Powerful Irma Threatens to Put South Florida Underwater, Spill Lake Okeechobee

Near category five strength Irma represents a major flood threat from storm surge and rainfall to South Florida. Due to its large size, strong winds, its movement toward shore atop rising seas, and ability to push a tall and wide-ranging surge of water over far-flung coastlines, Irma has the potential to put major cities like Miami under water. In addition, expected 10-15 inch rainfall over Lake Okeechobee threatens the integrity of an aging dike which, if overtopped, could result in severe flooding of inland communities.

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As of the 5 PM advisory from the National Hurricane Center, Irma was a top-strength category 4 hurricane packing 155 mile per hour maximum sustained winds and a minimum central pressure of 925 mb. The storm is presently tracking just north of Cuba along a westerly or west-northwest path. It is expected to turn north by Saturday, ultimately making landfall somewhere in South Florida.

Like Harvey, Irma is very moisture rich. Like Harvey, Irma is set to interact with a deep trough dipping down over the Eastern U.S. Like Harvey, Irma is tapping warmer than normal surface waters off Florida which is helping the storm to maintain a high intensity. And like Harvey we can confidently say that the record-breaking and long-lasting high intensity of Irma has been fueled by human-forced climate change — with some weather models indicating a risk that Irma could restrengthen on approach to Florida as it crosses over the 3.5 degree F warmer than normal waters of the Gulf Stream.

Unlike Harvey, Irma is expected to continue moving after making landfall. And this movement will prevent the kind of prolonged event that occurred during Harvey — with a tropical system raining out over the same region for days and days on end. That said, Irma’s extremely strong winds presently at 155 mph and what is likely to be a very powerful storm surge pose a threat to most locations along the Florida Peninsula — especially South Florida. As with other recent hurricanes like Sandy and Matthew, Irma presents an even greater threat from storm surge flooding due to higher overall ocean levels as a result of melting glaciers in places like Antarctica and Greenland. So Irma’s massive predicted surge is running in on a higher ramp than that of decades past.

(The NAM 3 kilometer model shows a very intense 896 mb storm off South Florida by 10 PM Saturday. This model forecast shows Irma strengthening to a very extreme Category 5 storm over the much warmer than normal waters of the Gulf Stream. Official forecasts from the National Hurricane Center still call for a weaker, but still strong and dangerous, Category 4 or 5 system threatening South Florida at about this time. Image source: Tropical Tidbits.)

According to the National Hurricane Center, preliminary expected storm surges range from 8-12 feet for SW Florida from Captiva to Cape Sable, 5-10 ft from Cape Sable to Boca Rato including the Florida Keys, 5-8 ft from Venice to Captiva, 3 to 6 ft from Boca Raton to the Velusia County line, and 3 to 5 ft from Anclote River to Tampa (note that both Florida coasts expect moderate to severe storm surges and that these totals are increased and expanded from the 2 PM NHC advisory).

To put these numbers in perspective, pretty much all of South Florida, including most of the city of Miami is below 10 feet above sea level. A 10 foot storm surge with breaking, wind-driven waves on top, would therefore have catastrophic impacts for this region (see graphic below). As Irma approaches, these already significant storm surge projections may rise further even as impacts from storm surge are likely to expand up the coast.

(A ten foot rise in base sea levels as could occur during Irma’s storm surge would put most of South Florida under water. Storm surge projections for this region are presently 8-12 feet and 5-10 feet. Note that storm surge impact can vary widely based on location and that changes in Irma’s projected path is likely to alter its storm surge related impacts. Image source: Climate Central.)

Though Irma has been compared with Andrew, we must note that Irma is a significantly larger storm — dwarfing the tiny but intense Andrew. As a result, Irma has the ability to deliver a lot more in the way of a powerful surge of water to both Florida coasts. And where Andrew’s damages were primarily due to extremely high winds, Irma’s damages are likely to come from both wind and water — with the potential for very severe storm surge and flood-based destruction.

In addition to the problem of Irma’s likely large and wide-ranging surge, a second issue is the fact that there’s some concern that an aging dike holding water back from communities near Lake Okeechobee might not withstand projected rainfall totals from Irma of 10-15 inches. Though not Harvey-level rainfall amounts, these rains would come in very intense bands over the course of perhaps one day. Such heavy rainfall could cause the lake to over-top the dike — resulting in severe flooding for downstream communities.

 

(Irma’s heaviest rains are expected to fall over Lake Okeechobee — adding to an already significant flood risk to South Florida. Image source: NOAA.)

The seventy year old dike is presently vulnerable not just due to its age, but also due to the fact that a construction project aimed as shoring up the dike is underway. This rebuild in progress makes the dike even more vulnerable to heavy rains and to large waves that would be stirred up on the lake by hurricane force winds. The Army Corps of Engineers has reassured the public that a dike breach is unlikely — as its most vulnerable section in the southeast has already been strengthened. Concern remains, however, that flooding from the dike could combine with a backing up of canals due to storm surge to swamp communities far inland from the coast.

(UPDATED — UPDATES TO FOLLOW)

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