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Huge Hurricane Helene accelerates toward Florida » Yale Climate Connections

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26 Sep

Published Date and Time: 2024-09-26 13:19:25


Massive Hurricane Helene is rumbling north-northeastward through the Gulf of Mexico, en route to what is likely to be a devastating landfall late Thursday night in the Florida Big Bend region. The hurricane’s huge area of strong winds – hurricane-force winds extended outward up to 60 miles (95 km) from the center, and tropical-storm-force winds extended outward up to 345 miles (555 km) at 11 a.m. EDT Thursday — were already pushing a storm surge in excess of two feet along most of the Florida Gulf Coast Thursday morning when the storm was about 300 miles offshore.

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. In a press release on Wednesday morning, reinsurance company Gallagher Re estimated that Helene might cause at least $4-$7 billion in damage. This may need to be revised upwards in light of the more severe inland rainfall risk now being predicted.

Helene steadily intensifying

At 11 a.m. EDT Thursday, Helene was located 255 miles (405 km) southwest of Tampa, Florida, moving north-northeast at 14 mph (22 kph), with top sustained winds of 105 mph (165 kph) and a central pressure of 960 mb. Satellite imagery and Southeast U.S. radar showed that Helene had a large area of heavy thunderstorms with very cold cloud tops, which were bringing heavy rains to most of Florida’s Gulf Coast. Helene’s heaviest eyewall thunderstorms were wrapped about 3/4 of the way around a prominent eye.

Helene has undergone one round of rapid intensification (defined as a 35-mph increase in winds in 24 hours), in the 24 hours ending at 2 p.m. EDT Wednesday, when it increased from 50 mph winds to 85 mph winds. The rate of intensification slowed on Wednesday night, thanks to some dry air from Mexico that wrapped into Helene’s core. The Hurricane Hunters reported at 10:15 a.m. EDT Thursday that the eye had finally closed off, which will potentially enable Helene to undergo another round of rapid intensification.

Track forecast for Helene

Helene is being steered by a high-pressure system to its east and an upper low that is cutting off (detaching from the jet stream) over the mid-Mississippi Valley. The upper flow between these systems will carry Helene northward to a Thursday-night landfall in Apalachee Bay in Florida’s Big Bend region. The timing of Helene’s landfall among the top six track models ranges from 9 p.m. EDT Thursday to 2 a.m. EDT Friday. The consensus among these models is that Helene will hit one of the least populated parts of the Florida coast, near the town of Perry (pop. 7,000). At this time, it appears that the capital of Florida, Tallahassee, will be on the weaker (left) side of Helene, and will escape the hurricane’s strongest winds.

Helene will be accelerating until landfall when it will be moving at a very fast forward speed of about 25 mph. This rapid motion will allow Helene to maintain hurricane strength over 100 miles inland, well into Georgia. As Helene penetrates into northern Georgia and weakens, the counter-clockwise flow around an upper-level low to the west will turn the storm to the west over Tennessee and Kentucky, where it is expected to slow, stall out, and dissipate by Friday night.

Intensity forecast for Helene

Now that Helene has fully closed off an eyewall, rapid intensification, defined as a 35-mph (56-kph) increase in winds in 24 hours, is a possibility. 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 is 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 upper low 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 evening.

The 11 a.m. EDT Thursday official forecast from the National Hurricane Center called for Helene to peak Thursday evening as a Cat 3 with 120 mph (195 kph) winds, but the top intensity models give a range of potential top intensities from a 100 mph (160 kph) Cat 2 (the HAFS-A model) to a 140 mph (225 kph) Cat 4 (the HMON model).

A widespread and destructive storm surge for Florida

Given its huge size and low pressure, Helene will generate 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.

During the morning high tide cycle, Helene brought a storm surge of nearly 3 feet to Ft. Myers and Cedar Key, 2.5 feet to Naples, 2.6 feet to the Tampa Bay region (at Clearwater Beach and Old Port Tampa), and 1.3 feet to Key West, causing some minor coastal flooding.

