Honeywell Primus 880 Pilot's Manual page 108

PRIMUSr 880 Digital Weather Radar System
Step
9 Avoid all rapidly growing storms by 20 miles.
When severe storms and rapid development are evident,
the intensity of the radar return may increase by a huge
factor in a matter of minutes. Moreover, the summit of the
storm cells may grow at 7000 ft/min. The pilot cannot
expect a flightpath through such a field of strong storms
separated by 20 to 30 NM to be free of severe
turbulence.
10 Avoid all storms showing erratic motion by 20
miles.
Thunderstorms tend to move with the average wind that
exists between the base and top of the cloud. Any motion
differing from this is considered erratic and may indicate
the storm is severe. There are several causes of erratic
motion. They may act individually or in concert. Three of
the most important causes of erratic motion are:
1. Moisture Source. Thunderstorms tend to grow toward
a layer of very moist air (usually south or southeast in
the U.S.) in the lowest 1500 to 5000 ft above the earth' s
surface. Moist air generates most of the energy for the
storm' s growth and activity. Thus, a thunderstorm may
tend to move with the average wind flow around it, but
also grow toward moisture. When the growth toward
moisture is rapid, the echo motion often appears
erratic. On at least one occasion, a thunderstorm echo
moved in direct opposition to the average wind!
2. Disturbed Wind Flow. Sometimes thunderstorm
updrafts block winds near the thunderstorm and act
much like a rock in a shallow river bed. This pillar of
updraft forces the winds outside the storm to flow
around the storm instead of carrying it along. This also
happens in wake eddies that often form downstream of
the blocking updraft
10 3. Interaction With Other Storms. A thunderstorm that is
(cont)
located between another storm and its moisture source
may cause the blocked storm to have erratic motion.
Sometimes the blocking of moisture is effective enough
to cause the thunderstorm to dissipate.
Severe Weather Avoidance Procedures
Radar Facts
5-58
Procedure
Table 5- 12 (cont)
A28- 1146- 102- 00