Photo: Hurricane Irma approaching, on September 5, 2017 (NOAA image).
It’s been a sobering week of flashbacks for Irma survivors, as our islands closely tracked Hurricane Dorian, especially its crawl and stall over the northern Bahamas. A close brush with the storm, especially given the conditions on Saba, was a snap back to the reality of peak hurricane season for anyone who needed it.
A hurricane with the unbelievable strength of Irma, lasting much longer than Hurricane Luis, is hard to fathom.
The record-setter, as well as the fact that 2019 became the fourth year in a row with a Category 5 hurricane forming in the Atlantic (the longest such streak on record), has the attention of the world.
International media houses have been reporting that Dorian, with maximum sustained winds of 185 mph, is the second strongest Atlantic hurricane ever in terms of windspeed, tied only with Labor Day (1935), Gilbert (1988), and Wilma (2005). The strongest Atlantic hurricane on record was Allen (1980), with maximum sustained winds of 190 mph.
But doesn’t this sound exactly like what we know about Hurricane Irma, which made landfall on St. Maarten/St. Martin at 185mph? The impressive and highly publicised number is a hard one to forget – if not from the eagerly awaited updates by the National Hurricane Center via the Storm Watch Team at Laser 101.1FM, then from the many news reports afterwards. Why then, is Irma not included in Dorian’s windspeed comparison?
On September 1, 2019, Philip Klotzbach, Research Scientist in the Department of Atmospheric Science at Colorado State University and author on the seasonal hurricane forecasts, published a table of the strongest Atlantic hurricanes since 1950 on Twitter. He cited Irma as having maximum sustained winds of 180mph, although he had reported 185mph in his Hurricane Irma Notable Facts Recap. The same lower number is being quoted in other reports and articles.
The Weekender contacted Joseph Isaac, Department Head of the St. Maarten Meteorological Department to ask about the discrepancy. As it turns out, it’s standard for the National Hurricane Center (NHC), under the National Oceanic and Atmospheric Administration (NOAA), to review all statistical data about hurricanes after the season has passed, and it’s always possible that a few points might be slightly off.
NHC, based in Florida, is tasked by the World Meteorological Organization to provide the official information in this region of the world. NHC combines data from equipment in different locations – shared by meteorological departments throughout the region – with other data gathered through aircraft and satellite.
Isaac explained that although the initial figures are quite reliable, all the data is delved into on a deeper level, using different tools and mechanisms, after the season is over.
Of course, limitations need to be considered. Equipment at different locations can be compromised or destroyed during the storm, which was the case for St. Maarten.
Hurricane Hunter aircraft, described as “high-flying meteorological stations”, are essential to collecting data, but cannot fly too low. Dropsondes, small cylinders dropped from the aircraft into the storm, collect all sorts of data about the atmosphere as they descend, including pressure, temperature, wind speed and wind direction, two times per second.
All of this data, in combination with the data from other sources, need to be sorted through and compared, over time.
In the National Hurricane Center Tropical Cyclone Report for Hurricane Irma, originally published on March 9, 2018, and available at www.nhc.noaa.gov/, described Irma’s max. sustained winds battering the island at 155 knots, which translates to 180mph (rounded). The report notes that this is 5 knots “lower than the operational assessment in favour of blending the flight-level and SFMR [surface wind measurement] reports.” One-hundred sixty knots translates to 185mph (rounded).
For insight into Hurricane Hunter aircraft and dropsondes, search for Dropsondes: Work Horses in Hurricane Forecasting - Science Nation on YouTube (www.youtube.com/watch?v=iMvydih9rlM).