Patrick, what does it mean when a tropical cyclone is considered "extratropical" or "post-tropical"? 
Because on one of its projected paths a few days ago, NOAA projected Danielle to become an extratropical hurricane. How is this possible? 
Warning: This is really long scientific explanation, so here it goes!
There are two types of cyclones in the world by structure. Extratropical storms are asymmetric and have fronts, so these cyclones are dictated by temperature differences and wind shear in their background environments. Yes, every time you see a low with a warm front and cold front in the mid-latitudes, it's an extratropical cyclone. Tropical storms are symmetric and don't have any fronts. They're able to form in areas of no wind shear and no temperature difference in their background environments. We can go further and classify by the temperature as to whether the cyclones are cold-core or warm-core. Upper-level ridges are overhead of warm-core storms, and upper-level troughs are overhead of cold-core storms. A phase space showing the four possibilities of cyclones types (asymmetric vs. symmetric and warm-core vs. cold-core) was made by a Florida State researcher a couple decades ago to help us understand the spectrum of behavior and structure available for storms.
It's perfectly okay for cyclones to transition from one phase to another in that phase space, and Danielle is undergoing an extratropical transition as we speak. It's transitioning from a warm-core symmetric (tropical) cyclone to a cold-core asymmetric (extratropical) cyclone. Structurally, it's making a major change. It'll lose the overhead upper-level ridge, and you'll soon see a cold front and a warm front form, which means you'll have temperature differences. A stronger temperature difference is directly related to stronger vertical wind shear, so the increasing shear will tear up the symmetry of the storm and turn it into the comma-like storms we know. It'll soon merge into the upper-level trough that's helping to steer the storm out to sea. It may produce hurricane force winds of 74 mph or greater, but it won't look anything like a hurricane as it will have lost all its tropical characteristics. NHC is then forced to declare it extratropical and cease advisories at that point.
Hope this helps, Andy! Maybe I should bring back my thread where anyone can ask any kind of weather question to me, and I would try my best to answer them.
