ROCHESTER, NY (WROC) – The jet stream is a fast-moving flow of air in the upper levels of the atmosphere that helps separate cold and warm air. There are entire textbooks dedicated to how a jet stream works and how it applies to the forecast, but we will be as concise as possible here.
The next time you walk outside, look up and imagine you are as high as a 747 jet engine flying from Chicago to London. That plane is flying about as high as the jet stream, somewhere between 25,000 & 35,000 feet. In the northern hemisphere a jet stream flows from west to east in two basic patterns. Zonal flow is horizontally west to east and helps separate the cold air to the north and warm air to the south. Meridional flow is when the jet stream becomes wavy and we start to get active weather. Remember that active weather comes when colder and warmer air clash.
The polar jet stream during winter is what provides us with arctic blasts and big winter storms. During summer this jet retreats northward and the subtropical jet stream and move northwards and provide moisture for big rain events.
DETAILS & APPLICATIONS OF THE JET STREAM:
The polar jet stream exists somewhere between 50°-60° latitude and can move at speeds higher than 250 mph. The subtropical jet is a bit slower and hangs around 30° latitude. As jet streams meander across the United States like a wave, they create patterns by forcing air up or down depending on the location of the jet stream.
Just like water, air is a fluid and acts as such. Just like waves on Lake Ontario, there are troughs and ridges of this long “river” of air that we are calling the jet stream. Imagine that on a scale of thousands of miles, high up in the atmosphere. When the jet stream sags southward, that means colder air pushes south and we get a trough. This is associated with stormy weather and is a signal that we can expect either clouds and rain in the summer or potentially snow and wind in the winter.
When the jet stream surges northward, colder air retreats and warmer air advances north, that is a ridge. This is often associated with clearing skies at the surface and calm weather. In the example below, THe jet stream is flowing in a trough pattern on the western and eastern half of the United States (dashed line) and ridge pattern in the middle part of the United States (zig-zag line).
The jet stream forecast becomes very useful when it comes to longer range forecasts. Certain models stretch out to fourteen days or more ahead of initialization. These models are often significantly off when it comes to surface forecasts, but are considered more accurate as to the general pattern aloft, where the jet stream lives. We can look at certain groups of models put together and when they show a collective pattern of either a zonal or meridional flow, we can make generalizations on what a general pattern will be for the next few weeks. While a surface forecast for a specific day will not be reliable, a jet stream pattern can indicate a wetter, drier, warmer, or colder pattern.
You can observe forecasts of the jet stream in many different places online, including here. It is always important to check the upper air pattern when forecasting as that will give clues as to what we can expect for the future.