As an operational forecaster, it becomes very helpful to remember common rules when putting together a forecast, and one of these rules deals with the position of the 700 mb trough.

If you know when the 700 mb trough will swing through, you'll have a better idea of when the weather will clear out. Take a look at the picture above. This picture deals with our Thanksgiving forecast later this week. Notice the position of the 700 mb trough in relation to our area.

Subsidence, or sinking air at the mid levels of the atmosphere, becomes responsible for the weather clearing up. The picture is for midnight heading into Thanksgiving morning. Looks like Thanksgiving will be cold, but dry, thanks to the speed of our next system.