Troughs are "U-shaped" dips in the jet stream; some are rather narrow and uniform while others are broad and uneven, often covering a large area of the country. Since the trough allows cool, dry air to spill through its interior, its leading edge is a cold front, igniting thunderstorms and heavy rain as it pushes into warm, moist air east of the trough. Over the past two days, such a cold front triggered severe storms and flooding rains from Texas to Michigan; here in central Missouri, we received 2-3 inches of rain.
As the trough moves eastward, areas within its "U" become cooler and drier, sheltered from the active borders. After a string of summer like days, we sit at 46 degrees F this morning and our afternoon high should remain in the sixties; such trough-related respites are always welcome during the hot, muggy days of July and August.
Finally, as the trough continues to move east, its back edge approaches as a warm front, ushering more seasonable temperatures into the region and triggering showers as this warmer, more humid air overrides the cool air within the trough; this third stage is expected to pass through Missouri tomorrow and Thursday. Should the trough slow down or stagnate due to strong high pressure to its east, any one of the three stages may persist for days or weeks, producing stationary fronts and, especially at its leading edge, flooding rains.
