Hurricane Ian poses serious risks to Florida Gulf Coast

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On Monday, Hurricane Ian was rapidly strengthening and is expected to make landfall in western Cuba early Tuesday morning. It will most likely become a major hurricane before moving along the Florida Gulf Coast. Whether or not Ian comes ashore on Florida’s west coast, the Tampa Bay area – which has not seen a major hurricane in more than a century – is facing one of its most dangerous hurricane threats in decades. People along Florida’s west coast need to take Ian with the utmost seriousness and consult local authorities for evacuation orders that could be extended quickly based on Ian’s progress.

Monday, February 2, 2012, Category 1 Ian was 120 miles west of Grand Cayman. Top winds were 85 mph and the wind direction was north-northwest, at 13 mph. Ian was bringing heavy rain showers to Jamaica, the Cayman Islands, and much of Cuba, as seen on Cayman Islands radar. The Cayman Islands radar did not show Ian's eyewall, and the peak winds at Grand Cayman on Monday morning sustained at 28 mph, gusting up to 44 mph.

Satellite imagery early Monday afternoon showed the symmetry, organization, and intensity of the storm’s heavy thunderstorms steadily increasing, and Ian had the look of a storm well on its way to becoming a major hurricane. On Monday afternoon, rainbands were already moving through South Florida.

Track the forecast for Ian

Ian’s track will be fairly straightforward through Tuesday. The storm will move over or close to Cuba's western coast early Tuesday. NHC has predicted that it will be a major hurricane at this point (see below). Havana will be on Ian's stronger right-hand side. The city will likely experience tropical-storm-force sustained winds between 40 and 60 mph. Hurricane-force winds are possible if Ian shifts to the right or is greater than expected by Tuesday. Major impacts can be expected across far western Cuba, including 6 to 10 inches of rain along Ian’s path and storm-surge inundation of 9 to 14 feet on Cuba’s southwest coast near and just east of Ian’s track.

The general track of Ian as it enters eastern Gulf of Mexico from Tuesday to Thursday has been agreed upon by most forecast models. Ian will move toward an upper-level trough (a dip of the jet stream) on Monday, which is expected to be from the eastern U.S. by Thursday. This steering pattern will keep the main threat of an Ian landfall away from the Florida Panhandle and west-central Florida.

Despite this general model agreement, small differences in Ian’s track angle could have major implications for impacts in Florida.

East versus west: The GFS model continued to follow Ian on Sunday night. It took him towards the Florida Panhandle. However, the European and UKMET (British models) have been consistently farther east, which has brought Ian across or very close to the Tampa Bay area (see Figure 1). Importantly, the GFS Monday run at 12Z shifted eastward more in line to the ECMWF (and UKMET) forecasts that both bring Ian into Tampa Bay.

Timing: The geography of Florida’s Gulf coastline means that a more westward track would keep Ian over the Gulf longer, thus increasing the amount of time Ian might be affected by the high wind shear and dry air expected to arrive around midweek (see below). In this case, a more northerly landfall could be much weaker in terms of peak winds, and it would occur later – perhaps as late as Friday on the Panhandle coast, versus a potential landfall in Tampa as soon as Wednesday if the eastern track proves more accurate.

A pause near Tampa? The trough across the eastern U.S. will have moved far enough east by Wednesday that it will exert less of a tug on Ian’s motion. The major models agree on a potentially crucial slowdown in Ian’s movement for about 24 hours, mainly from Wednesday into Thursday. Ian should resume a slightly faster northward motion around Thursday.

Forecast for Ian: Intensity

Ian has nearly ideal conditions for intensification through Tuesday morning: very warm water of 30-30.5 degrees Celsius (86-87°F) with a high heat content, light wind shear, excellent outflow channels aloft, and a moist atmosphere (a mid-level relative humidity of 70%). Ian’s crossing over the western tip of Cuba early Tuesday morning is likely to interrupt the intensification process only briefly, as the few hours Ian spends over land should not significantly disrupt its core. On Tuesday, Ian will likely resume intensification over warm waters of the southern Gulf of Mexico.

The aggressive Monday 11 a.m. ET National Hurricane Center forecast for Ian predicted rapid intensification. NHC predicts that Ian will change from a Cat 1 hurricane at 11 a.m. Monday to a Cat 3 storm with 120 mph winds by Tuesday morning. This is easily more than the minimum definition for rapid intensification (a 35-mph rise in winds within 24 hours). On Tuesday, further strengthening is expected over the southeast Gulf of Mexico. Ian will be a category 4 storm with sustained winds of 140 mph at its peak by Tuesday evening.

