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Extratropical Transition

When a tropical cyclone enters the subtropics and mid-latitudes, it can interact with other extratropical 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. Some of Western Europe’s worst extratropical storms were former ETs. Neither tropical cyclones nor ETs directly hit the west coast of Canada. However, some bad fall and winter storms (extratropical cyclones)--ex-typhoons that have tracked across the Pacific-- have hit British Columbia long after their ET stage. The most infamous case was ex-Typhoon Freda in 1962.  Because of its impact on the American northwest, it became known as the Columbus Day Storm.

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. ETstorms 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.

Simple graphic showing where extratropical transition occurs during the lifecycle of tropical cyclones
Simple graphic showing where extratropical transition occurs during the lifecycle of tropical cyclones

A 2001 study by Hart and Evans showed that extratropical transition happens more often in Atlantic Canadian waters than anywhere else in the world. Therefore, Canada’s main tropical cyclone problem is accurately predicting extratropical transition.

The table below shows the differences between tropical cyclones and extratropical cyclones. It helps explain the challenge of predicting the transition before it actually happens.

Differences Between Tropical Cyclones and Extratropical Cyclones
FactorTropical CycloneExtratropical Cyclone
Storm entityThe vortex of the storm is unconnected to the surrounding atmosphere, much like a cork floating in a riverThe vortex of the storm is connected to and is part of the surrounding atmosphere, much like the water near the drain in a bathtub
Maximum wind in the stormLocated in the lower part of the storm, below 2 000 feetLocated in the upper part of the storm, above 30 000 feet (known as the jet stream)
Maximum wind near the surfaceLocated close to the storm centre, just outside the eye of the storm … just to the right of the storm along the direction of motionLocated well away from the storm centre and generally in an elongated orientation along the direction of motion and to the right of the storm
Thermal nature of the storm’s coreWarm coreCold core
Driving energy mechanismEnergy driven by latent-heat release of condensation through deep convectionEnergy driven by baroclinic instability which results from warm and cold air coming in close proximity
RainfallForms in spiral bands around the stormForms in a large area predominantly to the left side of the storm in the direction of motion

To understand the challenge of forecasting ETs, consider Hurricane Ivan from September 2004. The rainfall and upper atmospheric moisture from a dying Ivan (inland over Tennessee) fed northward into a developing storm over eastern Canada. The result was a much more intense storm system over Atlantic Canada than would have otherwise developed, with Newfoundland waters experiencing hurricane-force winds and fatalities. Meanwhile, the lower atmospheric rotation of Ivan drifted back southward, through Florida, into the Gulf of Mexico, where it renewed itself as Tropical Storm Ivan, creating dangerous conditions all over again along some U.S. Gulf coasts.

Typical threat areas

As mentioned earlier, most tropical cyclones are in some stage of extratropical transition when they arrive in Canada or in Canadian waters. Even Hurricane Juan (2003), which was still more tropical than most by the time it reached Canada, was still undergoing ET when it arrived. This is important because the resulting weather and impacts are different than what is seen with a purely tropical cyclone. At the same time, the weather is also different from what might be expected from the purely extratropical storm systems that are common in Canada. The very simple graphic below shows the usual arrangement of the weather and oceanic impacts with typical ETs. Go to the section on Hazards and Impacts for details about the wind, rain, storm surge and ocean waves that are seen with post-tropical storms. For more information on the distinction between the terms "extratropical storm" and "post-tropical storm", visit the Post-tropical Cyclonespage.

The graphic explains the usual threat areas of each kind of weather that a post-tropical storm normally brings (explained in detail below).

This map shows the track, or path, of "Post-tropical Storm Whatzizname", a fictitious storm that is moving northeast through Newfoundland. The graphic explains the usual threat areas of each kind of weather that a post-tropical storm normally brings. The main area of rain (green) is located to the left of the storm track, whereas the worst wind, waves and storm surge are on the right. The highest surge (yellow) is at the coast and immediately to the right of the storm track. The highest winds (red) are generally much stronger to the right of the track and at a distance out from the track (the actual distance depends on a number of factors). The highest waves (blue) are also to the right of the track. They are well-removed from the track itself--at least as far out as the highest winds and sometimes twice that distance.

First Canadian hurricane flight – Hurricane Michael (2000)

In 2000 when Hurricane Michael was bearing down on Newfoundland, Canada flew its first hurricane-mission flight into the storm. The National Research Council (NRC) partnered with Environment Canada to fly NRC’s Convair 580 research aircraft into the hurricane to gather data for forecasters and researchers. Later flight missions into Karen (2001), Isabel (2003) and Juan (2003) gave important data that help us to understand the nature of ETs--but none of them were as important as Michael.

Hurricane Michael storm track
Hurricane Michael storm track
Convair 580 research aircraft. Photo: National Research Council
Convair 580 research aircraft. Photo: National Research Council

The NRC crew and Environment Canada research-flight specialists, including a CHC forecaster, flew into Michael when it was southeast of Nova Scotia on October 19. The flight pattern was designed so that researchers could gather data in all quadrants of the storm as well as through the hurricane’s eye.

National Research Council and Environment Canada research scientists conducting a research flight. Photo: National Research Council and CHC
National Research Council and Environment Canada research scientists conducting a research flight. Photo: National Research Council and CHC
Hurricane Michael dropsonde sequence and storm track map. Photo: Environment Canada © 2009
Sixteen dropsondes were deployed throughout the storm between 16:00 and 18:30 UTC(1:30-4:00 p.m. NDT ). © Environment Canada, 2009
Dropsondes graph for Hurricane Michael. © Environment Canada, 2009
Dropsondes graph for Hurricane Michael. © Environment Canada, 2009

Remarkable wind speeds were recorded on the right side of the storm by the seventh dropsonde, at 16:40 UTC (2:10 p.m. NDT ), shown here. Note the increase in wind speed between the surface and 500 metres; this is normally seen with tropical cyclones. Above that, however, one would expect to see the winds gradually decrease as the altitude increases. In the case of Michael, the hurricane was clearly showing that it was a storm in transition. What was surprising was the strength of the maximum winds: 70-72 metres per second, or 252-259 kilometres per hour.  Category 5 hurricane-strength winds begin at 252 km/h!

It was also surprising that these powerful winds were occurring through a very deep layer of the lower atmosphere, from 500 to 2 000 metres (almost 5 000 feet thick). This was the first time that researchers had obtained detailed wind data from inside a rapidly transitioning storm, and the results got everyone’s attention. If the very stabilizing effects of the cold Atlantic Canadian waters had not kept the high winds from reaching the surface, Michael could have been an enormously devastating storm for Newfoundland. Forecasters, therefore, have to forecast the power of the storm, but they also have to predict the stability of the lower atmosphere to figure out how much of the high wind will reach down to where we live and play.

Research about extratropical transition began in the late 1990s and continues today. The knowledge has been furthered in Canada by, among other things, research flights into Canadian-style tropical cyclones. Read the reports about: