Honeywell Primus 880 Pilot's Manual page 109

Step
Three of the most common erratic motions are:
1. Right Turning Echo. This is the most frequently
observed erratic motion. Sometimes a thunderstorm
echo traveling the same direction and speed as nearby
thunderstorm echoes, slows, and turns to the right of its
previous motion. The erratic motion may last an hour or
more before it resumes its previous motion. The storm
should be considered severe while this erratic motion
is in progress.
2. Splitting Echoes. Sometimes a large (20- mile or
larger diameter) echo splits into two echoes. The
southernmost echo often slows, turns to the right of its
previous motion, and becomes severe with large hail
and extreme turbulence.
If a tornado develops, it is usually at the right rear
portion of the southern echo. When the storm weakens,
it usually resumes its original direction of movement.
The northern echo moves left of the mean wind,
increases speed and often produces large hail and
extreme turbulence.
3. Merging
become severe, but often the circulation of the
merging cells interfere with each other preventing
intensification. The greatest likelihood of aviation
hazards is at the right rear section of the echo.
Severe Weather Avoidance Procedures
A28- 1146- 102- 00
PRIMUSr 880 Digital Weather Radar System
Procedure
Echoes. Merging
Table 5- 12 (cont)
echoes sometimes
Radar Facts
5-59