However, the conditions for intensification will diminish by Wednesday morning as a southwesterly flow west of Ian of upper-level winds brings an increase wind shear. The 12Z Monday run on the SHIPS model predicted that Ian would be affected by wind shear of 20 to 35 knots on Wednesday when the storm is expected close to Tampa. With dry air to the west of Ian at that time, the higher wind shear and drier air should cause weakening – and potentially rapid weakening – of the storm. As Ian moves further north on Thursday, wind shear will continue its upward trend. Ian will be stronger if it makes landfall earlier to the south. Ian would be weaker if it tracks more westward, which would result in a delayed landing further north. Many models predict a rapid weakening right before landfall on Thursday or Friday. If landfall occurs in Florida Panhandle, Ian could become a tropical storm.

Ian could cause major damage even if he made a delayed landing as a tropical hurricane in the Florida Panhandle. On Tuesday morning, the steering currents will weaken and Ian will be moving northward at 10 mph. By Wednesday, Ian’s forward speed will likely be about 7 mph, slowing to about 5 mph by Thursday. This slow motion will allow Ian build up a severe storm surge along Florida's west coast. This surge will survive multiple high tide cycles and can penetrate deep inland, up narrow creeks or estuaries. The storm will also make landfall near the coast due to the slow motion of Ian, causing the coast to experience extended battering winds and increasing wind damage.

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At 11 AM ET Monday, Sep 26, @NHC_Surge north Sarasota issued a storm surge advisory. This advisory included Tampa Bay and Clearwater north up to Tarpon Springs. Up to 10 feet of surge flooding possible with what's expected to be a large, slow-moving Hurricane #Ian pic.twitter.com/Rg0JoaSyFd

— Michael Lowry (@MichaelRLowry) September 26, 2022

NHC currently places Ian as a Category 4 hurricane with 140 mph winds Tuesday evening through Wednesday morning. It will be west of Key West in Florida. The 12Z Monday run of the DTOPS model gave a 91% chance that Ian would intensify by 65 mph in 48 hours, becoming a Cat 4 with 145 mph wind by 12Z Wednesday. The top two intensity models for making 4- and 5-day forecasts in 2021, the HMON and the HWRF, predicted with their 12Z Monday runs that Ian would reach category 4 strength with 140-150 mph winds on Wednesday.

Tampa Bay is facing a dangerous and destructive storm surge

Storm surge is expected to cause 5-10 feet of flooding in Tampa Bay at 11 a.m. EDT. This surge, which is typical of a Category 1 or 2 hurricane, could be very dangerous and destructive for the region. The National Hurricane Center Storm Surge Risk Maps provide useful information about the possible levels of inundation for different Saffir-Simpson hurricane category hurricanes. They also paint a disturbing picture of how vulnerable the Tampa Bay area is (Figure 2). For a category 1 hurricane, the map shows that Pinellas County – home to St. Petersburg, and nearly 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 be cut well before a hurricane hits.

For those living in Hurricane Alley at low elevation, it’s important to know your evacuation zone, your storm surge risk (using the NHC storm surge maps), and by using floodfactor.com, a tool first made available by the First Street Foundation in 2020. This tool allows you to enter an address and see the flood risk for that property. It is a valuable resource that was not available before. It’s free for non-commercial purposes.

Tampa Bay is highly susceptible to storm surges

Tampa Bay doesn’t get hit very often by hurricanes, because the city faces the ocean to the west, and the prevailing east-to-west trade winds at that latitude make it uncommon for a storm to make a direct hit on the west coast of Florida from the ocean. This is fortunate because the large area of shallow continental shelf waters offshore Tampa Bay (less that 300 feet deep out to 90 mile offshore) allows large storm surges to form. In a worst-case, with a powerful hurricane traveling north-northwestward at just the right speed parallel to the coast, the geometry of the coast creates a unique additional rise in the water level because of a phenomenon known as a coastal-ly trapped Kelvin Wave.

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Why are we so concerned about the risk of hurricane surge in Tampa Bay? Bathymetry. Bathymetry. pic.twitter.com/ZfnI89R8Nt

— Steve Bowen (@SteveBowenWx) September 26, 2022

The last time Tampa suffered a direct hit by any hurricane was 1946, when a Category 1 storm came up through the bay. The Tampa Bay Hurricane of October 25, 1921 was the last major hurricane to make landfall in the Tampa Bay Region. This low-end Category 3 storm with 115 mph winds at landfall brought a storm tide of 10 – 11.5 feet (3 – 3.5 meters), causing severe damage ($10 million 1921 dollars.) The only other major hurricane to hit the city occurred on September 25, 1848, when the Great Gale of 1848, the most violent hurricane in Tampa’s history, roared ashore as a Category 3 or 4 hurricane with 115 – 135 mph winds. In the area now known as downtown Tampa, a storm surge of 15 feet (4.6 meters) was seen. The peninsula where St. Petersburg is located in Pinellas County was also affected. This made St. Petersburg an island. Large parts of the few remaining human structures in the area were destroyed.

