When a tropical cyclone or hurricane enters the subtropics and mid-latitudes, closer to Canada; it can interact with weather systems that are there. These systems can include mature storm systems, leftover surface systems or disturbances in the upper atmosphere. In many cases, a change takes place, resulting in a new weather system that is a cross between a tropical cyclone and an extratropical cyclone. This process is called extratropical transition (ET), and meteorologists often simply call the resulting storms ETs.
Although with time the ET will end up being mostly extratropical, these systems always keep some tropical traits, even after moving across the entire Atlantic Ocean.
Almost all 'Canadian-style' tropical cyclones are ETs in some stage of transition. Hurricane Juan (2003) was a rare example of a tropical cyclone that was late in starting the transition process, arriving at Nova Scotia’s coast with most of its tropical qualities still intact. ET storms affecting Canada have often been as bad as, or even worse than, their earlier tropical versions.
Hurricane Hazel (1954) is the best-known Canadian example of a hurricane that wasn’t really a hurricane. The storm that hit Toronto on that fateful October night was actually a newly formed extratropical cyclone, but one that had completely ingested the energy and moisture of Hazel.
In November 2007, Hurricane Noel completed its ET stage long before arriving in Nova Scotia. But the extratropical storm that hit was as powerful as any Category 1 hurricane, and it delivered the most damaging waves felt along the coast in at least 50 years. Whether an ET weakens or strengthens depends on the timing and phasing of a number of atmospheric conditions.
Understanding these storms and the transition process is very important because as these storms transform, their structure, behavior and impacts also change. ETs are an ongoing concern for Canada because they are some of the most challenging storms to predict, and they can bring surprisingly severe weather. Forecasters tracking a hurricane have to go to a different 'play book' when they think that extratropical transition is taking place. They look for certain telltale signs such as the development of front-like characteristics, the erosion of the deep convection and cloud shield, and an expansion in the radius of gales, to name a few. When and where this transition takes place is important because the impacts are different.
Source: Canadian Hurricane Centre