During Winter, jetstream tends to be lower in atmosphere, so 300 hPa height is useful.

Left exit & right entrance regions bring upwards motion enhancing development of showers or thunderstorms.
Right exit & left entrance regions bring downwards motion weakening development of showers.

A little explanation where to find these zones:

Jet streaks pull in air upstream (to their west) in what is called the entrance region and throw it out downstream (to their east) in what is called exit region. They are further subdivided in a left or right zone as shown on the picture.