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Storm Ophelia was so unusual, it was literally off the charts

Storm Ophelia was so unusual, it was literally off the charts

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A graphic depicting the storm’s high speed winds spilled over its boundaries

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Graphic by the National Hurricane Center

Over the weekend, post-tropical cyclone Ophelia hit an unusual place, Ireland — so unusual that a graphic depicting the storm’s high-speed winds was abruptly cut off. That's because so few tropical cyclones travel so far northeast that the US National Hurricane Center (NHC) doesn't include the area in its wind forecast graphics.

Ophelia started out in the North Atlantic as a tropical cyclone that gathered strength as it traveled northeast, briefly becoming a Category 3 hurricane. The storm ventured farther east than any other Category 3 hurricane on record, the New York Times reports.

To help people prepare for a storm, scientists at the NHC prepare maps showing the color-coded probabilities of where high-speed winds might reach. If a point on the grid is highly likely to see sustained, tropical-storm-force winds, it’s colored in purple. If it’s unlikely, green. These maps include a thousand different plausible scenarios that could play out in the hours to days after the hurricane forms, says Michael Brennan, senior hurricane specialist at the NHC.

When scientists at the NHC wrote the program to show wind speed probabilities, they based its boundaries on where tropical cyclones had hit over the past few decades, says Mark DeMaria, the acting deputy director of the NHC. None had hit farther north than 60 degrees, or farther east than 0 degrees longitude — roughly in the middle of the North Sea. “The larger the grid, the longer it takes to run the computation — so it’s a tradeoff,” Brennan says.

Ophelia stayed within those boundaries, but the regions that could possibly be touched by its winds stretched beyond the edges of the map. That’s why that multicolored swoosh was cut off. “In hindsight, in figuring out what domain to use, we probably should have left a little bit of room to take into account the size of the storm,” DeMaria says.

Matthew Onderlinde, an NHC meteorologist, tweaked the code to expand the grid’s boundaries for windspeed probabilities.
Matthew Onderlinde, an NHC meteorologist, tweaked the code to expand the grid’s boundaries for windspeed probabilities.
Graphic by Matthew Onderlinde, National Hurricane center.

It’s rare for a storm traveling north to stay as strong as Ophelia did: Ocean waters usually get colder as you move up the map, and warm water is what fuels a hurricane’s strength. But this year, the ocean temperatures were warmer than usual, and the upper atmosphere was colder than usual — which combined to create a strong storm where they don’t typically occur, DeMaria says.

It’s hard to say whether similar hurricanes will continue occurring more frequently, but in the meantime, the NHC is considering extending its map’s boundaries. “We most likely will have it extend further east and a little bit further north for next season,” DeMaria says.

Update October 18th, 9:45AM ET: This post has been updated to include a picture of a windspeed probabilities graphic with expanded boundaries.