Thunderstorms can be categorized by their physical characteristics: the presence or absence of rotation, the number of location of updrafts and downdrafts present.
There is a continuous spectrum of types of storms that we observed. At times, it is difficult to place a storm into a specific category as a storm, or storm system evolves. However, there are five basic categories that are often useful in describing storms:
This storm forms when there is weak shear (change in wind speed or wind direction with height) in the atmosphere. Essentially, precipitation forming from rising air eventually becomes too heavy for the updraft to support its weight.
With the light wind and low instability, precipitation falls against the rising air cutting off the updraft and preventing further development. This is the same as the life cycle of a thunderstorm previously mentioned. Characteristics of this kind of storm include:
Pulse storms are also single cell storms due to weak wind shear. Usually not severe, but instability is sufficiently moderate so it supports more precipitation than its weaker version. The result is when the falling finally cuts off the updraft, the additional water content falls with a much greater force. These storms can create:
Any damage from pulse storms is isolated. Pulse storms are responsible for the expanding ring often seen on radar depictions. This ring is the gust front created by the falling rain and downburst winds.
These rings can act as minifronts in that they cause more uplifting air producing more pules storms. Many times, these outflow regions, seen on radar, intersect and form additional storms.
These are the most common type of thunderstorms. These are clusters of thunderstorm cells in different stages of life cycles. While individual cell life cycles last only 30 or so minutes, the cluster itself may last for several hours and move as a single unit. The clusters occasionally may contain a supercell.
There persistence stems from new updrafts forming in an area of persistent lifting where air converges in low levels, such as:
Typically, cells will develop in the lifting zone and move with the mid and upper level winds as it matures and dissipates, with new cells continuing to develop. In one typical scenario during the spring and summer months:
Given the right conditions, the cells can become severe within the multicellular cluster producing:
Frequently called squall lines, these are long line of storms where individual storm outflows merge to produce a continuous, well developed gust front. The line of storms is often oriented north-south or northeast-southwest and generally move toward east while individual cells, comprising the line, move northeast.
Like the multicellular cluster thunderstorms, new cells continually develop but this time on the leading, downwind, edge of the cluster. The new cell does not move quite as fast as the under cutting outflow so, the cells position relative to the line, shifts from the front to the back as it matures.
All the while, new cells form on the leading edge giving the line its eastward progression. These multicell line storms can produce strong downbursts producing "straight-line" wind damage where debris is often laying in straight lines parallel to the wind flow.
However, given the right environmental conditions, winds produced by stronger cells can cause a bulge in portions of the squall line. The bulges are called "bow echoes" or "bowing line segments" where, as seen from the radar, the line bows or arches.
Transient tornadoes an occur in squall lines in association with bow echoes. These tornadoes, however, tend to be weaker and shorter-lived on average than those associated with supercell thunderstorms.
However, the tornadoes tend to occur on the leading edge of squall lines and are often "rain wrapped" or appear with little discernible color difference with the background precipitation. Therefore, they can be extremely difficult to see which makes leading edge tornados very dangerous despite intensity.
The squall line maintains its intensity when the line's forward speed is close to the speed of the leading edge of the gust front. When the gusts front begins increasing its separation ahead of the precipitation, the updraft which feeds the storm begins to diminish causing the squall line to decrease.
