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A Lull In The Tropics – But Will It Last?


A year ago Friday, Hurricane Arthur pounded coastal North Carolina with 100+ mile-per-hour winds and heavy rain. A year later, however, the tropics, particularly in the Atlantic, are notably calm.

So the million dollar question is, with the peak of hurricane season quickly approaching (typically late August and early September), will it stay relatively calm through the rest of hurricane season?

Of course, this early in the season (hurricane season in the Atlantic basin officially started on June 1st), there are no firm answers; rather, there are simply clues which offer relatively limited insight into what might happen down the road. With that said, the good news is those clues are offering a decidedly optimistic forecast.

After two early tropical storms, Ana and Bill in May and June, respectively, the tropical Atlantic is calm, with little expected to develop over the next few days. But the main clue for optimism lies in sea-surface temperatures. In order to fuel hurricanes’ 100+ mile-per-hour (MPH) winds, sea-surface temperatures of 80° or higher are one of the primary prerequisites for hurricane development – but these are lacking across a key chunk of the Caribbean Sea and the western Atlantic, two areas critical to developing and sustaining hurricanes. As the adjacent map shows, sea-surface temperatures across most of the Caribbean are slightly below average, but they’re well below average across the central and western Atlantic. Temperatures there – while still months from their typical peak – are not even at that 80° threshold, an indicator that the current lull in the Atlantic tropics is likely to continue.

‘Cape Verde season’, a reference to the island chain off the west African coast where mid-to-late season tropical cyclone development often originates, is likely to be stifled by this lack of warm water. As tropical systems develop along the Inter-Tropical Convergence Zone (ITCZ), the elongated area of low pressure caused by the convergence of trade winds, they’re likely to dissipate, if not fall apart altogether by the cooler-than-average sea surface temperatures.

Wind shear, another prominent factor in tropical weather forecasting, is more difficult to forecast on a long-term basis, but the trends  here also appear to favor a quieter Atlantic hurricane season. Wind shear, the turning of winds both in terms of speed and direction at different levels of the atmosphere, is a huge inhibitor for tropical development, and it’s also helped to keep the eastern Pacific relatively tranquil over the last two-to-three weeks.  The presence of El Nino is likely to keep wind shear in the Atlantic high, further disrupting development in the Atlantic.

Official forecasts have called for a below-average season in the Atlantic basin, mostly as a result of the strengthening El Nino, and shorter-term trends appear to strongly agree with that assessment.

Stay with WeatherNation through tropical season.

Meteorologist Chris Bianchi – Photo – WeatherBell Analytics

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