Rain, snow, storms, wind, and everything in between can all be within a mid-latitude cyclone. In it’s most basic form, the mid-latitude cyclone is exactly as it sounds. A cyclone that is across the middle latitudes, like that of the United States.

These usually occur in transitions seasons, such as the fall and spring, but also occur in winter as cold air is better at plunging southward and colliding with warmer air. They become more rare in summer.

The model for how these worked was developed in the early 1900s, but these have been happening ever since there’s been an atmosphere and a jet stream. Below is a sequence of images that depict how a cyclone is formed.

The Norwegian Cyclone model, images via NWS.
  1. The clash of cold and warm air creates a stationary front. This is often where the jet stream aloft is flowing from west to east in the middle latitudes of the northern hemisphere.
  2. A wave of energy forms along the jet and puts a kink in the stationary front. This kink starts to force warmer air northward and colder air southward. Based on the coriolis force and where the warm/cold air is located, rotation starts to occur and a low pressure center begins forming at the surface.
  3. Warmer air northward starts to ride up over the colder, more dense air and a warm front forms. This front can often be associated with light precipitation, a wind turning southerly, and climbing temperatures. Cold air starts to plunge southward and plow under warmer air, creating a drastic change in temperature and can be associated with strong winds, heavy precipitation, and storms.
  4. Low pressure closes off and the slower moving warm front gets ‘caught’ by the faster moving cold front. That sweeps underneath the warm front which creates a triple point. This is where strong storms and very heavy precipitation can be found. An occluded front is where the cold front has overtaken the warm front. It is depicted in purple.
  5. The dissipation of the frontal boundary can come as the jet stream weakens or falls apart. Colder air disperses eastward and the entire system moves out of the region.
Example of a trough extending down from the upper-Midwest into Mexico and then back up into the Great Lakes. This setup created a mid-latitude cyclone across Texas.

The jet stream is clearly one of the most important parts of this whole process. It is the polar jet stream that helps keep the cold air to the north separated from the warm air to the south. These cyclones form in concert with upper-level troughs with the jet stream. When forecasting, it is important to look at the upper-level jet stream pattern to help identify where mid-latitude cyclones may form and bring inclement weather.