Interesting Weather Information

Tuesday, March 19, 2013

Thunderstorm Primer - Part 4 - Linear Mesoscale Convective Systems - The Line Echo Wave Pattern

A Line Echo Wave Pattern  (or LEWP - pronounced loop) is a Linear MCS (mesoscale convective system) that is composed of more than one bow echo. See my previous post for more on Bow Echoes.

Bow echoes can merge to form a LEWP, or a relatively straight squall line can develop the bow shaped bulges as shown in the schematic. The lows that develop initially start because of the mid-level Rear Inflow Jet (RIJ).  As the lows strengthen and heavy rains drive cool downdrafts, a very complicated process is taking place, and the net effect is that the RIJs strengthen. In turn the lows strengthen more.

At the apex where the southern end of one bow meets the northern end of another bow a strong, small-scale low pressure system can form leading to enough spin for isolated tornadoes. Supercells can form within LEWPs but are fairly rare. Most of the tornadoes associated with a LEWP are leading edge vortices and form near the low pressure systems, but not exclusively.

 Bow Echoes and LEWPs can become derechos, large, long lived squall lines which come in two varieties: serial derechos and progressive derechos. More on this in a later post.

In the next  Thunderstorm Primer post is a big blob or cluster of thunderstorms called a mesoscale convective complex.

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