Active Storms

Tropical Update

 

ATLANTIC TROPICAL ACTIVITY

There are currently no active tropical cyclones that pose a threat to the United States. This bulletin will be updated should a threat materialize. 

 

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Hurricane Irma Recap
Overview: Hurricane Irma broke multiple records on its two week trip across the Atlantic en route to its final landfall on the southwestern coast of Florida. The storm was classified as a Cape Verde hurricane, named for the island chain off west Africa where it developed. Some of the most damaging Atlantic tropical cyclones in history were Cape Verde hurricanes, including the Galveston Hurricane of 1900, the Great New England Hurricane of 1938, Donna (1960), Hugo (1989), Andrew (1992), Ivan (2004), and Ike (2008). Irma caused devastation on several islands in the northeastern Caribbean before bringing hurricane force winds to the entire Florida peninsula and tropical storm force winds to much of the southeastern U.S.   

Hurricane Irma Track and Intensity History
Source: NHC


Caribbean Impacts: Irma caused devastation throughout the northeastern Caribbean as it passed through the area as a category 5 hurricane. The eye of the storm first moved over Barbuda while near its peak intensity. A wind gust of 155 mph was recorded by an anemometer on the island before being blown away. It has been reported that 95% of the structures on Barbuda were either severely damaged or destroyed. 

Visible Satellite Image of Category 5 Hurricane Irma Over the Northern Leeward Islands
Source: NOAA


After passing over Barbuda, Irma caused catastrophic damage throughout the northern Leeward Islands, most notably on Saint Martin, Saint Barthelemy, Anguilla, and the Virgin Islands. Severe impacts were also felt further west in Puerto Rico, Hispaniola, the Turks & Caicos, the Bahamas, and Cuba. Because of the storm’s large size, damaging winds and significant storm surge occurred in areas well removed from the center of circulation. The winds were so strong that entire forests were stripped of their leaves as can be seen in the before and after satellite images below. 

Before and After Satellite Image of the Virgin Islands Showing the Loss of Vegetation
Source: NASA


Why the Models Changed: As Irma tracked over the islands of the northern Caribbean, forecast models called for an abrupt northward turn in the vicinity of Cuba.  For several days, it appeared likely that Irma would avoid the island of Cuba and turn northward in time for its center of circulation to track toward the Miami area and southeastern Florida. Because the peninsula is only ~50 miles across at its southern point, very minor changes in the timing and magnitude of this turn could have significant implications on whether Irma would track toward Florida’s east coast or its west. Furthermore, a path that remained over water would allow Irma to maintain its intensity as a category 4 or 5 hurricane as it approached the state. 

As Irma approached Cuba, its westward motion persisted for longer than anticipated. This led to a landfall on the northern shore of the island as a category 5 hurricane. The center of circulation remained over land for several hours, which led to significant weakening. By the time the storm turned northward and moved back over water, it had weakened to a category 3 hurricane. Because the northward turn was delayed by a few hours, forecast models shifted and began calling for Irma to track toward the southwestern shore of Florida. 

This delay of only a few hours changed the likely outcome from a category 4 or 5 hurricane hitting Miami on the east coast to a category 3 hurricane hitting Marco Island on the west coast. While winds in excess of 100 mph still occurred in Miami, the impact would have been much more severe had the former scenario come to fruition. 

U.S. Impacts: Irma regained some intensity as it moved away from Cuba and tracked toward the Florida Keys. On the morning of September 10th, Irma made landfall on Cudjoe Key as a category 4 hurricane. Later that afternoon, the storm made landfall on Marco Island as a category 3 hurricane. From there, the eye tracked up the coast and moved over the city of Naples before shifting further inland over central and northern Florida. The center of circulation passed east of Tampa, which spared the city of Irma’s most severe storm surge. 

Irma was a very large hurricane. Despite a landfall in the southwestern corner of Florida, much of the peninsula experienced hurricane force winds. Tropical storm force winds occurred as far north as Tennessee as Irma crossed the border from Florida into Georgia. The image below shows a size comparison of Hurricanes Irma and Andrew (1992). 

Size Difference Between Hurricanes Irma and Andrew
Source: NOAA


Wind
Hurricane force winds occurred well away from the center of circulation. The maximum recorded wind gust for Florida was 142 mph in Naples. A 109 mph gust was reported just north of Miami on the opposite coast from where Irma made landfall. Some other notable wind reports include:
  • 130 mph - Marco Island, FL
  • 120 mph - Big Pine Key, FL
  • 99 mph - Key Biscayne, FL
  • 94 mph - Cape Canaveral, FL
  • 91 mph - Key West, FL
  • 86 mph - Jacksonville, FL
  • 78 mph - Fort Lauderdale, FL
  • 76 mph - Parris Island, SC
  • 70 mph - Fort Pulaski, GA
  • 60 mph - Gatlinburg, TN
Large Extent of Irma’s Sustained Wind Field at Landfall (mph)

Storm Surge
Irma’s storm surge impact varied widely along both coasts of Florida and all the way up into coastal Georgia and South Carolina. The first area to be impacted was the Florida Keys. The islands to the north of the landfall location experienced the highest surge. In Marathon, there were reports of the sea level rising above U.S. 1, the only highway through the Keys. Parts of downtown Miami were also inundated by storm surge as Irma passed through the Keys.  

