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Not all parts of a hurricane dump rain equally. What scientists call "rain bands" are lines of thunderstorms surrounding the center or "eye" of a hurricane. They're what give hurricanes their spiral appearance. And they're the part of a hurricane that generally produces rain, with most if not all rain falling on one side of the storm. The rain bands correspond closely to the orientation of the land topography.{ meaning they change the shape of the storm}

The satellite appearance of Gustav is steadily improving. Visible satellite loops of Gustav show that the storm has recently assumed a more symmetric appearance, though the heaviest thunderstorms are still just on the south side of the eye. Upper level winds from the south are creating about 15 knots of shear over Gustav, restricting the upper-level outflow on the south side. The 35-mile wide eye is not very distinct, and the Hurricane Hunters reported that the eyewall was missing a chunk on the southeast side. However, the eye was elliptical this morning, and has now assumed a more circular, well-formed appearance. The outer spiral bands of Gustav are now visible on New Orleans radar, and some rain bands have already affected the Mississippi River Delta region.

http://www.wunderground.c...log/JeffMasters/show.html


UF geographer: New tools to forecast hurricane rainfall inland
All eyes are on where hurricanes make landfall, but the massive storms actually cause the most deaths inland, where severe flooding often surprises residents.

In a paper in the current issue of the journal Professional Geographer, Corene Matyas, an assistant professor of geography at the University of Florida, outlines new tools to predict how the storm's intensity, distance it has moved inland and landscape topography alters its "rain shields" - the bands of heavy rain so visible in Doppler radar images. Among other things, her tools proved adept at modeling observations that when hurricanes or tropical storms encounter the Texas hill country or the Appalachian Mountains, their rain shields tend to line up in the same direction and with the same orientation as the underlying topography.

Researchers are developing some models for forecasting inland rain patterns, but they have difficulty accounting for the lopsided or elongated shape the pattern often takes, with most if not all rain falling on one side of the storm. A common assumption is that rainfall will decrease as the hurricane moves away from the ocean, which is generally true but may be obviated by other weather systems and local landscape.

With hurricanes crossing Texas hill country, the rain shields tend to line up parallel to the main axis of the hills, running west to east. Storms near the Appalachians also line up parallel to the mountains, whose axis runs southwest to northeast, with the heaviest rain consistently occurring to the west of the track. This is due to a combination of the mountains and a wedge of cold and dry continental air forcing the moist air upward, causing the water vapor to condense and fall to the ground as rain. This phenomenon does not happen with the Texas storms, as the dry continental air masses over Texas are similar in temperature to tropical moist air masses that accompany hurricanes.
http://news.ufl.edu/2007/06/19/hurricane-rainfall/