Tropical cyclone

The long-term future of Tropical Storm Ian still remains frustratingly obscure, as computer models forecasting its track remain obstinately in disagreement.

The official 2021 NHC Atlantic track forecasts tended to have a northeast bias of 7-21 miles for one- to three-day forecasts (i.e., the official forecast tended to fall to the northeast of the verifying position). According to Jasper Deng, who manages tropical cyclone information at Wikipedia, this increase of 105 mph in 24 hours is the fifth largest in global records, topped only by Hurricane Patricia in the eastern Pacific in 2015 (120 mph), Typhoon Hagibis in the northwestern Pacific in 2019 (115 mph), Tropical Cyclone Ambali in the South Indian in 2019 (115 mph), and Tropical Cyclone Ernie in the South Indian in 2017 (110 mph). At 12Z, the JTWC rated Noru as a category 4 typhoon with 130 mph winds – a decrease in winds of 20 mph compared to six hours earlier, at 6Z. Tropical Storm Hermine became a post-tropical cyclone in the waters 580 miles north-northeast of the Cabo Verde Islands on Sunday morning. This is unusually close for a strong typhoon to be passing to the city, whose metropolitan area has a population over 21 million: The database of storms from the Joint Typhoon Warning Center (JTWC) lists only six major (category 3 and stronger) typhoons that have passed within 50 miles of Manila. For that reason, the skill curve in Figure 1 shows relatively low skill for NHC forecasts for short-term forecasts out to one day; skill increases for forecasts between one and three days, when persistence tends not to be a good forecast (hurricanes generally don’t move in a straight line at a constant speed for days on end). EDT Sunday, when Ian will be south of Jamaica, NHC predicts Ian will go from a tropical storm with 65-mph winds to a category 2 hurricane with 105-mph winds in 24 hours, exceeding the minimum definition of rapid intensification (a 35-mph increase in winds in 24 hours). Now is a good time to remember that hurricanes stray outside the boundary of the “cone of uncertainty” about one-third of the time. The 12 Sunday runs of two of our top track models, the European and UKMET, were particularly concerning, showing a landfall near or to the south of Tampa Bay. But by Wednesday conditions for intensification will become marginal, as a southwesterly flow of upper-level winds to the west of Ian brings an increase in wind shear; the 12Z Sunday run of the SHIPS model predicted a moderate 15-20 knots of wind shear would affect Ian on Wednesday, when the storm is expected to be to the west of Tampa. The spread in model track solutions for Ian continues to be unusually large, despite the continuous surveillance of the Hurricane Hunters and a doubling of the number of upper-air balloon soundings preformed at over 40 U.S. NHC currently has Ian topping out as a category 4 hurricane with 130 mph winds Tuesday night through Wednesday morning, when it will be to the west of Key West, Florida.