Question

How can meteorologists use the jet stream to predict the weather

Answer 1

Because if the plane is flying fast the air is thin and cold, and if it is going slow then that mean that the air is thick 

Answer 2

They used jet streams because they are formed by air masses which have temperature differences.

The locations of the jet streams are also crucial for airplanes, because can generate turbulence, or vary the destination arrival time.

But meteorologists tend to follow and study this streams because they carry cyclones systems in lower atmosphere.