Helene intensifies en route to an expected major landfall in Florida » Yale Climate Connections

Hurricane Helene entered the southeast Gulf of Mexico at midday Wednesday, en route to what is likely to be a devastating landfall late Thursday in the Florida Panhandle.

Hurricane warnings were in effect for much of Florida’s Gulf Coast, from Mexico Beach to the Anclote River, and storm surge warnings covered an even broader area, from Indian River south to Flamingo, including the Tampa-St. Petersburg and Fort Myers areas. Wind damage from this fast-moving and unusually large hurricane will be extensive and reach far inland into Georgia, and record- or near-record storm surge flooding is likely from Florida’s Big Bend to Tampa Bay. Helene’s torrential rains are also expected to bring highly destructive flooding into northern Georgia, western North Carolina, eastern Tennessee, and northwestern South Carolina.

Helene growing more organized

At 11 a.m. EDT Wednesday, Helene was located 85 miles (135 km) north-northeast of Cozumel, Mexico, moving north-northwest at 10 mph (16 kph), with top sustained winds of 80 mph (130 kph) and a central pressure of 979 mb. Satellite imagery and Cuban radar showed that Helene had a large area of heavy thunderstorms with very cold cloud tops, which were bringing heavy rains to western Cuba and Mexico’s Yucatan Peninsula. Helene had not quite closed off an eye, which was open to the southeast (Fig. 1).

Track forecast for Helene

There is not much mystery about where Helene is going until landfall. The storm will be steered by a high-pressure system to the northeast and a trough of low pressure across the Mississippi Valley that will evolve into a closed upper low centered over the Mississippi Delta by Thursday, with upper-level high pressure building to the north. As a result, Helene will carry out a large and rather unusual arc across the eastern Gulf of Mexico and Southeast U.S., at first moving north-northeast across the Gulf and then gradually angling northward across western Georgia and eastern Alabama and more sharply northwest and west into Tennessee by Friday and Saturday. 

Ensemble forecast models are now unanimously clustered on a projected landfall Thursday late afternoon or evening in the eastern Florida Panhandle, perhaps toward the west side of Apalachee Bay as indicated in the official forecast from 11 a.m. EDT Wednesday. Such a track would produce record or near-record storm surge in the Big Bend region and could bring record-strong winds to the Tallahassee area, with widespread tree falls, power outages, and other related damage well inland across the region. Damaging winds sustained at 40-70 mph (64-113 kph) would be possible across much of southern and central Georgia, with a 30% to 50% chance of tropical-storm-strength winds extending as far north as Atlanta.

Intensity forecast for Helene

Now that Helene is emerging into the Gulf of Mexico and has avoided making landfall in Cuba or the Yucatan Peninsula, conditions appear ideal for rapid intensification. However, the system does not yet have a fully formed inner core – radar data from Cuba and eyewitness reports from the Hurricane Hunters showed gaps in the eastern portion of Helene’s eyewall, where some dry air sucked in from the Yucatan Peninsula was interfering. Until Helene manages to fully close off an eyewall, the storm may only experience a modest rate of intensification, and not rapid intensification (defined as a 35-mph (56-kph) increase in winds in 24 hours).

Supporting rapid intensification will be an atmosphere that is very moist and climate-change-boosted ocean temperatures that are record- to near-record-warm – about 30-31 degrees Celsius (86-88°F). Helene will be tracking along the axis of the warm-water current called the Loop Current, which transports warm water from the Western Caribbean into the Gulf of Mexico. This current has a great deal of warm water that extends to great depth, giving Helene a substantial amount of ocean heat content to power rapid intensification. Wind shear is predicted to be in the low range, five to 10 knots, until just before landfall in Florida, and the trough of low-pressure pulling Helene to the north will provide a favorable upper-level outflow channel. These conditions should allow Helene to become a major hurricane by Thursday afternoon.

The 11 a.m. EDT Wednesday official forecast from the National Hurricane Center called for Helene to peak Thursday evening as a high-end Cat 3 with 125 mph (200 kph) winds, but the top intensity models give a range of potential top intensities, mostly from Cat 2 to Cat 4 (Fig. 2). The DTOPS model gave a 67% chance that Helene would intensify by at least 50 mph (80 kph) in the 36 hours ending at 8 p.m. EDT Thursday, making it a Cat 3 with 120 mph (195 kph) winds.

