Opinion

Cone of uncertainty

(Note: Having access to the internet will be helpful when reading this column.)

With respect to hurricanes, have you ever heard of the “forecast cone”? Well, I’m going to tell you where to find it, and how to interpret it, because it instantly clarifies where a hurricane is heading.

The National Hurricane Center issues a “forecast cone” graphic whenever they are monitoring a tropical depression, a tropical storm or a hurricane. The shape of the cone describes the projected potential paths, while the line running down the center has letters inside circles which relate to the projected intensity along the storm’s projected path. 

More on that in a bit. Google “Hurricane Charley forecast cone,” then “images,” then click on the one in the upper left. Next, click on that same image in the upper right, and you will be taken to a report, where you’ll find a larger version of the image on page 4.

Charley was “Exhibit A” as to why anyone within the forecast cone must be prepared. The centerline within the cone went right toward Tampa Bay.  However, while still remaining within the forecast cone, Charley turned right as it rapidly intensified, and made landfall farther south down the coast as a very small but very ferocious hurricane. 

People said meteorologists missed the forecast because the storm did not strike Tampa Bay. But it did impact the coastline within the forecast cone.  

I urge you to google “Hurricane Charley RaceTrac.” Just watch the 37-second video with sound up. When it suddenly sounds like a jet engine, those are gusts around 145 mph. 

While a forecast cone contains the current projected range of path possibilities of the storm’s center, it does not show the size (diameter) of the storm, and that’s very important with respect to storm surge. A larger diameter storm can drive a lot of water toward the coast, even if, like Sandy in 2012, it is “only” a category 1 hurricane. So storm surge is not just a function of hurricane wind speed, it is also a function of hurricane size. 

Storm surge is the vertical rise in water levels above normally dry ground. Visualize being in a swimming pool and extending your arm all the way out and then moving it forward. You’re able to move a lot of water that way. Compare that to trying to move water by just flipping your wrist while cupping your hand. The “arm all the way out” example would be the large-diameter hurricane. 

A larger diameter hurricane will produce storm surge along a longer stretch of coastline. However, it’s important to note that a hurricane does not need to have a large diameter to be incredibly destructive. Camille in 1969 is a great example of that. Camille was a small hurricane, but it was extremely intense, and generated a 24 foot storm surge on the Mississippi Gulf Coast, along with winds which gusted to 190 mph.

Back again to the forecast cone. The solid white part of the cone is the National Hurricane Center’s forecast of path possibilities out through three days. The edges of the cone represent the range of possibilities. The storm might track along the right edge, right down the center, or on the left edge. This is why the National Hurricane Center always tells folks not to focus on the centerline track.

The part of the cone that is not solid white shows the forecast out through days four and five.

Now let’s address the letters inside the circles that relate to the storm’s intensity. Along the centerline within the forecast cone, you might come across the letters D, S, H or M. They stand for tropical depression, tropical storm (the point at which a name is assigned), hurricane and major hurricane (Categories 3, 4 and 5). In common usage, all of these are based solely on wind speed. A tropical depression is upgraded to a tropical storm when the sustained winds reach 39 mph. A tropical storm is upgraded to a Cat 1 hurricane when the sustained wind reaches 74 mph. A hurricane is called a major hurricane when the sustained winds reach 111 mph. A category 5 hurricane contains sustained winds of 157 mph or stronger.

Remember, anytime there is a tropical system, starting with a tropical depression, the National Hurricane Center will issue a forecast cone graphic, along with their other forecast products. Just Google “NHC” for the center’s home page. Once you click on a storm they are monitoring, you’ll find the cone as the fifth graphic in from the left on the top row of icons. Under it, it will read “warnings/cone” and under that it will read, “static images.”

Once you’ve clicked on a storm, on the top line you’ll see “public advisory,” and then, fourth over, “forecast discussion.” They both are excellent. The discussion is pretty technical, but it is a fun read if you are really into weather. And here in the “big sky country of the East,” I think a lot of people are.

Ted Shapiro holds the Broadcast Seal of Approval from both the American Meteorological Society and the National Weather Association. An Alexandria, Va. native, he has been chief meteorologist at WAGM-TV since 2006. Email him at tshapiro@wagmtv.com.

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