"What is a short wave? No more than about half as long as a long wave. Short waves move more quickly, though. They travel eastward down the long wave and show up as a bump on the larger trend. This is where we label most of our upper troughs and ridges.
We also find that they are tied to maximum and minimum values in vorticity, with our troughs associated with higher vorticity and ridges with minimum vorticity. In this way, a short wave trough commonly indicates an axis of maximum vorticity. So what? This explanation ties in with weather in this fashion: air rises and clouds form ahead of a trough, while it descends and clouds dissipate in front of a ridge.
In fact vertical motion is responsible for this cloud co-relation. Also, these relations assist analysts in finding world map latitude longitude - troughs and ridges - by comparing to a satellite photo, especially where there is little other data to use, such as over the oceans."
Now look at the current 500MB map (commonly and historically used as a major tool in steering influences for hurricane development and movement)
An imaginary line drawn through the dips in the lines indicate the trough lines and one drawn along the upward peaks or bends show the ridges. The two to watch here is the trough from Canada to New Mexico and the ridge from the Atlantic coast through Floria toward Cuba's west end. The trough will draw Gustav up into the Gulf at the same time the ridge will force it to stay west of Florida's east coast. Right now they are betting on the high moving slowly north eastward while the trough moves generaly eastward and like you would push the bottom of a squeeze bottle to force the contents upward, Gustav will be pushed along the forecast path.