Eight days after leaving the coast of Africa, the tropical wave designated 98L finally developed, becoming Tropical Depression Nine (TD 9) in the southeastern Caribbean at 5 a.m. EDT Friday. While TD 9 was experiencing hostile wind shear that was limiting it on Friday afternoon, it is expected to rapidly intensify into a hurricane this weekend, and will affect Jamaica, the Cayman Islands, Cuba, and Florida over the next five days.
Satellite imagery and Curacao radar on Friday afternoon showed TD 9 bringing heavy rain showers to the ABC Islands and to the north coast of South America, but struggling with high wind shear: The low-level center was exposed to view, and the system’s relatively scanty heavy thunderstorms were confined to the southwest side of the center. The reason: Outflow from powerful Hurricane Fiona was bringing strong northeasterly winds over TD 9, creating about 20-25 knots of wind shear. Otherwise, conditions were favorable for development, with warm sea surface temperatures of 29-30 degrees Celsius (84-86°F) and a moist atmosphere (a mid-level relative humidity of 70%). At 11 a.m. EDT Friday, TD 9 was 515 miles east-southeast of Jamaica, with top winds of 35 mph, headed west-northwest at 14 mph.
Intensity forecast for TD 9
Continued high wind shear from Fiona’s upper-level outflow is expected to impede development of TD 9 through Saturday morning, and its close proximity to the coast of South America may also hinder it. Odds for development will rise by Saturday afternoon, when Fiona will be approaching Canada, allowing wind shear over TD 9 to drop to the moderate range, 10-20 knots. Highly favorable conditions for development are expected by Saturday night, when TD 9 will be in the central Caribbean: There, it will find very warm water of 30-31 degrees Celsius (86-88°F) with a high heat content, light wind shear, excellent outflow channels aloft, and a moist atmosphere (a mid-level relative humidity of 70%). The entire northern half of the Caribbean has been free of tropical cyclones all season, so these untouched waters (running about 0.5 degrees Celsius or 0.9 degree Fahrenheit above average for late September) will be particularly ripe for supporting any well-organized cyclone with favorable atmospheric conditions.
The two intensity forecasts from the National Hurricane Center so far have been unusually aggressive. The initial 5 a.m. EDT Friday forecast predicted that TD 9 would reach 110 mph winds by the end of the five-day forecast period. That forecast is only the third time NHC has made such an aggressive forecast in the first advisory for a new system. The other two cases were also for western Caribbean systems: Hurricane Ida in 2021 and Hurricane Iota in 2020. Both of those hurricanes ended up rapidly intensifying to category 4 storms, exceeding the initial intensity forecast from NHC.
The 11 a.m. EDT Friday NHC forecast for TD 9 was also very aggressive, explicitly predicting rapid intensification. Beginning on Sunday morning, when TD 9 will be south of Jamaica, NHC predicted TD 9 to go from a tropical storm with 50 mph winds to a category 1 hurricane with 85 mph winds in 24 hours. This meets the minimum definition of rapid intensification, which is a 35 mph increase in winds in 24 hours.
Continued intensification is predicted on Monday, when TD 9 will be passing through the Cayman Islands and approaching western Cuba. Crossing Cuba is likely to interrupt the intensification process, and if TD 9 passes over one of the more mountainous portions of the island (Figure 1), it is possible that the storm will take a day or more to recover and resume intensifying. In addition, TD 9 may be hampered at that time by an increase in wind shear, as an upper-level low to the west of Cuba brings a southerly flow of upper-level winds over the storm.
NHC currently has TD 9 topping out as a category 3 hurricane with 115 mph winds on Wednesday morning near the coast of southwestern Florida, but this forecast should be regarded as highly uncertain. TD 9 could plausibly be as weak as a category 1 hurricane, or as strong as a category 4, when it makes its closest approach to Florida next week (see Tweet below).
It would not be a surprise if TD 9 rapidly intensified at a greater rate than NHC is predicting: The western Caribbean is a notorious breeding ground for major hurricanes, and the top intensity model for making 4- and 5-day forecasts for Hurricane Fiona, the HWRF model, was predicting that TD 9 would reach category 4 strength in six days, as was the top intensity model from 2021, the HMON (Figure 2).
