Weather forecasts sometimes contain terminology that people either do not understand, or think they do, and then make poor safety decisions. A perfect example is recent Hurricane Florence. For days prior to landfall, it was clear that steering currents were going to collapse, and that the storm would make it to the Carolinas and then nearly stall for a while. This made torrential, flooding rains a near certainty.
However, what many people tend to focus on is “what category is the hurricane?” Hurricanes have 5 categories, and a Category 5 is the most intense, in terms of maximum wind speeds. If a hurricane is a Cat 4 or Cat 5 at sea, but by the time it arrives on the coast it is “only”‘ a Cat 1, many people let their guard down, their thinking being, “Oh, it’s just a 1 now, so we should be fine.”
It’s important to remember that the category assignment is based on maximum sustained wind speed. It is not related to how much rain it might produce. That total is a function of the storm’s forward speed.
Another cause of people letting their guard down, is use of a specific word, “downgraded.” When a hurricane’s winds diminish from, say, Cat 2 strength to Cat 1 strength, the word “downgraded” is usually used, as in, “The hurricane has been downgraded to a Category 1.” When folks hear “downgraded,” many think “less of a threat.”
In the case of Florence, as it was unloading tremendous amounts of rain in the Carolinas and southern Virginia, while crawling along at 2 mph, the storm’s winds dropped below hurricane strength (Cat 1 starts at 74 mph), and use of the word, “downgraded” happened again. The reports everyone down there heard said, “Florence has been downgraded to a tropical storm” (a tropical storm has winds of 39-73 mph).
Now don’t get me wrong, the meteorologists down there were HAMMERING home the point that life-threatening inland flooding would occur, so the message was certainly out there, but the combination of hearing the word “downgraded” a few times, in concert with the fact that the larger rivers don’t respond at once to very heavy rains, led many people to think it wasn’t “that bad” when, in fact, the worst of the storm, impact-wise, was still to come, inland from the coast.
I checked the river gauges as all of this was unfolding, and the “downgrade” to tropical storm from hurricane was announced when many of the rivers had 20 to 25 feet of vertical rise still to come. And most of that rise was going to happen over the next 48 hours.
When rivers rise that sharply, they spread out all across the countryside, and flood the homes of people who think they are a safe distance away. But water doesn’t care about distance. It’s all about elevation.
If you are ever in the market for a home, and it is anywhere near a river, make sure you know that river’s flood plain. One can find out where a flood would extend in a 1 in 50-year flood, a 1 in 100-year flood, a 1 in 500-year flood and so on.
Also, even if you live miles inland from the coast, a hurricane can drive a storm surge from the ocean, up a river. The Cape Fear river in Wilmington, N.C., rose 11 feet from storm surge (storm surge is a vertical rise in the water level which occurs when strong winds are blowing onshore — from the ocean toward the land). So if you lived along the river, it rose vertically a basketball hoop, plus one foot.
Back to the title of the column: precise use of language is essential when life-threatening weather is on the way, and since I don’t see use of the word “downgraded” going away anytime soon, I can only offer this advice: if you are in the path of a hurricane and you hear that word, do not let your guard down, because the worst of the storm’s impact, in terms of flooding, may very well be yet to come.
There is a great website where you can check river levels throughout New England. Just google “Northeast River Forecast Center.”
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 firstname.lastname@example.org.