Cyclogenesis

[1] Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyclone, and at any size from the microscale to the synoptic scale.

The process in which an extratropical cyclone undergoes a rapid drop in atmospheric pressure (24 millibars or more) in a 24-hour period is referred to as explosive cyclogenesis, and is usually present during the formation of a nor'easter.

Synoptic scale systems cover a portion of a continent, such as extratropical cyclones, with dimensions of 1,000–2,500 km (620–1,550 mi) across.

For example, a system does not necessarily transition from meso-alpha to synoptic scale when its horizontal extent grows from 2,000 to 2,001 km (1,242.7 to 1,243.4 mi).

[8] A preexisting frontal boundary, as defined in surface weather analysis, is required for the development of a mid-latitude cyclone.

[11] However, enhanced along-frontal stretching rates in the lower troposphere can suppress the growth of extratropical cyclones.

[12][13] Cyclogenesis can only occur when temperature decreases polewards (to the north, in the northern hemisphere), and pressure perturbation lines tilt westward with height.

Usually the cold front will move at a quicker pace than the warm front and "catch up" with it due to the slow erosion of higher density airmass located out ahead of the cyclone and the higher density airmass sweeping in behind the cyclone, usually resulting in a narrowing warm sector.

Such upward motions decrease the mass of local atmospheric columns of air, which lower surface pressure.

[24] As the occlusion process extends further down the warm front and away from the central low, more and more of the available potential energy of the system is exhausted.

Tornadoes form in the warm sector of extratropical cyclones where a strong upper-level jet stream exists.

The cycle begins when a strong thunderstorm develops a rotating mesocyclone a few miles up in the atmosphere, becoming a supercell.

As rainfall in the storm increases, it drags with it an area of quickly descending air known as the rear flank downdraft (RFD).

[30] As the mesocyclone approaches the ground, a visible condensation funnel appears to descend from the base of the storm, often from a rotating wall cloud.

As the funnel descends, the RFD also reaches the ground, creating a gust front that can cause damage a good distance from the tornado.