Here’s a guest-posting from ImpactWeather meteorologist and lead hurricane forecaster Chris Hebert.
Even though we’re just past the peak of the hurricane season (September 10th), the tropics are showing signs of shutting down for the year. Cold fronts (along with the jet stream) are dipping into the Gulf of Mexico, making conditions there quite unfavorable for development. Conditions across the Caribbean and Tropical Atlantic have not been favorable for development/strengthening all season, but there were still 14 named storms so far this season, which is above the 30-year average of 12 named storms. However, that doesn’t tell the whole story.
Of the 14 named storms, as of September 24th there were eight hurricanes but only one major hurricane. Of those eight hurricanes, only one formed in the deep tropics (Ernesto). The rest of the hurricanes formed in the subtropics (north of 25N). Pre-season indicators suggested that the environment across the tropics would be rather hostile – sinking air reducing thunderstorm activity and an abundance of dry air moving off the west coast of Africa. Given that only a single brief hurricane developed in the deep tropics, it’s clear that the tropics were indeed unfavorable for development this season. But why the high number of named storms?
There are two reasons. First, three of the named storms formed from non-tropical low pressure areas in the subtropics (Alberto, Beryl and Chris). It’s unusual for so many non-tropical lows to develop and eventually become tropical cyclones. The second reason is that though the deep tropics (south of 25N) were quite hostile toward development this year, the subtropics were not nearly as hostile toward development. Many struggling weak tropical storms formed in the Main Development Region (MDR), the tropical region between Africa and the Eastern Caribbean, only to finally reach hurricane strength once they moved out of the tropics.
Only two hurricanes affected any land areas. Ernesto hit the Yucatan Peninsula and Isaac hit southeast Louisiana, both as Category 1 hurricanes. The single major hurricane was Michael, with winds at the lower end of Category 3 strength (115 mph) for only a 6-hour period. The 2012 season was one of struggling tropical storms in the deep tropics and mid-latitude hurricanes.
Another way to measure seasonal activity is with something called Accumulated Cyclone Energy (ACE). ACE is a function of a tropical cyclone’s maximum sustained wind and how long it persists. A strong hurricane lasting many days will accumulate many more ACE points than a weak, short-lived tropical storm. In an average year, tropical cyclones in the Atlantic Basin total about 104 ACE points. Even with the above-average number of named storms in the Atlantic, the total ACE as of September 24th is only 91 points. And many of those 91 points were accumulated by Leslie (15), Michael (17), and Nadine (13) – three hurricanes that formed in the subtropics and persisted for quite a few days out in the open Atlantic. [Ed. note: As of today, Nadine is still active a full 15 days after the first bulletin was issued and is still racking up ACE points.]
As for the rest of the season, it’s possible we may see another tropical storm form in the central Subtropical Atlantic, possibly from a non-tropical feature. Closer to the U.S., the place to watch will be the southwestern Caribbean Sea and possibly the Bay of Campeche. But with strong westerly winds aloft across the Gulf of Mexico, the chances of any impact along the northern Gulf coast are quite low. Though the season may end up with a total of 15 or 16 named storms, it will probably only be remembered for Isaac’s impact on southeast Louisiana and southern Mississippi.