Track forecast for TD 9
The spread in model track solutions for TD 9 continues to be large, with impacts to Central America, Mexico’s Yucatan Peninsula, Jamaica, the Cayman Islands, Cuba, Florida, and the Bahamas all in play. As of Friday early afternoon, it appears that Central America is at relatively low risk of a direct hit, though heavy rains from the storm may affect coastal Honduras beginning on Sunday. Mexico’s Yucatan Peninsula is not in the NHC cone, but some of the members of the 12Z Friday GFS ensemble forecast showed TD 9 passing very close to the northeastern tip of the peninsula, so residents there need to pay attention to future forecasts.
The eventual track of TD 9 will depend heavily on what happens during the coming 24 hours, when the storm will be weak and subject to considerable shifts in position. These can occur because of relatively subtle changes in the steering flow, and from center reformations when wind shear rips into the core of TD 9. By Saturday, when TD 9 should have a stronger circulation and be a tropical storm, we will have a much better idea where it is going. It appears highly likely that Jamaica will miss a direct hit, but the island will still receive damaging rains of 4 – 8 inches. The Cayman Islands could receive a direct hit, and Cuba is highly likely to receive a direct hit, possibly by a major hurricane. The threat to Florida is more murky, with most of the peninsula south of the Panhandle in the NHC 5-day cone of uncertainty. Residents of the western Bahamas should also be paying attention, since they are also in the 5-day cone of uncertainty. In all these areas, preparations for the arrival of the storm should be given high priority.
Hurricane Fiona to thrash Atlantic Canada
One of the strongest storms in Canadian history is expected late Friday into Saturday, as category 4 Hurricane Fiona quickly transforms into a massive and powerful post-tropical cyclone just hours before landfall. Rare hurricane and tropical storm warnings have been issued by Environment Canada across a span of roughly 800 miles, from eastern New Brunswick and far southeast Quebec to Nova Scotia and most of Newfoundland and Labrador.
As of 11 a.m. EDT Friday, Hurricane Fiona was charging northeast at 35 mph with top sustained winds of 130 mph, about 250 miles north of Bermuda, according to the National Hurricane Center (NHC). Despite that considerable distance, Fiona’s sprawling circulation brought sustained winds of 64 mph to L.F. Wade International Airport in Hamilton just after 8 a.m. ADT, with a gust to 93 mph around 7 a.m. Intense rain squalls had mostly departed Bermuda by late Friday morning. Fiona was generating large swells that were creating dangerous rip currents along much of the coast of the U.S. and Canadian Maritimes.
Fiona’s wind field will expand even more before the storm reaches Canada. As Fiona accelerates northward over unusually warm midlatitude waters, it is expected to remain a potent hurricane into Friday evening, perhaps maintaining category 3 strength. An approaching cold front and upper-level storm will then trigger a phenomenon recently classified as an instant warm seclusion. Much like Hurricane Sandy’s transition to “superstorm” just before it struck New Jersey in October 2012, the approaching midlatitude storm will envelop Fiona and quickly become a vast post-tropical storm system, but with the hurricane’s warm core still tucked within (secluded). As with Sandy, the impacts of Fiona will be essentially those of a hurricane, even if the formal classification changes in the hours before landfall, which is why Environment Canada has gone with hurricane warnings.
As Fiona evolves Friday night, its central pressure may dip to astounding levels. Multiple models are predicting Fiona to reach Nova Scotia on Saturday morning with a central pressure around or below 930 mb, which would easily break the all-time record in Canada for sea-level pressure of 940.2 mb set at St. Anthony, Newfoundland and Labrador, on January 20, 1977.
A pressure below 930 mb is more on par with what one might expect in a small but intense category 4 hurricane. However, in this case of Fiona, the pressure differential across the storm – which drives the surface winds – will be dispersed over a much broader region, so winds of tropical-storm and even hurricane strength will cover a vast area.
In addition, a streak within the jet stream called a sting jet may also whip around the storm’s southeast side and descend to the surface, pushing even stronger upper-level winds to the ground in focused but damaging pockets.
