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greatest storm intensity and hazards (discussed below)
are attained during the mature stage.
The precipitation and downdrafts in an ordinary thunderstorm
are eventually responsible for its demise, as
the supply of heat and moisture is cut off, leading to the
dissipating stage. As the Cb dissipates, the mid-level
cloud becomes more stratiform (spread out). Remnant
cloud can linger for some time after the storm begins to
dissipate, especially the upper-level cirrus anvil, which
consists mostly of ice. In hazy conditions, or with Cb
imbedded in widespread cloudiness, judging the stage
of the Cb lifecycle can be difficult.
Thunderstorms frequently last longer than an hour. As
an ordinary thunderstorm reaches its mature stage,
cool and sometimes quite strong surface outflows are
created by Cb downdrafts reaching the ground and
spreading out. The outflows can act as a focus for lifting
warm, moist air that may still be ahead of the
advancing storm. New cells form in the direction of
storm movement, for instance, a Cb moving eastward
will generate new storms on the east side. The new cell
undergoes a mature and dissipating stage as it progresses
towards the back of what has become a cluster
of Cb. This is an example of a multi-cell thunderstorm,
which, depending on the vertical shear of the
wind, will continue to regenerate new cells as long as
unstable air exists ahead of the moving storm. Multicell
thunderstorms can last for several hours and travel
100 miles or more. As usual when dealing with
weather phenomena, there is no clear distinction
between an ordinary and multi-cell thunderstorm. For
instance, a thunderstorm may last one to two hours
after having undergone regeneration two or three
times. Pilots need to closely watch apparently dissipating
thunderstorms for new dark, firm bases that
indicate a new cell forming. In addition, outflow from
one Cb may flow several miles before encountering an
area where the air is primed for lifting given an extra
boost. The relatively cool air in the outflow can provide
that boost, leading to new a Cb, which is nearby,
but not connected to the original Cb. [Figure 9-17]
Another type of thunderstorm is the supercell. These
huge and long-lasting storms are usually associated
with severe weather: strong surface winds exceeding
50 knots, hail at least 3/4 inch in diameter, and/or tornadoes.
Supercells can occur anywhere in the United
States, but are most common in the southern Great
Plains. They differ from ordinary thunderstorms in
two ways. First supercells are much larger in size.
Second, they form in an unstable environment with
A B C
Figure 9-16. Lifecycle of an ordinary thunderstorm (A) cumulus stage, (B) mature stage, (C) dissipating stage.
9-15
large vertical direction and speed shear, which causes
the updraft to be tilted or even twisted so that is located
away from the main downdraft. This leads to an organized
circulation within the storm, hence the longevity
of a supercell. These dangerous storms should be
avoided. [Figure 9-18]
Thunderstorms sometimes exist in clusters, known to
meteorologists as Mesoscale Convective Systems
(MCS). Most commonly, MCSs form east of the
Rocky Mountains in the spring and summer months.
Squall lines are MCSs that are organized in a line or
arc, sometimes hundreds of miles long. A cold front or
upper-level trough advancing on unstable air can be
the forcing mechanism for squall-line development.
Strong lift can be found ahead of advancing squall line
and some extended cross-country flights have been
made using them. Unfortunately, they come with all
the dangers of severe thunderstorms, so their use is not
recommended. Another type of MCS has a similar
name, the Mesoscale Convective Complex (MCC),
which form as a cluster of storms not along any distinct
line or arc. When viewed from satellite, MCCs
reveal a circular or elliptical shape to the cirrus anvil,
which can be a few hundred miles across. Severe thunderstorms
are often embedded within the MCC.
Fortunately, the huge cirrus shield tends to suppress
thermals away from the thunderstorm, so soaring
pilots should not have the opportunity to approach an
MCS. However, since they can contain severe
weather, the forecast of a possible MCS or other severe
thunderstorm may inspire the glider pilot to avoid
leaving a glider outside overnight if other options are
available, e.g., an enclosed trailer. Securing the trailer
extra well would also be advisable.
Using the Skew-T and a morning sounding, the possibility
　
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