Helene’s 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. Should Helene arrive at high tide, record-smashing inundations atop dry ground of as much as 15 to 20 feet are possible along Florida’s Big Bend 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.

Storm surge forecast for Tampa BayStorm surge forecast for Tampa Bay
Figure 1. NHC gave a 10% chance that significant portions of the Tampa Bay area could see flooding of 6-9 feet (orange colors) above normally dry ground, with some low-lying areas getting over nine feet of inundation (red colors), according to the 5 a.m. EDT Sep. 26 NHC potential inundation graphic for Helene. These values are somewhat higher than the forecast done 24 hours previously. Residents should consult the official warnings and guidance from local officials, rather than the map above, in making evacuation decisions.

A record storm surge for Tampa Bay is 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. High tide at St. Petersburg, Florida, is 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).

The predicted timing of Helen’s passage offshore from Tampa until the time of landfall has slowed some since yesterday’s forecast, and will likely occur from about an hour before low tide until a few hours before high tide. Thus, we can probably subtract only about 0.5 feet (0.15 m) of inundation from NHC’s surge forecast for Tampa Bay, giving an inundation of 4.5-7.5 feet (1.4-2.3 m). The highest water levels in Tampa Bay will likely occur when Helene is making landfall, late Thursday night or just after midnight (see Tweet below).

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.

Helene will have outsized, potentially catastrophic impacts well inland

Between its massive size, substantial strength, and fast forward motion, Helene is tailor-made to produce inland impacts much greater than most hurricanes deliver.

Rainfall: A predecessor rain event (PRE) – which can develop hundreds of miles ahead of a northward-moving hurricane –  dumped a broad corridor of heavy rain along the east side of the southern Appalachians over the past day. Over the 24 hours ending Thursday morning, widespread 1- to 4-inch totals occurred in north Georgia, including the Atlanta area, and a focused corridor of 4- to 8-inch rains developed along the western end of the Carolinas and far eastern Tennessee.

Those rains will set the stage for even more trouble as Helene’s showers and thunderstorms push inland on Thursday and Friday. The high winds wrapping around Helene from the southeast will force moisture upslope against the southern Appalachians, leading to potentially catastrophic flash floods – in both rural and urban areas – and mudslides.

Rainfall predictions for Thursday through Sunday. The area of Northwestern Georgia and the Western Carolinas could see upwards of 8-12 inches of rain.Rainfall predictions for Thursday through Sunday. The area of Northwestern Georgia and the Western Carolinas could see upwards of 8-12 inches of rain.
Figure 1.  Projected three-day rainfall totals from Helene for the period from 8 AM EDT Thursday, September 26, through Sunday, September 29. This do not include preceding rains of up to 12 inches that fell from Wednesday into early Thursday over and near the southern Appalachians. (Image credit: NOAA/NWS/WPC via NHC)

For the period through 8 a.m. EDT Friday, the NOAA/NWS Weather Prediction Center issued a large Moderate Risk area for excessive rains along and east of Helene’s track, with an embedded High-Risk area extending from the coast of Florida through Atlanta, GA, to Asheville, NC (see Tweet below). 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.

Widespread 4- to 8-inch totals can be expected, and some 8- to 16-inch amounts may occur for the three-day period from Friday to Sunday morning across western North Carolina (see Fig. 1). A few storm totals in the 20- to 30-inch range – as projected by the National Weather Service office in Greenville-Spartanburg, South Carolina (see Fig. 2 below) – would not be surprising.

Rain predictions for western North Carolina and Northwestern South Carolina. Areas just south of Asheville could see upwards of 30" of rain through the weekend.Rain predictions for western North Carolina and Northwestern South Carolina. Areas just south of Asheville could see upwards of 30" of rain through the weekend.
Figure 2.  Projected storm-total rainfall totals from Helene across the area served by the Greenville-Spartanburg office of the National Weather Service. (Image credit: NWS/GSP) ot

Among many locations that could set records for multi-day rainfall are two cities at particular risk of deadly urban flash flooding from Thursday into Friday:

— Atlanta (current 3-day record 11.75 inches on Dec. 7-9, 1919)
— Asheville (current 3-day record 8.49 inches on Oct. 24-26, 1918)

The French Broad River at Asheville was predicted Thursday morning to crest at 19.4 feet on Friday, producing major flooding. This would be the second-highest crest on record, behind only 23.10 feet on July 16, 1916. Just to the southeast, the Swannanoa River at Biltmore is predicted to hit a record crest of 21 feet on Friday, topping the modern record of 20.70 feet from July 16, 1916. (A height of 26 feet reportedly occurred in April 1791.)

