Hurricane Forecast “Cone” Versus Actual Impacts – They Are VERY Different!

We have officially started the hurricane season as of June 1st, As I said in my last blog, early season tropical storms and hurricanes tend to form more readily in the Gulf of Mexico which means less time to prepare before landfall than if a storm moves across the Atlantic into the Gulf. Thus, we need to be prepared now.

But, today let’s discuss the hurricane forecast cone versus actual weather impacts, which are VERY different, and has confused many people. The hurricane forecast cone graphic below is from Hurricane Sandy in 2012. You have likely seen this type of forecast cone many times from the National Hurricane Center (NHC) and/or media.

Image 1

First, let me explain what this graphic actually means. “S” means tropical storm, with winds between 39 and 73 miles per hour. “H” means hurricane, with winds exceeding 74-110 miles per hour. If you ever see an “M”, that means major hurricane with winds greater than 110 miles per hour.

Second, the actual cone is where the expected center of the storm will be. It does NOT mean if you are outside the cone there is nothing to worry about. Historical records show that when comparing the forecast cone with what actually happened, the center of the storm ends up outside of the cone 1/3 of the time! So, lesson one to remember is not to focus on the forecast cone, which can change dramatically day to day right up to landfall.

The second graphic below is new, beginning this year by the NHC (www.nhc.noaa.gov).

Image 2

In addition to the forecast cone, you can overlay the area affected by tropical force winds (shaded in yellow). This yellow area will be valid ONLY for actual forecast issuance time (4 pm in this instance), NOT the entire path of the storm.

But, this is the point. Look again at the location of Hurricane Isaac at 4 pm, marked by the “X”, and look how much of the eastern Gulf of Mexico is actually observing 39-73 mph winds at the same time.

The wind impacts alone are well outside of the cone! We haven’t even discussed deadly flooding due to heavy rains, storm surge or tornadoes, which also often occur well away from the center of the storm.

In summary, DON’T focus on the forecast cone and think you are safe if you are outside of it. You also should not focus on the category of the hurricane (1-5), which is strictly wind based. Actual impacts (surge, flooding rains, high winds and tornadoes) typically occur well outside the cone, including category 1 storms. As my friends at National Weather Service (NWS) Mobile say, “It doesn’t take a major storm to produce major impacts.” I could not agree more.

When a tropical storm or hurricane does again affect Alabama, the best way to find out what will occur at your specific location will be through the media and NWS. Go to the NWS website at weather.gov. Once there, enter your zip code or town and state in the location box in the upper left hand corner.

Finally, there are some new NHC graphical products this year, and next blog I will go over them with you. Be safe and be prepared!

Edition 2

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