The long-term future of Tropical Storm Ian still remains frustratingly obscure, as computer models forecasting its track remain obstinately in disagreement. Rapid intensification of Ian into a major category 3 or 4 hurricane remains highly probable, but whether or not the storm will impact the heavily populated west-central coast of Florida – or the less populated Big Bend and Panhandle areas – remains a mystery.
At 2 p.m. EDT Sunday, Ian was 265 miles south-southeast of Grand Cayman, with top winds of 50 mph, headed west-northwest at 12 mph. Ian was bringing heavy rain showers to Jamaica and eastern Cuba, as seen on Cayman Islands radar. Satellite imagery early Sunday afternoon showed that the symmetry, organization, and intensity of the storm’s heavy thunderstorms were steadily increasing. Ian had the look of a storm well on its way to becoming a hurricane, and it may now have a vertically aligned core that will allow it to rapidly intensify.
Intensity forecast for Ian
Ian has nearly ideal conditions for intensification through Monday night: very warm water of 30 degrees Celsius (86°F) with a high heat content, light wind shear, excellent outflow channels aloft, and a moist atmosphere (a mid-level relative humidity of 70%). However, data from the Hurricane Hunters on Sunday morning showed that Ian’s vortex was still not aligned in the vertical, with the upper-level center displaced from the surface center. A tilted configuration does not allow for much intensification, but satellite imagery from Sunday afternoon suggests that Ian had solved this problem.
Intensification is predicted on Monday, when Ian will be passing by the Cayman Islands and approaching western Cuba. Crossing Cuba is likely to interrupt the intensification process only briefly, as the few hours Ian spends over Cuba will not significantly disrupt its core. Ian is likely to resume intensification over the warm waters of the southeastern Gulf of Mexico on Tuesday.
The 11 a.m. EDT Sunday National Hurricane Center forecast for Ian was aggressive, explicitly predicting rapid intensification. Beginning at 8 p.m. 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).
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. With dry air to the west of Ian at that time, the higher wind shear and drier air may cause a weakening of the storm. Wind shear will continue to increase as Ian heads farther north, and many of the models predict rapid weakening just before landfall on Thursday or Friday, if landfall occurs in the Florida Panhandle. However, Ian would be able to push a storm surge to the coast greater than its winds at landfall might suggest, as we have seen with several landfalling hurricanes in the Gulf. Examples include Katrina in 2005 (category 3 at landfall) and Ike in 2008 (category 2 at landfall).
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. The 12Z Sunday run of the DTOPS model gave a 93% chance that Ian would intensify by 75 mph in 72 hours. The top two intensity models for making 4- and 5-days forecast in 2021, the HMON and the HWRF, predicted with their 12Z Sunday runs that Ian would reach category 4 or 5 strength with 140-160 mph winds on Wednesday.
Track forecast for Ian
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. locations beginning on Saturday. One trend of note, though: The models have stopped shifting westward with their solutions, reducing the odds that a landfall will occur west of the Florida Panhandle, and increasing the odds that Tampa Bay will see severe impacts. 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.
The location of Ian’s landfall in Florida will depend upon the strength and orientation of the trough of low pressure pulling the storm northwards, and there are significant model differences on this point. So far, for the limited amount of time Ian has existed, the GFS model has out-performed all the other models – and the official NHC forecast – but that is no guarantee that the model is making good long-term forecasts for Ian: The GFS was the poorest-performing top-six model for 4- and 5-day forecasts for Hurricane Fiona (Figure 3). Now is a good time to remember that hurricanes stray outside the boundary of the “cone of uncertainty” about one-third of the time.
Which model should you trust?
For those puzzling over the various hurricane computer forecast models to figure out which one to believe, the best answer is: Don’t believe any of them. Put your trust in the National Hurricane Center, or NHC, forecast.
Although an individual model may outperform the official NHC forecast in some situations, the 2021 National Hurricane Center Forecast Verification Report documents that overall, it is very difficult for any one model to consistently beat the NHC forecasts for track and for intensity.
In 2021, the official NHC track forecasts for Atlantic storms were tough to beat, and none of the individual models outperformed the official forecast at any time period, when compared to a “no-skill” model called CLIPER5 (Figure 2). The CLIPER5 model (which combines the word “climatology” and “persistence” to show the nature of the forecasts it makes) is tough to outperform at short-term forecasts, since a hurricane will tend to keep moving in the same direction and at the same speed as at its initial point (this is called persistence). 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). Beyond three-day forecasts, NHC forecast skill starts to drop off, as the CLIPER5 model starts weighting its forecasts using climatology, which becomes tougher to beat at long ranges.
The GFS model was the best model in 2021, followed by the European model. The HWRF, HMON, COAMPS-TC, and CMC models did respectably for forecasts up to 72 hours; at longer time periods, the CMC and COAMPS-TC models performed poorly. 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).