Around 160,000 people lived in the four-county area when the 1921 hurricane struck Tampa Bay. Most of them lived in high-mountain communities. Today, the region is home to 3.5 million people. This number is increasing by approximately 50,000 per year. The sea level is about a foot higher today than it was in 1921. A storm surge from the same storm could cause more damage.

The majority of the population of the four-county Tampa Bay area lives along the coast in lowlying areas. About half of it is at an elevation less than 10 feet. More than 800,000 people live in evacuation zones for a Category 1 hurricane, and 2 million people live in evacuation zones for a Category 5 hurricane, according to the 2010 Statewide Regional Evacuation Study for the Tampa Bay Region. Given that only 46% of the people in the evacuation zones for a Category 1 hurricane evacuated when an evacuation order was given as 2004’s Category 4 Hurricane Charley threatened the region, the potential exists for high loss of life when the next major hurricane hits.

According to the second installment of a two-part series published in the Tampa Bay Times earlier this year, 11% of Tampa's properties are at risk of flooding during a Cat 1 hurricane. In Pinellas County (where St. Petersburg is situated, and home to nearly one- million people), this number is 20% – with nearly $30 billion in property. No Florida county has both more buildings and more value at risk, the article reported: “More than 700 essential properties like places of worship, gas stations, schools, government buildings and public utilities are at risk of Category 1 flooding. 500 more are at risk from Category 2 storms. Almost 400 hotel properties, most along Pinellas’ famed beaches, are similarly vulnerable.”  

Tropical Storm Eta 2020 in Tampa: a sobering experience

Even a strong tropical storm passing 70 miles west of Tampa Bay can generate a damaging storm surge, as discovered on November 11-12, 2020, when Tropical Storm Eta’s center passed about 70 miles to the west of Tampa. At the time, Eta’s sustained winds were 65 – 70 mph, and these were high enough to bring a storm surge of 3-4 feet above ground level to portions of Tampa Bay, said NHC. According to the 2022 Tampa Bay Time feature in Pinellas County, more than 1,400 homes were flooded in Pinellas County. One person was killed when a man was walking through floodwaters and electrocuted. 33 water rescues were also performed.

Eta inundated approximately 9,000 properties in the Tampa Bay region. The Tampa Bay Times article based on storm surge modeling results from NHC explained that Eta would have hit with a water level seven inches higher and 17,000 properties would have been affected. A 22-inch increase in water level would have inundated 43 500 properties. Those numbers do not bode well for the future habitability of the Tampa Bay region with the current forecasts of sea-level rise fueled by global warming – and they raise serious concerns over potential impacts a Hurricane Ian could cause this week.

The worst-case scenario

A truly catastrophic worst-case scenario was portrayed by this morning’s 6Z (2 a.m. EDT) September 26 run of the HMON model, the top intensity model of 2021. The 66-hour forecast showed a slow-moving category 4 Ian descending on Tampa Bay. This combination of intensity and forward speed could cause a 15-20 foot storm surge to Tampa Bay, rendering St. Petersburg an island. It is possible that there would be catastrophic damages of more than $100 billion and a large loss of lives. For weeks, power, water, sewerage, and medical services could be severely cut off or lost. (The 12Z Monday run for the HMON model is nearly identical to the 6Z Monday runs discussed here.

Residents of Tampa should ask themselves if this scenario is a realistic outcome for Ian, given that it is only a forecast for 66 hours.

A best-case scenario

A plausible best-case scenario was portrayed by this morning’s 6Z (2 a.m. EDT) September 26 run of the HWRF model. The model's 66-hour forecast showed Ian moving northwards 200 miles west of Florida's coast on Wednesday. This would bring winds below tropical storm force to Tampa Bay. The model showed Ian weakening rapidly on Thursday and reaching the Florida Panhandle Friday morning as a tropical hurricane with 45 mph winds. This track and intensity would still cause a storm surge to the west coast and Big Bend region of Florida, but the total damage could stay below $5 billion and Ian might not be retired. The 12Z Monday run by the HWRF is less likely to be a best-case scenario as it brings Ian closer Tampa Bay and includes the distinct slowdown shown in other models.

Bottom line: Hope for the best, prepare for the worst.

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