A Tropics question or two

Elsa's winds leveled homes in Barbados last Friday
Elsa's winds leveled homes in Barbados last Friday

Elsa ramped just up to hurricane strength Friday as it battered Barbados. This statement came out from their weather office:

“The Meteorological Service of Barbados reported a sustained wind of 74 mph and a gust to 86 mph.”

Those winds along with the forward speed of the storm at 29 mph brought down houses as you can see from the cover picture. That brings me to this viewer question from Matt: “Frank, I’ve seen reporting on a 29 MPH forward speed for this storm. Questions...is this unusually fast? Is there an upper limit to fwd speed ? Does the forward speed add a +/- to wind damage on the left/right sides of the center?”

Thanks for the question, Matt, and to answer the first part, 29 mph for a storm is certainly a ‘racer’ but not that unusual for tropical storms and weaker hurricanes as they are steered more by upper level winds and that will move them along pretty quickly. With that in mind, the upper limit to a forward speed would be dependent on those steering currents. A couple of fast ones I found were the “Long Island Express” hurricane (1938) which barreled across the New York City area at 47 mph. The fastest on record for the Atlantic Basin is Tropical Storm 6 from 1961 which moved at 70 mph across Maine and New Brunswick.

The fastest moving tropical storm on record moved at 70mph over Maine

As to the danger in fast-moving storms, the right side (”dirty side”) of the storm is where the most energy is found -- the highest storm surge, which is wind-driven, and strongest winds. Additional wind strength will be found on that dirty side and you can find an excellent Weather Channel article right here.

Obviously, a fast moving storm is capable of huge amounts of damage, especially to power lines, transformers, roofs, and trees (which fall on houses). A fast-moving storm can cut a path that looks like a giant tornado moved through.

But the Faster, the Drier

Another viewer asked for this reminder:

“Frank: I’ve heard you tell of a way to forecast the amount of rainfall from a hurricane by the forward speed but I can’t remember the formula. I know that the slower it moves, the more rainfall it will produce. Would you please give me that formula again. I follow your page as well as the KPRC weather app. Thank you. Eileen”

Eileen, you are exactly right. The slower it moves, the bigger rain producer it will be. My rule of thumb on this is to divide 100 by the forward speed of the storm to get an idea of how many inches of rain the storm will produce. So a storm moving at 20 mph produces 5″ of rain (100/20=5) while a storm moving 10mph would produce 10″ of rain.

My thanks to Ron Stagno, formerly of the National Weather Service Houston, for sharing this with me back in 1989 when I first got to town!

Like all rules of thumb though, there will be exceptions and any training from a tropical storm can bring in a lot more rain than expected.

As to Elsa:

ELSA satellite courtesy NOAA

This storm is expected in Florida tomorrow night and Wednesday. The current forecast (11am Monday) calls for a 3-5′ storm surge. Rainfall from Elsa will impact portions of the Florida Keys, the Florida Peninsula and the coastal Southeast this week. Amounts of 3 to 5 inches with localized maximum amounts up to 8 inches are expected across Southern Florida which may result in isolated flash, urban, and minor river flooding. Isolated tornadoes will also be a threat.

I think the big story out of Elsa is likely to be power outages and wind damage with isolated flooding.


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About the Authors:

KPRC 2's chief meteorologist with three decades of experience forecasting Houston's weather.

Amanda Cochran is an Edward R. Murrow award-winning journalist. She specializes in Texas features, social media news and local crime.