Here is a list of some of the top hurricane forecast models used by NHC:
Euro: The European Center for Medium-range Weather Forecasting (ECMWF) global forecast model
GFS: The National Oceanic and Atmospheric Administration (NOAA) Global Forecast System model
UKMET: The United Kingdom Met Office’s global forecast model
HMON: Hurricanes in a Multi-scale Ocean-coupled Non-hydrostatic regional model, initialized using GFS data
HWRF: Hurricane Weather and Research Forecasting regional model, initialized using GFS data
COAMPS: COAMPS-TC regional model, initialized using GFS data
If one averages together the track forecasts from three or more of these six models, the NHC official forecast will rarely depart much from it. These six models are used as input to various “consensus” models, such as “TVCN,” often referenced in NHC discussions for a storm. Improved consensus modeling techniques are one major reason NHC track forecasts have improved so much in the past 30 years.
Typhoon Noru shocks the Philippines with a Cat 4 landfall
Typhoon Noru put on a spectacular, unexpected, and extremely dangerous rapid intensification feat in the waters just east of the Philippines on Saturday, becoming one of the fastest-intensifying cyclones in modern Earth history. Noru’s peak 1-minute wind strength, as gauged by the Joint Typhoon Warning Center (JTWC), increased from 50 mph (medium tropical-storm strength) at 2 p.m. EDT Friday to 155 mph (top end of the category 4 range) at 2 p.m. EDT Saturday. 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). All of these record-intensifying tropical cyclones occurred in the past seven years. Warmer oceans from human-caused climate change greatly boost the odds of extreme rapid intensification events like these (see Jeff Masters’ 2020 post on such trends in the Atlantic).
Noru made landfall on the Philippines’ Luzon Island about 35 miles northeast of the capital of Manila shortly before 12Z (8 a.m. EDT) Sunday. 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. Noru was on a west- to west-northwest track late Sunday local time that would take it about 30 miles north of Manila. 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. Noru’s small size may allow it to pass far enough north of Manila to limit winds there to tropical storm strength, but this heavily populated area should be prepared for potentially severe impacts from the typhoon.
Noru will emerge into the South China Sea after spending about eight hours crossing over Luzon, whose rugged terrain of Luzon is expected to disrupt the typhoon to category 1 status. Re-intensification into a category 3 storm is expected by Tuesday night (U.S. EDT), when Noru will make landfall in central Vietnam.
Noru is Earth’s second Cat 5 storm so far in 2022. The first was Super Typhoon Hinnamnor, which peaked with 160 mph winds at 8 a.m. EDT Tuesday, August 30, according to the Joint Typhoon Warning Center (JTWC). This year is running behind the usual pace for Cat 5s: The 1990-2021 average is 5.3 Cat 5s per year, according to ratings by the National Hurricane Center and Joint Typhoon Warning Center.
Tropical Storm Gaston dying near the Azores in central Atlantic
Tropical Storm Gaston, with sustained winds of 50 mph at 11 a.m. EDT Sunday, is expected to become post-tropical on Monday as it heads west at 12 mph away from the Azores Islands.
Tropical Storm Hermine dies in far eastern Atlantic
Tropical Storm Hermine became a post-tropical cyclone in the waters 580 miles north-northeast of the Cabo Verde Islands on Sunday morning. Hermine’s remnants are predicted to bring total rainfall amounts of 3 – 6 inches to the Canary Islands through Monday.
One other wave to watch: 99L
A tropical wave in the central Atlantic, several hundred miles west of the Cabo Verde Islands, was designated Invest 99L by NHC. The wave is in a dry environment (mid-level relative humidity of 50%), but sea surface temperatures are warm and wind shear is moderate, which could allow some slow development. Satellite imagery showed that 99L had developed a broad surface circulation, but had a limited amount of disorganized heavy thunderstorm activity. The wave has only limited model support for development as it wanders mostly northwest at less than 5 mph, far from any land areas. In its 2 p.m. EDT Sunday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 20% and 30%, respectively.
Sunday evening update: Increasing risk of a damaging storm surge in Tampa Bay
While the exact track and intensity of Ian as it approaches Florida will determine whether or not Tampa Bay receives a damaging storm surge, the current NHC cone of uncertainty – combined with the current intensity forecast – gives Tampa Bay a high chance of receiving a damaging storm surge, characteristic of a category 1 hurricane or stronger. The National Hurricane Center Storm Surge Risk Maps are helpful to see the levels of inundation one might expect for various Saffir-Simpson category hurricanes, and they paint an alarming picture of how vulnerable the Tampa Bay region is (Figure 5). For a category 1 hurricane, the map shows that Pinellas County – home to St. Petersburg, and over one million people – may become completely cut off from the mainland. Portions of all four connecting bridges, as well as the main highway leading north – U.S. Highway 19 – will potentially be under as much as six feet of water. These evacuation routes would likely be cut off well before a hurricane arrives.
If you live in Hurricane Alley at low elevation, it’s important to know your evacuation zone (see Tweet below), your storm surge risk using the NHC storm surge maps and floodfactor.com, a tool first made available by the First Street Foundation in 2020 (see an example in Figure 6). This tool allows one to type in an address and see the specific flood risk for that property, and is a fantastic new resource of a kind that was never available publicly before. It’s free for non-commercial purposes.
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