Above is the model prediction for tomorrow morning's chart at 500 mb (about 19,000 feet). Notice that the solid heavy lines, that show the height of the 500 mb surface (and act to force the upper-level winds), contain a trough centered near the MS/AL border tomorrow morning.
This little trough is known as an "upper-level disturbance". It is also sometimes called "an impulse", "upper-level energy", and other things by us meteorologists. We associate upper-level disturbances with clouds and rain, generally. And, that is what we are forecasting for tomorrow. But, what is an upper-level disturbance?
In a nutshell, the schematic below shows what is going on. The wind generally flows parallel to the isobars (or the lines of equivalent height at upper levels, they are proportional to isobars). So, in the case of a trough, the air flows around the trough, down the west side, then turns east in the middle, then turns back north on the east side. The thing is, the wind is generally in balance (pressure gradient and Coriolis force) when the isobars are straight. But, in the trough itself, centrifugal force (like you feel in the car when you go around a curve, you feel thrown to the outside) throws air to the south, getting things off balance, and the air has to slow down. Once the air gets around the trough and back into straight flow east of the trough, it speeds back up.
The thing is, as the air speeds up on the east side of the trough, from 40 to 45 to 50 mph in the above example, the air is spreading out, or diverging. Imagine if you had 3 cars lined up at a stop sign. If the front one starts going 50, the middle one 45, and the back one 40, they will spread out. But, air tends to stay in balance, and spreading out of air without something to compensate it would cause a rapid drop in pressure (sort of like a vacuum). So, when you have upper-level divergence like this, air moves up from below to fill in the gap. Upward motion in the atmosphere is often associated with cooling of air to its dewpoint, producing clouds and rain.
On the back side of a disturbance, the air is converging, or piling up, so it must flow downward to avoid a buildup of pressure. So, on the back side of the disturbance, you generally have improving weather, as descending air warms and evaporates clouds and rain.
Dr. Tim Coleman
Fox 6 Severe Weather Expert and Weather Blogger
Follow me on twitter. I am at timbhm.