How Hurricanes Form
What is needed:
· Warm water (at least 80° F)
· High humidity
· Moist air
· Warm surface temperatures
This is why hurricanes can be tracked for a while in advance; they typically form off the coast of Africa in the Atlantic ocean, by the equator, and travel over the open ocean. This gives the continuous evaporation and condensation cycle that is necessary to the tropical storm to get stronger. As the water condenses, it gives off latent heat and this decreases surface pressure.
Now the air begins to rotate around the low pressure area, continuing the evaporation and condensation cycle.
Irene in particular is encountering an area of dry air, which will help to weaken her. However, the risk of storm surges (which are usually what cause the most significant damage when a hurricane is due to make landfall) is still high and extremely likely.
These are exactly what they sound like. Surges of the coastal waters being pushed by the wind and low pressure (this is responsible for about 5% of the surge). With Irene pushing on toward densely populated parts of the North Eastern United States, this is a huge issue regardless of her category being downgraded; between high sea-level populations and underground transit systems, the damage is estimated to be in the multibillion dollar range (Associated Press, 26AUG11).