Severe Weather Watcher Handbook

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The gust front ushers in a cool gusty wind, either just ahead of the heavier precipitation or along the flanking line. Too much outflow can push the gust front far ahead of the storm, eventually cutting off the inflow. The storm will then stop regenerating and weaken. A detached gust front/shelf cloud may become a roll cloud (a long horizontal tube) before evaporating, or may rebuild new cumulus towers that will spawn a new storm later. But if the flanking line remains intact and connected to the original storm, inflow will continue and the storm can revive and reassert its former strength and structure.

Types of storms

There are three simplified storm types:the single cell; the multicell, and the supercell. Each has a distinct structure, circulation pattern, and set of updraft/regeneration charateristics. Thinking about their differences can help you interpret the clouds you see.

A single-cell thunderstorm is one whose life cycle portrays the growth and collapse of a single updraft cell. The cloud forms, grows to maturity, produces a heavy downpour, and then decays as downdrafts suffocate and dilute the original warm inflow. Such storms are common  in mid-summer and usually last no more than an hour. They are ordinary,harmless events that almost never produce any severe weather.In rare cases,a strong single-cell storm called a pulse storm can produce brief damaging winds during its collapse phase.

The multicell storm type is the most common and consists of successive, separate updraft pulses which maintain a more or less steady state for the system's overall strength, structure,and appearance. The pulses may be very close together - causing the storm's characteristics to be quite uniform over time - or they may be widely spaced, resulting in a storm that cycles repeatedly through stronger and weaker phases.These pulses in the system can be seen from a distance by watching the spacing and growth rate of individual cumulus towers along the flanking line as they merge with the main storm cloud.Each one may be part of a solid, continuous base but they mature as separate steps, adding their distinguishable contribution to the storm - precipitation pattern and anvil structure. When all factors are considered, the cloud system can be quite varied and provides an excellent opportunity to observe the convective life-cycle in continuous evolution. Multicell storms can cause all the severe effects, but they produce infrequent and short-lived tornadoes.

9  A typical multicell storm with a succession of rising towers along the flank (lower left). As each one matures, it spreads out to become the anvil at the top and the current core with heavy rain below. The rain and cool downdrafts then provide a "barrier" against which the warm air rises to form the next tower down the line. If the updrafts are strong enough, there may be a lowering on the inflow-outflow boundary under the storm.

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