in the vicinity of a jet streak, especially near the left exit region or right entrance region of the jet streak. In the winter, this can result in some areas receiving substantial snowfall over a relatively short period of time, while other areas not far away will see comparatively less snow.
Jet Stream Effects on Snowfall and Mountain Weather
When moisture is present, areas located near or just north of the jet stream (in the Northern Hemisphere) are most favored for heavy snowfall. On a more localized scale, areas just to the south of the jet stream near the right entrance region of a jet streak are also favored for snowfall.
When the winds in the jet stream are strong enough and moisture levels are significant, it can result in heavy snowfall rates not only across the typically-favored higher elevation western slopes of mountain ranges, but also valley areas or downwind slopes of mountain ranges that usually receive less snowfall.
The jet stream also can result in strong winds in the mountains. This is true for any location near the jet stream, including areas just to the north and just to the south. Areas located just to the south of the jet stream often miss out on more significant precipitation, but winds can be very strong in these regions even when conditions are dry.
In the summer months, the jet stream tends to be weaker but is often still a factor. Even relatively weak jet streams can act as a “trigger” mechanism for thunderstorm development when moisture and instability are present. This can result in strong and fast-moving thunderstorms, and also “training” of thunderstorms in which storms redevelop and track over the same areas.
In the Eastern U.S. (primarily the Southern Appalachians), the jet stream can also be a factor in springtime severe thunderstorm outbreaks as jet stream winds are typically stronger at this time of year compared to the summer, but instability is also greater and temperatures warmer compared to the winter.
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