A widespread and destructive storm surge for Florida

The current forecast for #Helene will bring life threatening surge to parts of the Bay Area. If this materializes, records will be broken for Tampa Bay and the Nature Coast. If you typically flood, flooded during Idalia, you’ll likely flood again, with perhaps more water. @WFLA pic.twitter.com/KzvbennrKB

— Jeff Berardelli (@WeatherProf) September 25, 2024

Like a giant stepping into a bathtub, Helene’s entry into the Gulf of Mexico today has already elevated water levels by over a foot along most of Florida’s Gulf Coast. Given its large size and low pressure (its central pressure of 979 mb at 11 a.m. EDT Wednesday is low for a Category 1 hurricane), Helene will be a very large storm capable of generating a significant storm surge hundreds of miles to the east of where its center passes. In fact, Helene is so large that a storm surge of one to three feet (0.3-0.9 m) is expected on the other side of the Florida Peninsula from northeastern Florida to Charleston, South Carolina. With such a sprawling storm, it’s especially crucial not to focus just on the “skinny line” of the center’s path.

NHC noted that Helene was in the 90th percentile for size for its current latitude, and predicted that tropical-storm-force winds would extend out up to 290 miles (465 km) from the center of the storm Thursday afternoon through evening; hurricane-force winds were predicted to extend out 45 miles (75 km). This huge wind field, combined with the large area of shallow waters off the west coast of Florida, is likely to produce a record storm surge for over a 100-mile-plus swath of the Florida Gulf Coast.

Helene’s surge and battering waves will cause significant damage and erosion to beaches and dunes in Florida’s Big Bend that were heavily impacted on August 5, 2024, by Category 1 Hurricane Debby and on August 30, 2023, by Category 3 Hurricane Idalia, which brought a storm surge of eight to 12 feet (2.4-3.7 m) to the Big Bend and three to five feet (0.9-1.5 m) to Tampa Bay. Property damage from Helene’s surge along the Big Bend coast will likely be lower than for Idalia since that hurricane wiped out a lot of coastal structures and there has not been time to rebuild.

A record storm surge for Tampa Bay possible

The greatest surge damage from Helene is likely to occur in the heavily-developed coastal areas from Tampa Bay northward approximately 30 miles (50 km), where water levels five to eight feet (1.5-2.6 m) above dry ground are predicted if Helene hits at high tide. The current timing of Helen’s passage offshore from Tampa until the time of landfall is forecast to straddle the time of low tide (see tide discussion below); thus, we can probably subtract a foot of inundation from NHC’s surge forecast for Tampa Bay. Thus, an inundation of four to seven feet (1.2-2.1 m) for Tampa Bay should be expected.

The tide gauge with the longest period of record in the Tampa Bay region is at St. Petersburg, with data going back to 1947. The all-time high-water mark there is four feet (1.2 m) above normally dry ground, so even at the lower end of Helene’s predicted surge corrected for the tide, a new all-time water level record would be set.

Here are the top 10 water levels at St. Petersburg since 1947 as measured above mean higher high water, or MHHW:

1. 4 feet, Aug. 31, 1985, Hurricane Elena
2. 3.8 feet, Aug. 30, 2023, Hurricane Idalia
3. 3.58 feet, Oct. 8, 1996, Tropical Storm Josephine
4. 3.52 feet, Jun. 18, 1982, Subtropical Storm One
5. 3.37 feet, Nov. 12, 2020, Tropical Storm Eta
6. 3.16 feet, Dec. 17, 2023, Winter storm
7. 2.74 feet, Sep. 6, 2004, Hurricane Frances
8. 2.91 feet, March 13, 1993, Storm of the Century
9. 2.74 feet, June 25, 2012, Tropical Storm Debby
10. 2.58 feet, June 25, 2012, Tropical Storm Debby

