Cat-5 Irma Threatens Northern Caribbean and Southeast U.S. with Destructive Storm Surge

MAIN POINTS

1) Hurricane Irma has already generated an 8-foot storm surge on Barbuda and is forecast to generate a 15-20 foot storm surge in the Turks and Caicos and Southeastern Bahamas;

2) Irma will bring the third destructive storm surge to the Bahamas in three years and may rival Hurricane Joaquin's 15-ft (4.57 m) storm surge, which also struck the southeastern Bahamas two years ago;

3) Irma is forecast to remain a powerful category-4 or -5 hurricane as it approaches the Florida Peninsula. The most reliable models predict a sudden turn towards the north, but uncertainty is fairly high regarding the timing and track of that turn, as well as potential impact to Florida and/or other southeastern states;

4) All interests from Florida through North Carolina should closely monitor the progress of Hurricane Irma. The region from northeast Florida (St. Augustine) through all of the Georgia coast and southwest South Carolina is particularly vulnerable to storm surge, whether or not Irma makes a direct landfall in that region.

Hurricane Irma threatens to strike the northern Caribbean and Southeast U.S. with a destructive storm surge

NORTHERN CARIBBEAN

Powerful Hurricane Irma made landfall in the northern Leeward Islands last night, making a direct hit on Barbuda. At the time of landfall, Irma's maximum sustained winds were estimated at 185 mph.

A NOAA Tides and Currents tide gauge in southwest Barbuda observed a storm surge of 7.95 ft (2.42 m), shortly after the eye crossed the island and intense winds changed direction. For much of the afternoon and evening, northwest winds increased in velocity, but on the back side of the eye, intense winds from the south-southwest caused a rapid rise in sea levels.

The NOAA Tides and Currents gauge on Barbuda remained operational through most Irma's passage last night, providing valuable wind, air pressure and water level data.


This station observed maximum sustained winds from the northwest at 103 knots, gusting to 134 knots at 1:54AM local time. The minimum pressure bottomed out at 921 mb at 2:18AM, followed by a maximum storm surge at 3:12AM, as air pressure rapidly began rising and winds shifted from the south-southwest.

Storm surge levels spiked at the NOAA Tides and Currents gauge on the southwest coast of Barbuda, after winds changed direction and rapidly pushed a storm surge in from the south-southwest.


As Irma tracks north of Puerto Rico and the Dominican Republic, and is expected to make a direct landfall on the Turks and Caicos, as well as the Southeastern Bahamas, variable storm surge levels are forecast throughout the region.

The highest storm surge levels are expected in the Turks and Caicos, and the Southeastern Bahamas. In these areas surge levels could reach 15 to 20 feet, particularly where broad, shallow lagoons are situated upwind of islands. Along the northern coast of the Dominican Republic and Puerto Rico, storm surge levels are forecast to range from 3 to 5 feet.

Storm surge forecast from the National Hurricane Center, as Wed Sep 6 at 700 Eastern Time.

Irma threatens to generate a destructive storm surge in the hurricane-weary Bahamas, where substantial salt-water inundation is likely to occur for the third consecutive year.

Last year, Hurricane Matthew generated destructive storm surge and waves in the Western Bahamas. Storm surge levels reached as high as 8 ft (2.44 m) on the south coast of Grand Bahama Island and New Providence. Powerful waves and storm surge also struck the east coast of Andros Island.

In 2015, Hurricane Joaquin struck the southeast Bahamas with a 15-foot (4.57 m) storm surge at Crooked Island. Irma may generate a severe storm surge that rivals Joaquin's in this same region of the Bahamas.

The broad reef-lagoon features of the Bahamas make it more susceptible to storm surge inundations than most places in the Caribbean. Broad lagoons in parts of the Bahamas stretch for more than 10 miles (16 km) in one direction, providing large, shallow pools of water that powerful hurricanes can displace locally. Storm surge builds more efficiently where water depths are shallow, and these reef-lagoon systems provide a setup for rapid storm surge generation.

Hurricane Joaquin devastated the southeast Bahamas with destructive winds and a storm surge reaching as high as 15 ft (4.57 m) in 2015. 

UNITED STATES

Irma is forecast to remain a powerful category 4- or 5 hurricane as it approaches the Florida Peninsula. The most reliable models predict a sudden turn towards the north, but uncertainty is fairly high regarding the timing and track of that turn, as well as potential impact to Florida and/or other southeastern states.

The GFS (American) model run from 200AM Eastern Time this morning generally shifted Irma's track to the east. The graphic below shows the GFS ensemble model run, with the black line depicting the ensemble mean. This model run shows why coastal interests as far north as North Carolina should closely monitor Irma.

GFS (American) model run from 200AM eastern time this morning. The ensemble mean track (black line) has shifted east of the Florida coast.

The ECMWF (European) model run from 800PM Eastern Time yesterday evening depicts a model ensemble mean that is slightly west of the GFS model, generally taking Irma closer to the southeast Florida coast, and potentially making a landfall. This model run also indicates the possibility of elevated wind/ storm surge threat for Georgia and South Carolina.

ECMWF (European) model run from 800PM eastern time yesterday evening shows an ensemble mean approaching the southeast Florida coast.

These two reliable global weather models show why all interests from Florida through North Carolina should closely monitor the progress of Irma.

I am particularly concerned about storm surge impacts in the following places:

1) Florida Keys or Southeast Florida IF Irma makes landfall in these places;

2) South Carolina/ North Carolina coast, east of landfall location IF Irma makes landfall here;

3) The stretch of coast from northeast Florida (St. Augustine) through all of coastal Georgia and southwest South Carolina, WHETHER OR NOT IRMA MAKES LANDFALL HERE. Even if Irma does not make a direct landfall in this region, the shallow bathymetry (offshore water depth) and concave coastline shape in this region is very efficient for generating high storm surges and Irma may generate a destructive storm surge here.

I will write more about this in my next post.

-HH