Winds: Helene’s expansive circulation and rapid motion will bring high winds much further inland than usual for a Gulf Coast hurricane landfall. One recent analog is Hurricane Michael (2018), which made landfall as a fast-moving, rapidly intensifying Category 5 storm. Michael produced devastating tree damage and massive power outages, especially from the Florida Panhandle into southern Georgia. Helene’s impacts could be similar in magnitude, but likely focused to the east of Michael’s damage swath and extending much further north.

Tallahassee and Atlanta would be especially vulnerable to such wind damage, although it appeared on Thursday that Helene’s center may track just east of Tallahassee. This shift would reduce the high-wind threat significantly but could still cause extensive urban tree- and power-line damage. If this eastward shift occurs, the worst of the winds may be focused across eastern Georgia, potentially including the cities of Valdosta and Augusta. Helene’s sheer size and strength imply that power outages across the Southeast could easily exceed 1 million customers at some point.

In addition to lower elevations in Georgia, powerful and damaging wind gusts can also be expected across the southern Appalachians, especially along ridge lines and atop peaks, as it will take time for Helene’s circulation to spin down, especially at altitudes well above sea level. Soils in these areas have been softened by the rains of the last few days, further enhancing the risk of uprooted trees.

Pressure: If current model projections hold, Helene may produce all-time low values of barometric pressure across multiple stations in the Florida Panhandle and Georgia. Records at risk include:

— Apalachicola, FL (984.1 mb)
— Tallahassee, FL (976.6 mb)
— Macon, GA (978.0 mb)
— Atlanta, GA (981.4 mb)

Tropical Storm Isaac forms in the remote eastern Atlantic

Tropical Storm Isaac was born on Thursday night in the central North Atlantic at an unusually northern latitude:  37.1 degrees north and 54.1 degrees west, or nearly 1,500 miles west of the Azores. As of 11 a.m. EDT Thursday, Isaac’s top sustained winds were 50 mph, and the storm was moving east at 12 mph.

The near-record warm North Atlantic is giving Isaac a boost: sea surface temperatures of around 27 degrees Celsius (81 degrees Fahrenheit) are about 1°C above average for this time of year, and also 1°C above the traditional minimum threshold for tropical development. Despite a fairly dry mid-level atmosphere (relative humidity around 45 percent), cold air aloft will help boost instability and perhaps keep enough showers and thunderstorms going for Isaac to approach or even reach hurricane strength by Friday. Isaac will most likely track well north of the Azores over the weekend, and its remnants may approach Spain early next week.

Another system may develop in the central tropical Atlantic by Thursday evening or Friday, slowly gaining strength as a tropical storm as it heads west to west-northwest over the next several days. The next name on the Atlantic list is Joyce. The average formation dates of the ninth and tenth named storms (1991-2020) are September 16 and 22, so even if Joyce develops this weekend, the Atlantic will still be running behind its average pace for named-storm formation.

Just off the coast of Mexico, John gains hurricane status for the second time

Hurricane John, which officially dissipated inland on Tuesday after becoming the first major hurricane to make landfall in Mexico further southeast than Acapulco, has redeveloped while moving northwest just off the Pacific coast. As of 11 a.m. EDT Thursday, John was centered about 30 miles offshore southwest of Playa Azul, packing top sustained winds of 75 mph. John is expected to move onto or near the coast later Thursday as it continues northwestward. Rainfall totals of 10 to 20 inches can be expected along the rugged coastal terrain, adding further to the multi-day risk of flash floods and mudslides plaguing the coast this week. According to Reuters, at least five deaths have been attributed to John.

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