Rapid intensification is defined by a tropical cylcone that strengthens by 35 mph or more within 24 hours.
It is not quite clear how rapid intensification occurs, as studies are still ongoing. However, tropical systems have been known to rapidly intensify when they enter an extremely favorable environment. This environment consist of very warm water, low wind shear, and lots of mid-level moisture.
Deep warm water can also cause rapid intensification. We often see rapid intensification of tropical cyclones in the Gulf of Mexico. This is where we see some of the warmest sea-surface temperatures and where some of the deepest warm water is located.
Rapid intensification is very dangerous. First off, the phenomena is hard to forecast, as weather models have a hard time picking up on all the variables that can lead to rapid intensification.
Since rapid intensification is hard to predict in tropical systems, it also makes it hard to prepare for. Imagine you are preparing for a category one hurricane approaching the coastline.
You wake up the next morning and all of a sudden you have a category four hurricane on your doorstep. This makes the preparedness and evacuation process much harder, as sometimes it might be too late. This is why it is always important to monitor forecast closely.