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to the downwind side of the existing thunderstorm system.
1st Edition, 1st Revision
5-12 18 Sep 03
COLLINS AVIATION WEATHER
MultiScan™ Radar Thunderstorms
N NOTE
Multi-cell thunderstorms have a normal life span of 3-4 hours.
STEADY-STATE THUNDERSTORMS
Steady-state thunderstorms do not follow the normal one-hour growth
and decay cycle for a typical thunderstorm. As the name implies, the
environmental factors that create these type of storms can give them
a long active life, from 6 to 8 hours. These storms can be 5 to 10 NM
in diameter and can grow to 50,000 - 60,000 feet. They can produce
updrafts that exceed 90 knots, 6-inch hail, damaging surface winds,
and large tornadoes.
N NOTE
In the following two paragraphs, numbers in parenthesis refer to
information found in figure 5-10.
Steady-state thunderstorms begin to form when warm, humid air begins
to rise from the earth’s surface (#1). As the air rises, mid-level winds
(#2) capture a portion of the rising air and redirect it to the surface as
a gust front (#3). A person standing on the surface will sometimes
experience this gust front as a blast of cool air as a thunderstorm
approaches. The gust front acts as a “plow” that lifts additional warm
moist air up into the storm system. This cycle of warm moist air being
lifted into the system by a gust front that is formed by mid-level winds is
what gives the thunderstorm its long life and is the reason it is referred
to as a steady-state storm.
Steady-state thunderstorms can generate tremendous updrafts that
force the storm to very high altitudes. The updrafts often cause the
top of the storm to “overshoot” (#4) the altitude where the tropopause
begins. Normally, weather does not form in the tropopause. Very
high energy levels are associated with weather that overshoots this
boundary. In a mature storm, upper level winds (#5) will cause the top
of the storm to form an anvil top (#6) downwind of the main body of the
storm. It is common for hail to be “thrown” out of the top of the storm
and in the direction of the anvil top. Thus, even though it may appear to
be clear under the anvil top, this area should be avoided due to its high
probability for hail (#7). This is also one reason thunderstorms should,
whenever possible, be circumnavigated on the upwind side.
1st Edition, 1st Revision
18 Sep 03 5-13
AVIATION WEATHER COLLINS
Thunderstorms MultiScan™ Radar
WARNING
Extreme turbulence, large hail, and tornadoes are characteristics
of steady state thunderstorms.
Figure 5-10 Steady-State Thunderstorm Structure
THUNDERSTORM CHARACTERISTICS
GUST FRONT/TURBULENT OUTFLOW
Gust fronts occur when a downdraft of cool air from a thunderstorm
comes in contact with the ground and spreads out laterally. Someone on
the surface in the path of the approaching the storm would experience
a gust of cool air much like a cold front passage. The passage of a
gust front is accompanied by strong gusty winds that may on occasion
exceed 55 knots.
Gust fronts are a form of turbulent outflow that emanates from the
thunderstorm. The turbulent outflow can create areas of severe
turbulence in much the same way that severe turbulence is associated
with frontal boundaries. Figure 5-11 shows a NEXRAD picture of a
thunderstorm with the corresponding turbulent outflow. Also note that
the gust front/turbulent outflow precedes the thunderstorm and is in the
1st Edition, 1st Revision
5-14 18 Sep 03
COLLINS AVIATION WEATHER
MultiScan™ Radar Thunderstorms
direction of cell movement. This is another reason that, when possible,
thunderstorms should be avoided to the upwind side.
Figure 5-11 NEXRAD Thunderstorm With Turbulent Outflow
THUNDERSTORM VAULTING
Thunderstorm “vaulting” occurs when thunderstorm updrafts are
so strong that large amounts of moisture are trapped high in the
thunderstorm cell and an area of potential energy is formed (figure
5-12). Little precipitation occurs at the bottom of the cloud due to the
fact that downdrafts are all but eliminated. This is especially possible
as thunderstorms transition from the towering cumulous stage to the
mature stage of development (♦page 5-9). At lower altitudes during
climb out, it is occasionally possible to miss the true extent of the
thunderstorm threat due to the fact that the radar beam may scan
below the high-energy area created by the vaulting of moisture high
in the storm.
N NOTE
MultiScan’s high beam scans along or slightly above the aircraft’s
climb out flight path to ensure that all potential areas of severe
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Collins Weather Radar operator’s guide(30)
