After an oddly quiet couple of weeks, things are starting to change in what’s been long projected to be a frenzied Atlantic hurricane season. The last named system in the Atlantic, Hurricane Ernesto, became extratropical on August 20 while racing east of Newfoundland, and it’s very unlikely we’ll have the next named storm before Labor Day weekend. However, long-range models are becoming increasingly confident that the deep Atlantic tropics will soon spawn a system worth tracking, and there’s plenty of reason to believe that a busy September and/or October lie ahead.
In its Tropical Weather Outlook issued at 2 p.m. EDT Tuesday, August 27, the National Hurricane Center gave odds of 20% that development will occur in the next seven days over the western tropical Atlantic, with near-zero odds in the next two days. There’s no tropical wave of concern bubbling right now, but models are increasingly suggesting that a disturbance will emerge along the Intertropical Convergence Zone – the semi-permanent band of low pressure, converging air, and showers and thunderstorms (convection) that oscillates north and south around the equator, currently extending from near the West Africa coast to the Caribbean islands. If such a disturbance does form, it would most likely track west to west-northwest toward the Lesser Antilles around the middle of next week.
Unless any surprises crop up, this means we could go a full two weeks from August 20 to September 3 without a single named storm. Given that we’re just a couple of weeks away from the traditional September 10 peak of the Atlantic season, that’s an impressive hiatus for any year – never mind a year expected to be hyperactive. In fact, according to hurricane expert Michael Lowry, this two-week period has included at least one Atlantic tropical or subtropical system in every year since 1956.
Why has the Atlantic seemed – or actually been – so quiet lately?
There’s both truth and fiction in the claim that the 2024 Atlantic season hasn’t lived up to expectations of a hyperactive season that were set months ago by seasonal forecasters.
True, there have only been five named systems thus far, compared to an average of six by this point. However, by most other measures, the Atlantic season through August 27 has actually been busier than the 1991-2020 average. This includes accumulated cyclone energy (55.1 units, versus an average of 28.4), named storm days (24.5 vs. 20.3), hurricanes (3 vs 2.1), hurricane days (12.0 vs. 5.5), and major hurricane days (4.5 vs. 1.2). However, that edge is entirely due to the record-smashing, long-lived run of Category 5 Hurricane Beryl in late June and early July. Without Beryl, this would have been a run-of-the-mill season to date, if not a below-average one. And the relatively modest number of named storms is an especially startling change from the trend in recent seasons for a cascade of weaker named storms apart from truly serious hurricane threats.
As for why the Atlantic has laid low in recent days, there’s been no end of speculation and pondering, both among storm watchers on social media and leading hurricane researchers on email lists. Here are some of the factors being discussed:
African waves are farther north than usual. Several of the systems coming off West Africa that typically serve as seedlings for Cabo Verde hurricanes have left the coast at a higher latitude than usual. As a result, they’ve ingested plenty of Saharan dust and dry air, hindering their development, and they’ve been traveling over cooler waters than usual. Unusually strong mid-level northeasterly winds across the eastern Atlantic, related to a persistently positive North Atlantic Oscillation, have further choked off these disturbances.
The Main Development Region is too stable. Sea surface temperatures remain at record to near-record highs across most of the Main Development Region (MDR) of the Atlantic tropics, stretching from Africa to the Caribbean. However, despite this oceanic and surface-atmosphere warmth, temperatures a few miles high have been even more unusually warm this season. The showers and thunderstorms that stoke the heat engine of a hurricane rely on instability: relatively warm, moist air at low levels and relatively cold air at high levels. The exceptional warmth at levels such as 200 millibars (about 39,000 feet) seems to be more than overwhelming the effect of the surface warmth, leading to some of the most stable MDR conditions in decades.
There’s too much easterly wind shear. For incipient tropical cyclones to have their best chance of vertical growth, winds at lower and upper levels should be as similar as possible. Sometimes the upper-level winds are westerly, which creates unfavorable vertical wind shear when superimposed on the typical easterly trade winds near the surface. In recent weeks, there’s been a more unusual hindrance to development: the easterly upper flow across the MDR has actually been stronger than the easterly trade winds – so strong, in fact, that it’s been counterproductive for hurricane development.
An Atlantic version of La Niña is forming. Just like the equatorial Pacific spits out El Niño and La Niña events, the equatorial Atlantic has its own quasi-cyclic oscillation that sends warmer or cooler water across the eastern end of the basin, causing what are known as Atlantic Niño and Atlantic Niña events, centered between sub-Saharan Africa and South America. The Atlantic version isn’t as well known or as globally influential as the Pacific one, but it can affect the short-term evolution of the Atlantic hurricane season. This year the eastern equatorial Atlantic has swung from Niña (warm) toward Niño (cool) conditions at the fastest pace in 40-plus years of monitoring. Such a swing would normally tend to enhance southeasterly trade winds across the eastern MDR and help stoke hurricanes, but it hasn’t done so – at least not yet, though this could change. For a great explainer, see the recent post at climate.gov by Franz Philip Tuchen (CIMAS/NOAA/University of Miami).
The consensus among the experts is that September will bring more typical atmospheric conditions to the region, allowing for a busy peak month of the Atlantic hurricane season.
Typhoon Shanshan intensifies to a Cat 3 while grinding to a near-halt
Meanwhile, in the Pacific, residents of Japan’s southernmost main island of Kyushu continue to be on high alert for the expected arrival on Thursday of Typhoon Shanshan. At 11 a.m. EDT Tuesday, the Joint Typhoon Warning Center (JTWC) said that Shanshan had intensified to a high-end Category 3 storm with 125 mph winds with a central pressure of 937 mb, located 313 miles south of Sasebo, Japan, headed northwest at a mere 1 mph.
Satellite loops showed that Shanshan had a solid area of heavy thunderstorms surrounding a large eye whose diameter had increased to 30 miles because of an eyewall replacement cycle. Shanshan has likely peaked in intensity, as its slow motion will allow the typhoon to upwell cooler waters that will cause weakening. The Joint Typhoon Warning Center and the Japan Meteorological Agency predict that Shanshan will make landfall in Kyushu near 12Z Thursday. However, Shanshan is in an area of weak steering currents, and the JTWC warns that their track and intensity forecast is of low confidence. Worryingly, the 12Z Tuesday run of the GFS model predicted that Shanshan would wander for the next 12 days over southern Japan and the adjacent waters, making multiple landfills.
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