Hurricane Iota of November 2020, which operationally was assessed as a Category 5 hurricane and was originally considered to be the latest Category 5 Atlantic hurricane on record, had its Tropical Cyclone Report (TCR) released on Tuesday. The report downgraded its maximum sustained winds to 135 knots (155 mph) – making it a high-end Category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale. Iota, the final named storm of the record-breaking 2020 Atlantic hurricane season, caused significant damage to Nicaragua, and the hurricane was attributed to 67 deaths. In addition to Iota’s downgrade, there is an area of interest being monitored by the National Hurricane Center for potential development in both the North Atlantic and Eastern Pacific basins.

Why was Iota downgraded?

There is one primary reason why Iota’s maximum sustained winds were downgraded: uncertainties about the reliability of the Stepped Frequency Microwave Radiometer (SFMR) in intense tropical cyclones. National Hurricane Center Senior Hurricane Specialist, Stacy Stewart, notes in Iota’s TCR: “ongoing research suggests there is a high bias of SFMR wind speeds in these high wind regimes.” This research has not yet been completed, and could lead to other peak maximum sustained wind estimates being changed of recent intense hurricanes. The report also notes that significant braking waves near the islands of Providencia and Santa Catalina may have inflated Iota’s SFMR winds. Iota’s peak flight-level winds were 147 knots, which using a 90% reduction, supports an intensity of about 132.3 knots (152.2 mph) – nearly 5 mph below the Category 5 threshold of 157 mph. Hurricane Matthew of 2016, despite having a maximum SFMR value of 143 knots (164.6 mph), only had a maximum flight-level wind estimate of only 137 knots (which supports an intensity of 123.3 knots, or 141.8 mph). As a result of the research findings, Matthew, currently assessed as a 145-knot Category 5 hurricane, is likely to lose its Category 5 status when the research is finished. It is likely that the NHC will be more hesitant to upgrade future hurricanes to Category 5 status based on SFMR values alone, and NHC may no longer upgrade a hurricane to a Category 5 without flight-level winds of at least 150 knots. It may take multiple years for the research to be completed. The National Hurricane Center often notes in their tropical cyclone reports that their best track estimates may have an error of up to 10%.

It is interesting to note that with Iota’s downgrade, the Atlantic Category 5 streak has been reduced from five to four years (and will likely be knocked down to three years in the future with Matthew’s potential downgrade coming). The 2020 Atlantic hurricane season, despite a record-breaking 31 named storms, no longer has a Category 5 hurricane (and it will likely stay this way, since data suggests Hurricane Eta getting upgraded to a Category 5 is extremely unlikely). Iota’s downgrade also no longer makes it the latest Category 5 hurricane on record in the basin, and the record now returns to the 1932 Cuba hurricane, which previously held the record before Iota’s operational Category 5 upgrade.

Current Tropical Outlook

The Eastern Pacific and Atlantic both have one system being monitored being by the National Hurricane Center (NHC) for potential development. Neither system appears to be a threat to land. In the eastern Pacific, an area of low pressure may form south of the Gulf of Tehauntepec on Thursday, and the NHC gives this system a 10% chance of development within 48 hours and a 30% chance within five days. In the Atlantic, an area of low pressure is expected to form Thursday northeast of Bermuda, and the NHC gives it a 10% chance of subtropical or tropical development within 48 hours, and a 40% chance within five days. Neither system has been designated an Invest as of this time. The next name on the Eastern Pacific naming list is Blanca, and the first name on the Atlantic naming list is Ana.