Severe Storms/Mid-latitude Cyclones
Extratropical cyclones, widely referred to as mid-latitude cyclones, are synoptic scale low pressure systems that form between 30° N and 60° N latitudes or 30° S and 60° S latitudes.
Characteristics of a Mid-Latitude Cyclone
Stages of Cyclogenesis
Cyclogenesis is a term used to describe the intensification or development of a cyclone, and is commonly used to describe the life cycle of a mid-latitude cyclone.
The first stage of cyclogenesis, the stationary stage, is named so due to the presence of a stationary front. The surface winds of the air mass converge, creating the low pressure system.
In the second stage of cyclogenesis, the wave stage, cold and warm fronts become established as the cyclone begins to rotate. In the northern hemisphere, the cold air mass moves southward on the west side of the cyclone and the warm air mass moves northward on the east side of the cyclone (the opposite is true for cyclones in the southern hemisphere).
During the third stage of cyclogenesis, the open stage, the cold air mass moves quickly to the southeast and the warm air mass moves gently to the north.
The fourth stage of cyclogenesis, the occluded stage, the cold air mass overtakes the warm air mass and the occluded front begins to form (specifically a cold occlusion). The triple point of a mid-latitude cyclone is present at this stage and is located where the occluded front, warm front, and cold front intersect.
In the fifth and final stage of cyclogenesis, the dissipation stage, the cold front will overtake all of the warm air in the low-pressure cell and the system begins to fall apart.
Other Modes of Formation
Lee-Side Lows - low pressure systems that form on the downwind (lee) side of a mountain range.
In the U.S., there are three main storm tracks of which mid-latitude cyclones develop through. These storm tracks have different development and movement speeds, which are dependent on the amount of moisture. The more moisture a system has, the slower it will develop and move.
- Alberta Clipper - originates in Alberta, Canada. Alberta Clippers tend to have relatively low moisture, although sometimes there's a strong inflow of moisture from the south. Alberta Clippers tend to develop and move quickly, sweeping through the Great Lakes region and curving northward toward Maine.
- Colorado Low - originates near Colorado. Colorado Lows tend to have intermediate levels of moisture, and therefore intermediate development and movement speeds as they sweep across the Ohio River Valley and move into the Atlantic just south of New England.
- Inside Leader - originates near the Four Corners region. Inside Leaders dip south near the Gulf of Mexico, gaining large amounts of moisture, therefore developing and moving slowly. Inside leaders move northward near the Appalachian Mountains and through New England.