Gustav: The science of the forecast

August 28, 2008

Any long-range weather outlook will become less accurate as the length of the forecast increases. For the case of Gustav, as with any tropical cyclone, we have to consider several factors:

1) Sea surface temperature: The ocean must contain enough heat to power the vast wind engine that is a hurricane. Generally, the water must be 80 degrees Fahrenheit or greater, and this warmth must extend for some distance below the surface.

2) Land masses in its path: Gustav was greatly hindered by the mountains in southwestern Haiti as it crossed that nation.  The topography interferes with the storm circulation and also causes the system to “rain itself out” as the moisture-laden tropical air is forced to rise over the slopes…making for torrential rainfall and landslides. Storms may occasionally make sudden, unexpected course changes as they weaken over land. The frictional drag of terrain on the circulation can also generate a wobbling motion.

3) The amount of wind shear in the atmosphere: Think of a mature hurricane as 10 CD’s stacked one on the other. Now imagine a ribbon of fast wind cutting through the top of this stack at around 30,000 feet, removing the top few discs.  When strong winds cut into a hurricane, they literally “shear” it apart and it can disintegrate within a day or two if this wind shear continues.

4) Atmospheric pressure systems:  A high pressure zone will “block” a hurricane and divert it around the edges.  In opposite fashion, a trough of low pressure will tend to “attract” the tropical storm or hurricane its direction.

5)  Mystery:  Since weather observations are spaced out and taken only every 12 hours or so in the upper atmosphere, these tropical systems will at times behave other than predicted. This is the limit of our science at the moment. That’s why the hurricane forecast map shown on KXII has a large “zone of error” which broadens the further out in days you go. You’ll see the same idea in play with the National Hurricane Center (NHC) graphics if you click the link on this weather page. It’s the best accuracy we have to forecast hurricanes in 2008. Perhaps it will improve in the future.






















So, when forecasting these beasts, we have to calculate the right sea temps, how much land is in the way (and what the topography is like) the potential wind shear, movement and strength of pressure systems, and be on the alert for subtle weather features. A weak, undiscovered trough can quickly lead to a bungled hurricane forecast. In the meteorological world, it’s going to be another head-scratcher (and hair puller) the next few days.

Take Care,

Steve LaNore

Chief Meteorologist



















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