Radar Tutorial
Chapter 1, Page 3: Line Echo Wave Patterns
Line Echo Wave Patterns (LEWP) are fairly common. They often occur when a squall line begins to pick up momentum into
an warm moist air mass. They can be indicative of severe winds. On occasion, tornadoes can form. Certain characteristics
of LEWP images can suggest the possible the existence of severe weather. Should certain characteristics of LEWPs occur,
spotters could be directed to verify the actual occurrence of these conditions. The following discussion will help the
reader identify certain severe weather radar signatures and suggest how the actual occurrences can be verified.
A
The most common severe weather associated with LEWPs are straight line winds. Such winds cause some of the most wide
spread damage that occur in land-based severe weather. The radar image will seem to bow out at one point. In a radar
loop, you might see the center of the bow accelerating out from each end. This is often called a "bow echo" radar
signature. See "A" in the image on this page.
Often one can see this sort weather coming before it arrives in the field as well. There may be an obvious shelf cloud
or roll cloud preceding the rain. You'd expect to see the gust front which maybe severe as the shelf or roll cloud
passes overhead. Remember, not every bow echo, shelf or roll cloud is associated with strong winds. But the
possibility will be there.
B
Breaks in the line (see "B" in the image on this page) can be locations where rotation (mesocyclones) can develop. Though
less frequent than bow echoes, tornadoes can occur at these breaks in the line. Breaks in the line may not be so
obvious from the field due to hazy conditions from all the moisture in the air. But a break in the rain or possibly
in the clouds may be visible from afar.
C
Perhaps the most likely place one might find rotation and therefore a tornado is in the southern most cell of a LEWP.
Its sometimes called a "tail end charlie". There the cell may take on traditional dimensions in the field complete
with a rain-free base at the southwestern edge and a recognizable inflow pattern.
D
At "D" in the image above, a mesocyclone can also develop. This type requires a bow echo just to the south of the
northern most cell. The "rotating comma head" may form at the very top of the line. There are stronger winds to the
south and weaker winds to the north, setting up a cyclonic circulation and a possible tornado area. This sort of
phenomena is relatively rare in the northern part of the US. It is very difficult to detect in the field unless the
spotter happens to be in the right place.
E
Another possible area of tornado development is at the trailing point between two bow echoes. Here there are converging
winds and rotation may occur significantly enough to form a mesocyclone and possibly a tornado. Again, spotters will
have to be on top of this one to see it coming.
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