There are many ways to categorize a hurricane.
The storm's central pressure is one way. The height of its storm surge is another. How much rain it drops is another. And, of course, the strength of its wind is another. But what causes a hurricane to strengthen or weaken? The answer is simple enough, but multifaceted.
In order for a tropical wave or disturbance to organize and become a tropical system, ocean water temperatures need to be above 80 degrees, and the wind aloft needs to be light. If those favorable conditions persist, then the system progresses to the hurricane stage. As that storm approaches places where we live, or have family or property, then interest in the storm's strength really heightens.
There are many factors that impact whether that hurricane will strengthen or weaken, or remain in a steady state. The most obvious is the ocean temperature, and it's more than just the surface temperature…we meteorologists look at the upper ocean's heat content. If the storm moves over cooler waters, then there will be some weakening. Conversely, warmer ocean waters are like throwing gasoline on a fire. Warmer waters mean that more water vapor evaporates up into the storm, which is then turned into rain. But there's more to that story: Condensation releases energy…and that's energy added to the hurricane's system. What becomes especially interesting is if a hurricane becomes stationary. When that happens, cooler water from deep below the ocean surface upwells, and a cooler pool of water can form underneath the hurricane, which can weaken it. In fact, sometimes, a tropical system moving over the same path as a previous hurricane encounters these areas of cooler water, which can affect their own development.
And remember that there are numerous currents in the ocean and Gulf of Mexico, and some of these currents carry warmer water that, if a storm passes over, can cause unexpected intensification.
Another factor the determines a tropical cyclone's strength is wind shear, which is a change in the wind speed (or direction) with height. Tropical systems develop in areas of low wind shear, because this allows those intense thunderstorms near its center to develop vertically. When there is stronger wind aloft, those thunderstorms tilt, and some of the energy from condensing water vapor doesn't remain in the tropical system. One good way to see if a tropical storm or hurricane is experiencing wind shear is simply to look at the satellite images. If you see a rather circular, concentric area of clouds around the center (we call this a CDO, or central dense overcast), then there is probably very low wind shear. However, if the CDO is significantly lopsided, then the storm is probably experiencing wind shear, which would either weaken it or limit its development.
So what causes the wind shear? Sometimes it's just the jet stream aloft getting closer to the system. Other times, it can be something seemingly innocuous like a small upper level disturbance or low pressure area nearby. But the wind environment in proximity to a hurricane is a key part of the strength forecast.
All of the above are closely monitored by the Hurricane Hunter aircraft that you've heard about. Those brave men and women fly in and around hurricanes, dropping probes to acquire data about the upper level wind and ocean temperatures near these storms.
Finally, while hurricanes thrive over warm ocean waters, they begin an immediate deterioration after moving over land. But sometimes, even just being close to land impacts a storm.
As you can see, there is a LOT that we meteorologists need to consider when discussing with you the anticipated strength of an approaching hurricane. It's a tough job, and rapid intensification is probably the single most difficult aspect of a hurricane forecast.