The thing to remember here is that most things happen above the ground before they happen on the ground and what flies overhead has a direct bearing on what will happen later at the surface. The arrows are wind direction and the length of the arrow indicated speed. The longer the arrow the stronger the wind. The shaded area is the region of strongest winds or jet. Think of this as being a suspended curtain of horizontal wind separating generally colder polar air to the north and warmer tropical air to the south.
Air coming from the Pacific is generally moist, while air coming overland is often dry.
A jet south of Montana opens us for mostly a polar influence with cool and often very cold temperatures. A jet north of Montana allows warm tropical air to invade the region. A jet overhead means that we are having a spot of weather, generally harsh. This is still way up there. The white lines are lines of equal pressure and the little flags indicate wind direction.