"During June, the jet stream developed kinks in its air current that served to shuttle moisture-packed frontal systems and their associated thunderstorms toward the East Coast while leaving the West high and dry. These kinks are called troughs and ridges. A ridge is a large area of above normal atmospheric pressure that pushes the jet stream northward of its usual position. A trough is a low-pressure area that allows the jet stream to dip southward. Since early June, the eastern half of the U.S. has been sandwiched between two ridges: one over the West and the other over the western Atlantic, creating a trough in between. "
(See a map of the upper level circulation here.) (NOAA)