Tide forecast for Tampa Bay

High tide at St. Petersburg, Florida, is Thursday morning at 8:42 a.m. EDT and again at 1:58 a.m. EDT Friday; low tide is at 5:39 p.m. Thursday. The difference in water level between high and low tide is about two feet (0.6 m). During Hurricane Idalia in 2023, the highest storm surge in Tampa Bay began when the center of the hurricane passed due west, then peaked when Idalia made landfall about 150 miles (240 km) to the north-northwest. We can expect a similar pattern for Helene, though potentially with higher water levels. If the timing of the current NHC forecast is correct, a storm surge in excess of three feet (0.9 m) will begin affecting Tampa Bay around 2 p.m. EDT Thursday, when the storm will be due west of the city. The surge will increase to its maximum level around 8 p.m. EDT Thursday, when landfall is expected in the Big Bend region. This six-hour period of the highest storm surge would straddle low tide, so that would be fortuitous.

Heavy rains from Helene could set records and produce devastating floods

Even though Helene will be moving at a relatively fast pace as it crashes ashore, its large size, ample moisture, and unusual left-angling track will still lead to torrential rains and a widespread flood threat. The projected track of Helene – onshore and then northward and westward toward a strong cut-off upper low – is unusual if not unprecedented for a major hurricane striking the Florida Panhandle. This trajectory would exacerbate the flow of moisture onshore and upward against the higher terrain of the southern Appalachians. Storm totals of eight to 12 inches are projected from northern Georgia, including the Atlanta area, into far northwest South Carolina and western North Carolina.

Well ahead of Helene itself, a predecessor rain event (PRE) along a cold front will dump widespread 2-4 inch rains with local 4-6 inch totals across much of the Southeast into Thursday morning, setting the stage for major subsequent problems with Helene. The NOAA Weather Prediction Center has issued a High Risk area for excessive rain near the intersections of Georgia, South Carolina, and North Carolina, warning that life-threatening flash flooding will be increasingly likely. A broader Moderate Risk area extends from northwest Georgia, including the Atlanta area, to parts of western South and North Carolina. Only about 4% of all days have High Risk areas, but these days lead to more than a third of all U.S. flood-related fatalities and 80% of flood damage.

The situation will worsen as Helene approaches and arrives. For the period from Thursday into Friday morning, the NOAA Weather Prediction Center issued a much larger Moderate Risk area along and east of Helene’s path from the Florida Panhandle into western North Carolina, and another High Risk area close to the location of Wednesday’s, where multiple models indicate that two-day totals of 10-11 inches (254-279 mm) are possible.

Along with widespread flash flooding and some eventual river flooding, landslides will be a distinct threat over the southern Appalachians. Western North Carolina may get additional upslope-driven rain on Friday as Helene’s circulation slows down and moves westward across the Tennessee Valley.

Atlanta’s largest three-day rainfall total in records dating back to 1878 is 11.75 inches on December 7-9, 1919. The highest total over the last century was 8.89 inches from Hurricane Opal on October 3-5, 1995. The population of metro Atlanta has more than doubled since 1995, to around 6.1 million, so a similar or larger rainfall than Opal produced could lead to much greater urban impacts from flash flooding.

In data going all the way back to 1869, the highest three-day rainfall total for Asheville, North Carolina, is 8.49 inches (October 24-26, 1918). Helene has a clear chance of breaking this record. At Greenville-Spartanburg (airport records dating back to 1962), Tropical Storm Jerry produced a three-day total of 14.47 inches on Aug. 25-27, 1995, but no other event has generated more than seven inches across a three-day period.

Damaging winds far inland

Because of Helene’s expected large size, Category 3 intensity, and fast forward motion at landfall, damaging winds will extend much farther inland than is usual for a hurricane. Tallahassee may be at particular risk if Helene takes a track into the eastern Florida Panhandle. In data going back to 1945, the strongest sustained wind and wind gust on record for Tallahassee is 58 mph (August 8, 1962) and 83 mph (September 11, 1990), respectively. Helene could exceed both of these values. Given Tallahassee’s substantial population and extensive tree canopy, widespread power outages and tree-fall damage are quite possible. Similar damage occurred near and northwest of Tallahassee into southern Georgia during the rapid landfall of Hurricane Michael in 2018.