Even though Irma made landfall in southwestern Florida, the maximum storm surge occurred all the way on the opposite corner of the state in the Jacksonville area. A height of 7.5 feet was recorded at Fernandina Beach, which is near the Florida/Georgia border. Tidal gauges in the area broke their all-time records. Further north, storm surge levels of ~5 feet were recorded along coastal Georgia and South Carolina. Because of Irma’s track, these areas experienced persistent onshore winds for the duration of the event. This was amplified by days of Irma pushing seawater in this direction as it tracked through the islands of the northern Caribbean. 

The highest surge on the west coast occurred from Naples southward. This area experienced several hours of onshore flow as Irma’s eye passed to the west. Areas to the north of Naples experienced a reverse storm surge, where the sea fell below normal levels due to persistent offshore flow. The Tampa areas experienced this reverse storm surge for several hours as Irma passed to the south. Because Irma made an abrupt northward turn before tracking toward southwestern Florida, there was less time for water to pile up on the right side of the storm. In addition, the storm made landfall at an oblique angle to the coast, which lessened the storm surge severity. 

Storm Surge Heights
Source: Hal Needham/NOAA Tides and Currents


Records
Hurricane Irma broke numerous records. Because of its intensity and impact, the name will undoubtedly be retired from the tropical cyclone list that repeats every seven years. Irma was the second category 4 hurricane to make landfall in the U.S. in a two week span after Harvey made landfall in late August. This marked the first time the U.S. has ever had two category 4 hurricane landfalls in one season. Some additional records include:
  • Strongest Atlantic hurricane on record outside the Caribbean and Gulf of Mexico
  • Sustained winds of 185 mph for the longest time of any tropical cyclone on record worldwide (37 hours)
  • Third strongest landfalling hurricane on record worldwide after making landfall on Barbuda, Saint Martin, and Tortola with winds of 185 mph
  • Strongest landfalling hurricane on record in the Atlantic basin, tied with the 1935 Labor Day Hurricane
  • Only category 5 hurricane on record to impact the northern Leeward Islands
  • Second hurricane to make landfall on Cuba as a category 5
Satellite Image of Irma as its Outer Bands Approach Florida
Source: NOAA

 

Hurricane Harvey Recap
Overview: 
Harvey made landfall on the evening of August 25th near Rockport, TX as a category 4 major hurricane with maximum sustained winds of 130 mph. This ended the longest stretch of time without a major hurricane landfall for the U.S. in recorded history. Prior to Harvey, the last major hurricane to directly impact the U.S. was Wilma in 2005. Harvey was the strongest hurricane to make landfall in TX since Hurricane Carla of 1961. 

Satellite Image of Harvey at Landfall

Winds: Significant wind damage was reported near where the center of circulation made landfall, especially in the towns of Rockport, Fulton, Portland, Port Aransas, and Aransas Pass. The maximum observed wind gust was 132 mph in Port Aransas. A storm chaser who experienced Harvey’s landfall in Rockport reported extreme winds followed by an eerie calm as the eye passed over. Eyewitnesses reported that stars could be seen and crickets heard as the eye passed through the area. 

Strongest Recorded Wind Gusts
  • 132 mph - Port Aransas
  • 125 mph - Copano Village
  • 110 mph - Lamar
  • 108 mph - Rockport

Harvey Wind Speeds
Source: Kinetic Analysis Corporation


Rainfall: Hurricane Harvey led to catastrophic flooding throughout southeastern TX and western LA as it stalled over the region for 6 days. There were widespread reports of totals in excess of 40” throughout the Houston metro area and southeastern TX. Some areas received more than their average annual precipitation from Harvey in less than a week. The highest recorded amount was 51.9” near Highlands, TX. Harvey now holds the record for the highest rainfall total from an Atlantic tropical cyclone. The catastrophic flooding will go down in history as one of the worst natural disasters for the U.S. 

Highest Rainfall Totals
  • 51.9” - Highlands, TX
  • 49.4” - League City, TX
  • 49.3” - Friendswood, TX
  • 49.2” - Dayton, TX
  • 47.4” - Beaumont, TX 
Harvey Rainfall Totals
Source: National Weather Service


Tornadoes: There were numerous tornadoes spawned by Harvey in TX, LA, MS, AL, and TN. Because the storm remained in the region for nearly a week and spawned tornadoes on a daily basis, it will go down in history as one of the most prolific tornado producing tropical cyclones on record. More than 50 homes were damaged by a tornado in the southwestern Houston metro area. There were other reports of sporadic damage from short-lived tornadoes throughout the region. 

Storm Surge: A record breaking storm surge of 6.7 feet was recorded at Port Lavaca, where some coastal flooding was observed. There were numerous reports of a storm surge in excess of 5 feet. The seawalls at Galveston were tall enough to prevent any substantial storm surge flooding in that area. 

 

NEXT UPDATE
This bulletin will be updated when a tropical cyclone poses a threat to the United States. 
Author: Kevin Sharp
Informational Resources

NATIONAL HURRICANE CENTER

CENTRAL PACIFIC HURRICANE CENTER

NATIONAL WEATHER SERVICE

ICAT MORATORIUMS

ICAT DAMAGE ESTIMATOR