Understanding Tropical Season Graphics

By Jim Stefkovich, Meteorologist, Alabama Emergency Management Agency
CLANTON –Wednesday, 8 am May 29, 2019

It’s not a question “if” Alabama will have tropical activity, but “when”. So, when it happens, let’s go over some basics to help you become better prepared.

The graphic above is from Tropical Storm Alberto, which directly impacted Alabama in May 2018. The forecast cone represents the forecast area of where the center of storm will be and the dotted line is just the center of the cone. However, actual impacts occur well outside the cone.

In addition, based on the past five years of research, the center of the storm remains within the cone approximately 2/3 of the time. But, this also means that for any five day forecast, the center of the cone will NOT remain in the cone. Why is this important to know?

Because of what is called “anchoring”. At times, people don’t keep up to date with changing forecasts and anchor on a particular forecast which may be a few days old. They are then surprised to see changes right before landfall and not prepared.

That’s why it’s important to constantly keep up to date with the latest forecasts from the National Hurricane Center (NHC) and your local NWS office.

The forecast cone continues to become smaller and more accurate. However, much work still needs to be done in understanding rapid intensification changes, especially at landfall.

Another useful product is the arrival time of “sustained” tropical-storm-force winds (39 mph – 73 mph) as shown below.3

This arrival time is when substantial damage can begin with downed trees, power lines and building damage. All outdoor preparations should have been completed by this time or you should have already evacuated to a safer location.

There is a difference between sustained winds and gusts. Again, the earliest time of arrival is for sustained winds of 39 mph – 73 mph which lasts for a minute or more. Wind gusts can last a few seconds.

The point is, a tropical system’s rainbands often contain damaging high wind gusts, flooding rain and tornadoes well away from the sustained tropical-storm-force winds, and you should consider having your action plan completed before the earliest time of arrival of the sustained winds.4

The bottom line is you need to focus on the potential impacts of any storm and not the category. Keep up to date with NHC forecasts https://www.nhc.noaa.gov and your local NWS office. Go to https://weather.gov and click on the area of the state you live. You will be directed to the office that serves you and they will provide specific impact and safety information.

The NHC and NWS are the only “official” forecasts. Beware of social media forecasts from untrusted sources. Unfortunately, I’ve seen bizarre predictions on social media that people have taken as truth. This does nothing but cause confusion and inaction.

Finally, keep up with your local Emergency Management and elected officials with the information they are providing through the web and social media.

To learn more about developing or reviewing your tropical action plan, go to https://www.ready.gov/hurricanes and https://www.weather.gov/wrn/hurricane-preparedness

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