Blow-out Tides

During Hurricane Irma, there was a phenomenon observed that is more typically associated with tsunamis. What happened was that, In some places, the water went out of the bays for a time on Florida’s Gulf coast.

News stories incorrectly reported this as the hurricane sucking the water away from shore, but what was really going on was something called blow-out tides. They occur when very strong winds blow from the land toward the water. Then, when the wind switches direction, the water comes rushing back, plus some! That “plus some” is the storm surge. To really see the concept, grab a sheet of paper and a pen.

Draw a tall and narrow letter “U”, about two inches high. You now have the basic shape of the Florida Peninsula.

Now, a quarter-inch up from the bottom of your “U”, draw an arrow through the “U”, from left-to-right, starting outside the “U”, cutting all the way through it, while remaining parallel to the top and bottom of the sheet of paper. Now go a further quarter-inch high up the “U”, and do the exact same thing, only this time draw your arrow from right to left.

Finally, inside the “U” and attached to the right-to-left arrow you just drew, draw an arrow straight up through the center of the “U”. This represents the northward movement of the hurricane’s wind. We are assuming that north is the top of your sheet of paper.

So again, assuming that North is the top of your paper, what you have is an east-to-west wind arriving at a location, followed by a west-to-east wind, as the hurricane’s eye moves north of your latitude.

We drew a U-shape for the Florida peninsula with smooth edges. But in reality, if you were to look at a map of Florida, you will see a coast on both sides that has its share of bays and inlets.

And as our zone of strong wind, switching 180 degree in direction from east to west (just as we drew it) moves northward, it can do some astounding things. Like emptying bays, and then refilling them with more water than before! There is video of the blow-out tides online. Just google on “Irma empties bays”.

Interestingly, Jacksonville, in Florida’s northeast, far from Irma’s landfall, had a much more significant flood than had been expected. And it all gets back to our wind example from the “U” that we drew. The persistent strong wind from the east, while the center of the hurricane was south of them, effectively impeded the discharge of the St. John’s river, which was carrying a tremendous volume of water from Irma’s torrential inland rains, toward the Atlantic. With nowhere to go, the river flooded Jacksonville, and the flooding was exacerbated by the upriver surge, caused by the easterly wind. The city saw its worst flooding since 1846.

In the Florida Keys, the surge damage and wind damage was quite severe. Surge levels may have been between 10 and 15 feet on Marathon and Big Pine Key. as those locations caught the eyewall of the hurricane. The eyewall is where the strongest winds are. The highest gusts I saw reported were 142 mph at Naples, in SW Florida and 130 mph at Marco Island, also in SW Florida.

Incidentally, the highest surge on record in the the US was when the Mississippi coast was besieged by a 28 foot storm surge during hurricane Katrina back in 2005. That’s a vertical rise in the ocean level of a basketball hoop. And another one. And eight more feet – almost three stories.  I would urge you to google on “Katrina surge”, then click “videos” and play the 2:26 one. You’ll be astonished.

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 tshapiro@wagmtv.com.

Get the Rest of the Story

Thank you for reading your 4 free articles this month. To continue reading, and support local, rural journalism, please subscribe.