Longwaves and Shortwaves
Downwind of LW trough is conducive to cyclone development (red shading) These
are large areas with modest-levels
of upper-level divergence!
One longwave
typically spans North
America (3). About 3-5 longwaves
are present on any given day.
They are move very little from
day to day.
Ahrens, Meteorology Today, 5th Ed.