Understanding Tropical Forecasts to Avoid Confusion

By Jim Stefkovich, Meteorologist, Alabama Emergency Management Agency

CLANTON –9 am September 12, 2018

If you look at the graphic above, it’s obvious a lot of activity is ongoing! A trough of low pressure over the south-central Gulf has a 60% chance of becoming a tropical depression before landfall Thursday or Friday along the western Gulf Coast. 3-8 inches of rain are expected across the southern third of TX. This system will have no impact on AL.

Isaac will move into the Caribbean on Friday. There are no indications that it will survive while continuing to move westward, and even if it does begin to curve northward into the southern Gulf of Mexico, this will not occur until at least September 19th.

All eyes are on Hurricane Florence. This will be a deadly and life-threatening hurricane. Information on Florence and the other systems can be found at https://www.nhc.noaa.gov

But, the purpose of this blog is to educate you about using these graphics and how to interpret them properly.  First, look at the current forecast track of Florence. The “X” marks the current location, “M” is major hurricane (sustained winds above 110 mph), “H” is Hurricane, “S” is Tropical Storm and “D” is Tropical Depression.

Notice the forecast cone. This cone IS NOT the bounded region of impacts. Impacts (high winds, flooding rain, storm surge and tornadoes) often occur well outside the cone. The cone represents where the center of the storm may be. In addition, approximately 1/3 of the time, the center actually ends up outside the cone.

To further illustrate this, look at the red and yellow shaded area below. The red is where hurricane force winds are located, and yellow are tropical storm force winds. Both of these are well outside the cone. This will be the case at landfall as well. Some locations may have winds around 70 mph up to 150 miles from the center, 13 feet of storm surge and rainfall approach 40 inches.

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Always use the “earliest” arrival time for tropical storm force winds as the time to have all preparations completed. Rain bands typically arrive well ahead of the tropical storm force winds, and once the winds arrive, it is dangerous to be outdoors, in a vehicle or a building that is not sturdy.

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Finally, in 35 years of being in the weather business, the common misperception and mistake people make starts out with, “I’ve lived here X number of years and nothing has happened.” Or, “I lived through hurricane “X” and we were fine.”

Both of these statements lead to inaction which can lead to injury and death. Never compare one storm to the next. Each is different. Also, the landscape where you live has likely changed over the years (population, construction, roads, etc.). These changes have altered rainfall runoff/flooding areas as well as the amount of damage from a storm that happened years ago.

The best thing you can do is stay up to date on the latest forecasts, and absolutely heed the advice of local officials. “Hide from the wind and run from water”. If you’re told to evacuate, EVACUATE! Things can be replaced. Lives can’t.

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