Landfall and impacts
As of 11 a.m. Friday, NHC was predicting Fiona will reach the coast of eastern Nova Scotia on Saturday morning as a post-tropical cyclone packing top sustained winds just east of its center of 100 mph. Much more concerning is the predicted west-to-east breadth of hurricane-force sustained winds of at least 74 mph (around 170 miles) and tropical-storm-force sustained winds of at least 39 mph (around 680 miles). These values suggest an exceptionally large storm by both tropical and extratropical standards, not far below what Hurricane Sandy of 2012 brought (see Tweet below).
Widespread rains of 3 to 6 inches (75-150 millimeters) are expected over Atlantic Canada, with a corridor of even heavier rain likely along the track of Fiona’s core. The storm will be slowing down near the time of landfall, which will intensify the rain and wind impacts. “Some districts have received large quantities of rain recently, and excessive runoff may exacerbate the flooding potential,” Environment Canada noted. Huge waves can also be expected over a broad stretch of coastline, perhaps topping 39 feet (12 meters) in eastern parts of the Gulf of St. Lawrence. A significant storm surge is likely near and just east of the landfall point, especially if landfall coincides with the Saturday-morning high tide. The complex geography of Atlantic Canada, together with Fiona’s size, means that a variety of coastlines may also be affected by surge apart from the immediate landfall location.
Hurricane Juan, the costliest in Canadian history, made landfall near Halifax, Nova Scotia, at category 2 strength in 2003 with 100-mph sustained winds, wreaking some $200 million (USD 2003) in damage. Fiona is expected to strike a less populated area, with central winds that could be slightly less intense than Juan’s, but it is a much larger storm than Juan. Thus, there is some risk of structural damage wherever Fiona’s winds and gusts are strongest, but many more Canadians are likely to be affected by downed trees and power outages, perhaps on an unprecedented scale for this region.
Some important context: Fiona is passing over unusually warm North Atlantic waters where a marine heat wave has been under way. Part of this historic event will be the coincidence of an upper-level low coming in at precisely the right time to merge with Fiona, but Fiona’s base strength as a hurricane so far north is directly related to its unusual supply of oceanic fuel – something that’s becoming more likely on our human-warmed planet.
Tropical Storm Gaston nears the Azores in central Atlantic
Tropical Storm Gaston was bringing gusty winds and heavy rains on Friday to the Azores Islands, where 2 – 6 inches of rain are expected from the storm. As of 2 p.m. EDT Friday, Gaston had top winds of 65 mph, and was headed southeast at 9 mph toward the central Azores. Given an array of less-than-ideal conditions, Gaston will probably weaken, as it enters a region with higher wind shear and colder waters. Gaston is expected to become post-tropical by Saturday night.
Unusual tropical depression forms in far eastern Atlantic
A tropical wave that moved off the west coast of Africa on Friday became Tropical Depression Ten (TD 10) at 11 a.m. EDT Friday. The depression is expected to move on a somewhat unusual northerly track just off the African coast between the Cabo Verde Islands and Canary Islands.
Tucked in an area with light wind shear of only around 10 knots into Friday night, TD 10 will benefit from a moist mid-level atmosphere (relative humidity of 70 percent) and warm SSTs of around 27 degrees Celsius (81°F). NHC predicts TD 10 to become a minimal tropical storm by Saturday morning, before quickly increasing wind shear, cooler ocean temperatures, and drier air destroy the system by Sunday. As a post-tropical cyclone, TD 10 will likely reach the western Canary Islands by Sunday or Monday, perhaps bringing rains of 2 – 4 inches of rain. It’s possible that TD 10 will become the northernmost tropical cyclone on record in the far eastern Atlantic just before its post-tropical conversion.
One other wave to watch: 99L
A tropical wave in the central Atlantic, several hundred miles west-southwest 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 very limited amount of 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 8 a.m. EDT Friday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 20% and 30%, respectively.
The next two names on the Atlantic list of storms are Hermine and Ian. It appears quite likely that TD 9 and TD 10 will take these names, but which one gets which name was still an open question as of midday Friday.
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