Weather Blog: Chris becomes extratropical, but what does that mean?

Posted: Jul 09, 2018 6:50 AM Updated:

Chris was a hurricane just yesterday, but just 24 hours later it looks and is something completely different. The National Hurricane Center issued its last advisory for Chris at 11 a.m. since it is undergoing what they call "extratropical transition."

What does it mean when a storm becomes extratropical? First we have to understand what a tropical cyclone actually means.

Tropical cyclones are any storm system that grows by evaporating warm ocean water into it and rotates around a center of low pressure at sea level. Tropical depressions, tropical storms and hurricanes are all tropical cyclones. To make things more confusing, hurricanes are called typhoons in the Western Pacific and cyclones in the South Pacific and Indian Ocean. No matter what they're called, they can all be lumped together as tropical cyclones. Because tropical cyclones regularly cause hazardous weather, they're tracked extensively and given names.

 

Chris however is now an extratropical cyclone. You may not have realized it, but you're impacted by extratropical cyclones all the time. This is another name for the regular storm systems we see all the time north of the tropics. These storms grow because of differences in temperature across large distances. Because of this, they track in the regions between the warm tropics near the equator and frigid Arctic, right where we live in the United States. Because they're so common, we often just call them storms or storm systems when we talk about them on TV.

 

 

Let's go back to Chris. Even yesterday, Chris was being influenced by a large kink in the jet stream, a belt of high winds that circle the globe high above the ground. This part of the jet stream was pushing Chris to the north and eventually began to tear apart the structure of the storm from the top down. At the same time this was happening, Chris tracked out of the warm waters of the Gulf Stream current. Ocean temperatures went from the low 80s to the low 40s! Because of these factors, there was no way Chris could survive as a tropical cyclone.

 

The energy from Chris couldn't just disappear though, so it essentially morphed into a new storm to take the rest of that energy further north. The structure of Chris on Thursday is classic for an extratropical system, shaped like a comma with clouds to the west and north of the center. Chris will continue to create high seas, high winds, and heavy rain even though it's no longer tropical.

 

 

The track for Chris takes it north over Newfoundland and near the nation of Iceland over the next few days. Even though Chris is no longer a tropical cyclone, it will maintain tropical storm force winds through at least the weekend. Eventually, Chris will weaken and merge with another system, ceasing to exist somewhere north of Europe next week.

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