11 months ago


Costly Hurricanes Return

Costly Hurricanes Return to the U.S. Mainland: Harvey and Irma – Major Impacts from Drastically Different Storms by D. Andrew Murray Senior Instructor in Meteorology Forecaster and Hurricane/Severe Weather Specialist Coastal Weather Research Center/Department of Earth Sciences, University of South Alabama An eleven-season lull in major hurricane strikes on the U.S. mainland ended in 2017 as both Harvey (Texas) and Irma (Florida) made landfall on U.S. soil. In fact, 2017 marked the first time in U.S. history that two hurricanes of at least category 4 intensity made landfall in the United States within the same year. Although Harvey and Irma represent two of the most destructive hurricanes ever to hit the U.S. mainland, these storms were quite different in structure, track, and resulting damage. Harvey became a tropical storm on August 17 over the open Atlantic Ocean but weakened to a tropical wave in the Caribbean Sea due to strong wind shear. The wave moved into the Bay of Campeche, where it re-intensified into a major hurricane as it approached the TX coast. The storm strengthened through eventual landfall near Port Aransas late on August 25. Harvey caused significant wind damage along portions of the SE TX coast; however, its worst impacts were yet to come. Immediately after landfall, upper-level winds around the storm weakened, and Harvey meandered around SE TX and SW LA as a tropical storm for 4-5 days, even briefly moving back over the extreme northwestern Gulf of Mexico. Exceptionally heavy rains persisted across SE TX and SW LA during this time over the eastern half of Harvey’s circulation. Virtually all of SE TX and W LA received at least 10” of rainfall, with the region from Houston to Port Arthur, TX, seeing 40-50” accumulations with a maximum just over 60 inches. Devastating flooding resulted, particularly in the Houston area. The life cycle and structure of Irma was quite different from Harvey. While Harvey was confined within the western Gulf of Mexico as a major hurricane, Irma became a major hurricane over the open Atlantic, peaking at an incredible 185 mph as it moved through the northern Leeward Islands. It remained powerful as it passed near Puerto Rico and through the Turks and Caicos and the SE Bahamas. After hitting Cuba, Irma began a slow turn towards the north in the direction of S FL and the FL Keys. Irma briefly weakened due to land interaction with Cuba but re-intensified to a category 4 storm, moving directly over Cudjoe Key to the east of Key West, FL, early on September 10. Devastating storm surge affected the FL Keys and even impacted much of the SE FL coast, including downtown Miami. Strong winds also caused substantial damage to the lower and central Keys. Irma then continued north and made its final landfall on Marco Island near Naples later that afternoon. Storm surge heights near 10 feet occurred along the coast of SW FL in the Everglades, and damaging winds affected the FL Peninsula and S GA over the next day as it continued northward. Irma’s sprawling size also caused storm surge from 6-8 feet along the coasts of NE FL and SE GA and from 4-6 feet all the way to the central SC coast. These two storms produced widespread damage, but for different reasons. Although Harvey did produce some storm surge and reached category 4 intensity, by far its worst impacts were a result of the catastrophic flooding which resulted from the prolonged heavy rain event that followed. Irma’s worst effects on the U.S. mainland were from storm surge along the coast from FL to SC and from widespread wind damage due to its large circulation. Together, these two storms made the 2017 hurricane season one of the costliest for the United States. The GOES-16 satellite captured the eye of Hurricane Harvey as new thunderstorms erupted on the southwest rim of the eyewall. This magnified picture was taken at 5:43 p.m. CDT on August 25 when Harvey was 45 miles east of Corpus Christi, Texas. At the time of this image, Harvey was moving NW at 8 mph with sustained winds of 130 mph and a central pressure of 27.79”. Courtesy of University of Wisconsin/CIMSS - 22 -

Over 29 Years of Service On The Eastern Shore Mandy Irman or Christy Uteg - 23 -