You know it’s summer time when the “omega pattern” is observed at 500mb over the United States. Okay, so this pattern can occur anytime of the year, but usually occurs between the months of June to early August. What is the omega pattern, you ask? Below is a diagram and general description of this phenomena:
The most active or “severe” weather occurs east of the trough (L’s) axis. Because the ridge is so strong in the central U.S., the short wave troughs usually have to climb over the ridge. Think of it as the ridge or “H” being a big hill. The jet stream carries these L’s over the hill, essentially. And because the most active weather occurs east of the trough axis in the Northern Hemisphere, Wyoming, North Dakota, South Dakota, Iowa, northern Nebraska, Minnesota, Wisconsin & northern and western Michigan often have severe weather during the summer time, depending on the location of the ridge axis. This can be said about the northeastern states too, but usually not as severe at the central/northern Plains and Great Lakes regions.
All of the quiet and hot weather occurs in the central United States during this pattern. Ridges are associated with subsidence, or sinking air. Below is a forecast model forecasting this pattern for Sunday evening, June 25, 2012: