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    Catastrophic storm surge, flooding rainfall

    Vaseline Posted on

    Helene’s surge impacts are without living precedent and highly likely to happen.

    Ryan Truchelut
     |  WeatherTiger

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    Hurricane Helene, an alliterative threat to life and property across much of Florida, remains on a collision course with the Florida Gulf Coast. Track expectations have changed little since Tuesday, and the strongest hurricane landfall to strike Apalachee Bay since the 1840s is expected to hit Thursday evening.

    Due to the intensity and extraordinary size of the storm, Helene will bring catastrophic, life-threatening surge to the west-central Florida and Big Bend coasts, and a core of destructive winds to a broad swath of North Florida and Georgia that likely includes Tallahassee.

    ➤ Spaghetti models for Hurricane Helene

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    As of mid-day Wednesday, the center of Helene is entering the southern Gulf of Mexico, about 600 miles south-southwest of Tallahassee and 475 miles southwest of Tampa. Helene’s movement as tracked this morning by Hurricane Hunter aircraft and Cuban and Mexican radar is now just west of due north at about 10 mph.

    Unfortunately, Helene is shooting the gap through the Yucatan Channel, and the center of the hurricane will not move over land. That is bad news, as land interaction was a possible speed bump to Helene’s continued intensification.  Rather, the hurricane has unfortunately followed yesterday’s NHC forecast track and most of the model guidance over the past day, and remained over water.

    The NHC track forecast has been remarkably consistent since Advisory Number One on Monday, and still shows a steady march north across the southern Gulf, followed by an acceleration north-northeast Thursday into a narrow channel of brisk steering winds between an upper-level low to the northwest and a ridge to the northeast.

    A landfall somewhere in Apalachee Bay on Thursday evening is probable, most likely between 6 p.m. and midnight. This is just a touch west of yesterday’s track.

    Rapid intensification for ‘gigantic’ Hurricane Helene up to Category 4 is possible

    As of 11 a.m. on Tuesday, maximum sustained winds have increased to 80 mph, making Helene a hurricane. Reconnaissance observations and radar suggest Helene is assembling a well-organized inner core this afternoon. Unfortunately, this sets the hurricane up for rapid intensification that may continue into Thursday afternoon.

    Helene will be crossing a plentiful source of oceanic heat, tracking longways down a finger of the Gulf’s infamous Loop Current that is currently heated to a stupifying historical extreme. At the same time, it now appears likely the spacing between Helene and the upper low will be such that the low will mostly ventilate, not shear, the inner core of the hurricane during the day on Thursday.

    This means yet another hurricane rapidly intensifying in the eastern Gulf on approach to land. It’s not possible to know how high this rapid intensification cycle will loft sustained winds, but a peak between high-end Category 2 and Category 4 sustained winds is in line with the 11 a.m. NHC forecast of a high-end Category 3 and typical forecast errors.

    There is some dependence of track on how fast Helene strengthens, with a bit slower strengthening perhaps tilting landfall odds towards western Apalachee Bay, and faster/more intensification possibly nudging landfall toward eastern Apalachee Bay or the northern Nature Coast.

    But we shouldn’t be thinking in terms of exact track or categories — really, ever, but especially with Helene. Hurricane categories are defined by the strongest winds at the surface anywhere in the storm, which are usually only are found in a small slice of the eyewall over water.

    Helene will be the worst hurricane impact in living memory for many Floridians, including those in Tallahassee

    In reality, Helene is a gigantic storm that will produce damaging winds, catastrophic surge, and flooding rainfall over most of Florida and well inland into Georgia and the mid-South, so “category” is going to understate total impacts. The familiar NHC cone estimates the region where there is a two-thirds probability the center of the hurricane will track. The NHC cone is, say it with me, NOT AN IMPACTS CONE.

    Severe and life-threatening impacts from Helene will occur hundreds of miles from the cone confines, especially on the eastern half of the storm.

    The anticipated landfall on Thursday is a truly unprecedented scenario for North Florida. Between Michael’s Category 5 landfall in Mexico Beach in 2018 and an 1896 Category 3 hit on Cedar Key, the only other major hurricane to make landfall anywhere in Apalachee Bay since 1851 is last year’s Idalia — which as a relatively compact storm, is a poor comparable to the gargantuan Helene.

    A major hurricane has never on record moved into Leon County, which the current NHC forecast shows.

    There is no modern reference point for Helene. Between Tampa Bay and roughly Cape San Blas, this will very likely be the worst hurricane impact in living memory. Let’s go one-by-one through each the wind, surge, and rain hazards to try to grapple with the myriad threats Helene poses to Florida and beyond.

    Wind: A tropical storm warning for almost the entire state; but North Florida will see a catastrophic blow

    In terms of winds impacts, there are two stories to Helene.

    The first is the certainty that tropical-storm-force gusts capable of knocking out power and damaging trees will occur over almost all of Florida starting overnight tonight in the Keys, South Florida early tomorrow morning, and North Florida by early tomorrow afternoon.

    A tropical storm warning is in effect for literally the entire state other than the hurricane warning zones (other than Destin and Pensacola), plus southeastern Alabama and south Georgia. Periodic wind gusts of 40-60 mph, highest in bands, are expected in the tropical storm warning area.

    Helene’s expected tropical-storm-force winds are expected to extend up to 300 miles east of the center on Thursday. These wind radii are larger than more than 90% of comparably strong storms.

    Helene’s 20 mph+ forward speed at landfall will also rocket damaging winds far inland, and north Georgia, including Metro Atlanta, plus portions of the Carolinas, which are also under a tropical storm watch as of Wednesday afternoon.

    The second wind story is about the extent and severity of Helene’s truly destructive winds, which will occur over a more limited area closer to the center of circulation. Wind gusts of 75 mph are expected in the hurricane warning area, which includes the Central and Eastern Florida Panhandle, Big Bend, and Nature Coast, plus southwest Georgia.

    Around a day out from landfall, it’s usually too soon to put a fine points on local wind expectations for the potential core strike zone, as maximum winds can ramp up or down quickly and usually occur in a limited area.

    This time, however, with the noose tightening on Apalachee Bay, there is unfortunately much higher confidence than usual that destructive winds associated with Helene’s core will occur in the eastern Panhandle and Big Bend. The highest chances of catastrophic wind gusts exceeding 115 mph extend along the coast from roughly Steinhatchee to Apalachicola, then arc inland through Tallahassee and north to Thomasville, reflecting the most probable path of Helene’s wide eyewall Thursday night.

    This could change. For instance, it’s theoretically possible in Tallahassee that Helene edges just far enough south and east of us such that the 100 mph+ gusts that cause structural damage will miss us. However, unlike in recent near misses, I wouldn’t bet on such luck holding out again this time. I’ll be tracking the core in real-time tomorrow to make the final call.

    So: if you’re in Apalachee Bay, the Big Bend, and Tallahassee, your realistic worst-case scenario is not thinking about what Michael was like for you. It’s thinking about how it was in Panama City inland to Marianna, and making your evacuation decision accordingly.

    If you are in the hurricane warning region and want to evacuate, head west. Pensacola, Destin, and vicinity is far enough.

    If you can safely leave, especially if you are in a house with tall trees overhead, you probably should. Weather will be fine to travel west through Wednesday night into Thursday morning. Also, turn off your water main if you’re leaving town. Thank me later.

    Storm surge: The biggest killer and Helene’s surge will be cataclysmic

    Water is the killer. There is no hurricane hazard more deadly or destructive than storm surge. No matter the precise track, Helene will be a widespread, cataclysmic surge event unlike any modern-era hurricane for Apalachee Bay and west-central Florida. This includes life-threatening surge in Tampa Bay, even with the expected track of the center passing around 100 miles offshore.

    Helene’s eastern windfield will be both intense and enormous, pushing a wall of water north onto the shallow continental shelf of the northeastern Gulf that no last-minute fine adjustments in track or maximum winds will be able to alter. Storm Surge Warnings are in place for the entire Florida Gulf Coast from Cape San Blas to the Everglades.

    These storm surge warnings include some areas up to 10 miles inland from the Gulf Coast itself in the Big Bend, in recognition that the peak NHC modeled storm surge of 10-15 feet will overspread huge areas of low-lying “dry land” adjacent to Apalachee Bay and the northern Nature Coast.

    The scheduled high tide on Thursday night at around 11 p.m. in Apalachee Bay is ill-timed, coinciding with the strongest onshore flow during and post-landfall, further increase the expected inundation.

    As usual, areas west of the landfall point past Cape San Blas will see much lower, limited surge with offshore winds. However, if you are east of the landfall point, a major surge event is essentially locked in at this point by the size of Helene. This includes in Tampa Bay, where the NHC is predicting 5-8 feet of storm surge.

    This would be the worst surge event since the 1921 Tarpon Spring hurricane in the Tampa area, and 3-5 feet of surge is also expected in Southwest Florida. Remember, if you are evacuating to escape a surge threat, you don’t need to drive to Mississippi to avoid a life-endangering situation.

    You just need to go a few miles inland, to somewhere 30 feet above sea level rather than 6 feet. Additionally, those numbers in Tampa Bay could go even higher and push above 8 feet if Helene tracks more like 50 miles west of you, which would also increase the surge in Southwest Florida.

    Bottom line, if you get an evacuation order, you Just. Need. To. Leave. Go several miles inland to higher ground. But go. Helene’s surge impacts are without living precedent and highly likely to happen. Just go inland!!

    Rainfall: Helene will be a bullet but bring widespread flood risk

    When people think of hurricane threats, excessive rainfall and flooding aren’t necessarily the first things that come to mind. But they should be — nearly one-third of tropical cyclone-related casualties are caused by freshwater flooding.

    Helene will be accelerating by landfall to a forward speed of 20 mph or more, which increases the inland wind threat and lowers the flash flood threat. However, Helene is so large that widespread 4-8 inches rainfall totals across North Florida, much of Georgia, and north into the Appalachians equals flash and freshwater flood potential from Thursday into the weekend.

    A flood watch is in effect from South Florida to Destin area and north to western Virginia for this reason. Local totals of 8 inches plus are possible in the eastern Panhandle, as well as in the mountains of North Georgia and western North Carolina, where there is a high risk of flash flooding.

    As far as timing, periodic rain bands will overspread the southern half of Florida overnight, and the remainder of the state Thursday morning. Look for rains to be heaviest and most persistent between early Thursday and early Friday in the eastern Panhandle, Big Bend, and west-central Florida. In other words, today’s rainfall will be scattered and intermittent, so finalize your evacuations and preparations prior to daybreak Thursday.

    Words fail to capture the level of threat Helene poses

    Honestly, folks, I’m running out of adjectives to describe the severity of the total situation with Helene. Like something from a lingering nightmare, it is hard to convey the extent of all the threats in play. Unfortunately, this ain’t a dream no more, it’s the real thing. React. Prepare. Evacuate. Take care of yourself. Take care of your neighbor.

    Everyone across Florida and north into the Southeast U.S. should be closely monitoring the latest information from the National Hurricane Center, your National Weather Service office, and local emergency management. The expected worst-hit areas in Florida have through tonight make preparations, so use your time. I will be back tomorrow with a region-by-region threat breakdown ahead of landfall.

    Good luck, Florida. Yet again, we’ll need it. Keep watching the skies. 

    Dr. Ryan Truchelut is chief meteorologist at WeatherTiger, a Tallahassee company providing forensic meteorology expert witness services, and agricultural and hurricane forecasting subscriptions. Visit weathertiger.com for more information. Email Truchelut at [email protected]

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