As of November 30th, the official 2017 Atlantic Hurricane Season has come to a close. By many measures, it was one of the most destructive and intense seasons on record, and included two of the Atlantic’s top 20 strongest hurricanes ever observed (Irma and Maria). Upwards of $200 billion in damages were done to the United States alone, with total worldwide damage in excess of $360 billion. The season featured two Category 5 hurricanes (only the sixth time a single season has had multiple Cat 5s) and several landfalls by Category 5s on the Caribbean island nations of Barbuda, St. Maarten, the British Virgin Islands, the Bahamas, Cuba, and Dominica, with an additional high-end Category 4 landfall made by Maria on Puerto Rico. And of course, despite being “only” a Category 4 at landfall, Hurricane Harvey devastated portions of coastal Texas with the heaviest rains ever recorded in a U.S. tropical cyclone, and may ultimately be responsible for upwards of $150 billion in damage around Houston.

In addition to the damage, rainfall and Category 5 landfall records set this year, the 2017 season was impressive by other metrics as well. Accumulated Cyclone Energy, or ACE, is one standard by which tropical cyclone seasons are measured. ACE is proportional to the maximum wind speed of a storm and added every 6 hours over the life of a given storm (the period of time between one full advisory by the National Hurricane Center), so it is a measure of both the strength and intensity of a tropical cyclone. In the Atlantic, an average (mean) season accumulates approximately 100 ACE units per season, a season is considered above-average when ACE exceeds 120 units, and a season is considered “hyperactive” when ACE exceeds 150 units. With a year-to-date (1 Dec 2017) cumulative ACE of 226, the 2017 season fits squarely into the “hyperactive” category. This ranks as the 7th most active season on record as calculated by Colorado State University’s Phil Klotzbach, behind only 1933, 2005, 1893, 1926, 2004, and 1995. Table 1 shows how this season stacked up against the long-term average and some of the other top hyperactive seasons on record (data reproduced from http://tropical.atmos.colostate.edu/Realtime/index.php?arch&loc=northatlantic).

YearNamed StormsHurricanesMajor Hurricanes (Cat 3+)Accumulated Cyclone Energy
201717106226
193320116259
200528157245
189312105231
19261186230
20041596227
199519115227
195016116211
19611187205
Long-term Average11.96.32.7104

2017 Atlantic hurricane season summary map

Figure 1. Tracks of all 2017 Atlantic tropical cyclones, including depressions, named storms, and hurricanes. Key: blue triangle = extratropical remnants, light blue circle = depression, dark blue circle = named tropical storm, yellows, oranges, and reds = hurricanes from Category 1 (yellow) to Category 5 (red). Image credit: https://commons.wikimedia.org/wiki/File:2017_Atlantic_hurricane_season_summary_map.png.

As the map of this season’s tropical cyclone tracks illustrates (Figure 1), a number of these storms had direct U.S. impacts. At MetStat, we’ve produced precipitation analyses of all of the hurricanes and tropical storms that impacted the mainland U.S. this year*. These storms include:

1. Tropical Storm Cindy, which made landfall near Cameron, Louisiana on June 22nd, spreading heavy rain far to the east of its center.

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Figure 2. MetStorm preliminary analysis of precipitation associated with Tropical Storm Cindy.

2. Tropical Storm Emily, which swept across Florida from west-to-east on July 31st.

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Figure 3. MetStorm preliminary precipitation analysis of Tropical Storm Emily.

3. Major Hurricane Harvey, which made landfall as a Category 4 during the evening hours of August 25th near Port Aransas, Texas, then proceeded to sit over Texas or just offshore for 5 days while producing astounding rainfall totals. These results are still preliminary and will be updated with additional data in the future, but we detail our analysis so far here.

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Figure 4. MetStorm preliminary precipitation analysis of Hurricane Harvey.

4. Major Hurricane Irma, which made a double landfall in the Florida keys and mainland on September 10th. Our full analysis is covered in much more detail here.

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Figure 5. MetStorm preliminary precipitation analysis of Hurricane Irma.

5. Major Hurricane Maria, which devastated Puerto Rico when it made landfall on the morning of September 20th. At present we are unable to obtain hourly precipitation data for Puerto Rico, and have therefore not yet conducted an analysis. There are also considerable issues with what data does exist, as rainfall initially thought to be record-shattering was later deemed invalid, as the violence of Maria’s winds shook automated gauges with such force that they recorded several times more rainfall than actually occurred. See this Science News article on other potential problems with rainfall reports from Maria. The National Weather Service in San Juan, Puerto Rico, was able to produce a preliminary storm-total analysis based on what little data there was (Figure 6). We hope to produce our own analysis in the near future when time and data availability allow.

Maria Rainfall PR and VI NWS

Figure 6. NWS San Juan preliminary rainfall estimate from Hurricane Maria.

6. Hurricane Nate, which made a double landfall at the mouth of the Mississippi River late on the night of October 7th and near Biloxi, MS during the early morning hours of October 8th. It moved northward through the Gulf of Mexico with record speed, preventing it from having the time to strengthen into a devastating major hurricane.

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Figure 7. MetStorm preliminary precipitation analysis of Hurricane Nate.

Each of these storms brought high-impact rainfall to at least localized areas, to say nothing of the damage caused by strong winds and storm surge along the coasts. Parts of Texas, Florida, Puerto Rico, Cuba, and many Caribbean island nations will be recovering from the 2017 season for months and years to come. A great summary of some records and notable oddities from this past season can be found at the Category 6 Wunderground blog by Dr. Jeff Masters and Bob Henson. As we transition out of tropical weather season and into winter in North America, we’ll be shifting our focus towards winter storms. As always, watch this space for analyses of major storm events, detailed reviews of concepts important to our work here at MetStat, and even an exciting announcement or two. Thanks for reading!