We’ve arrived at the peak week of hurricane season – the traditional midpoint of the Atlantic’s yearly activity is September 10 – but you wouldn’t know it from the Tropical Weather Outlook issued by the National Hurricane Center, or NHC, on Monday, September 8. The headline: “Tropical cyclone activity is not expected during the next 7 days.”
As we reported last Thursday, September 4, NHC had given a disturbance in the eastern tropical Atlantic known as Invest 91L an 80% chance of developing over the subsequent week and upped the odds to 90% on Friday morning. But 91L was never able to fully organize, and by early Sunday, it was no longer being tracked by NHC. There’s no obvious single factor that did in 91L, though an unexpectedly stable atmosphere in the deep Atlantic tropics may be part of the mix.
Michael Lowry has an excellent discussion on why 91L may have faltered and how Google’s DeepMind AI model did the best job on the system.
With the demise of Tropical Storm Fernand August 28, it’s now been 11 days since any named storms have prowled the Atlantic. It’s not that unusual to get a one- or two-week pause in the Atlantic hurricane season, although it’s a bit more uncommon for things to go quiet for too long when we’re this close to the climatological peak.
Just last year, there was a 20-day break between August 20, when Hurricane Ernesto was declared post-tropical, and the emergence of soon-to-be Hurricane Francine as a tropical storm on September 9. The interval without any named-storm formation ran from August 13 to September 8 – the first time since 1968 that this particular 27-day period had seen no named storms develop. As of Monday, we’ve racked up a string of 16 days free of named-storm formation in the Atlantic, and if the NHC’s current outlook verifies, that will extend to 23 days by next Monday.
One striking aspect of these peak-season pauses: They’ve both occurred despite the widespread extent of unusually warm to record-warm sea surface temperatures.
Meteorologist Eric Webb mused on the pause last year in this tweet of September 3, 2024:
This Atlantic Hurricane Season is teaching &/or reminding folks (including myself to a degree) that we are hyper fixated on/overweighting local, tropical [sea surface temperature anomalies] for seasonal forecasting. This season also could be offering us a glimpse into what future hurricane seasons may potentially look like.
Owing mainly to the strength and duration of former Category 5 Hurricane Erin, which nimbly avoided any landfalls, the Atlantic is only running about 20% below the average accumulated cyclone energy, or ACE, for this point in the season, according to statistics updated daily by Colorado State University. But Erin is the only hurricane among the season’s six named Atlantic systems thus far. Based on a 1991-2020 climatology, the average dates of formation of the third and fourth Atlantic hurricanes each year are September 7 and 16.
The chasm in strength between former Category 5 Erin and this year’s other Atlantic systems is vast. Erin, which revved up top sustained winds of 160 mph, is the Atlantic’s only system this year to pack top winds of at least 65 mph.
Looking ahead in the Atlantic
In its biweekly outlook issued September 3 for the period September 3-16, the forecast team at Colorado State University led by Phil Klotzbach called for a 65% chance of near-normal activity. The next week or two will see a partial reversal of the upper-level pattern that’s favored deep upper troughs in the eastern half of the United States and ridging in the west, with that pattern gradually morphing toward more of a western trough and eastern-U.S./western-Atlantic ridge. Such a pattern could favor a higher chance of a U.S. landfall if any system(s) take shape – a very big “if” at this point.
The operational GFS model has been consistently suggesting a system might move northward from the Western Caribbean into the Gulf of Mexico late next week, and a few GEFS ensemble members have also shown unsettledness toward the Bay of Campeche and western Gulf later next week, but this is far beyond a reliable forecast window. The GEFS also suggests recurvature is the most likely fate of any development further east. The European ensemble model is distinctly less bullish about overall activity next week, including any potential Gulf system.
Kiko expected to stay safely north of Hawaii
Hurricane Kiko’s multiple bursts of intensity have kept forecasters on their toes, but the storm’s very predictable track is good news for Hawaii. Now a weakening Category 1 hurricane with top winds of 85 mph (140 km/h) as of 11 a.m. EDT Monday, Kiko is continuing to move steadily west-northwest at 15 mph (24 km/h).
Kiko is projected to pass around 150-250 miles (240-400 km) north of the Hawaiian Islands on Tuesday and Wednesday as a weakening tropical storm. Sustained tropical-storm-force winds aren’t predicted to reach as far south as the islands, especially since Hawaii lies on the weaker side of Kiko, but several days of big swells and life-threatening rip currents can be expected. No rainfall impacts from Kiko are being predicted by NHC for the Hawaiian Islands.
Kiko’s top winds peaked in the Category 4 range twice: 145 mph late Wednesday, September 3, and 140 mph late Friday, September 5. There was a dip to minimal Cat 3 strength (115 mph) in between as dry air infiltrated and temporarily weakened Kiko.
New storm possible this weekend in the Eastern Pacific off the coast of Mexico
For the Eastern Pacific waters off the coast of Mexico, NHC is predicting that the next named storm could occur this weekend or early next week. Recent runs of the GFS and European model have been predicting that a tropical wave will develop then and move west-northwest, parallel to the coast of Mexico. In their 8 a.m. EDT Monday Tropical Weather Outlook, NHC gave two-day and seven-day odds of development of 0% and 30% to this future system. The next name on the Eastern Pacific list of storms is Mario.
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