September 10th marks the climatological peak of the hurricane season, as indicated by the graphic below. In a typical hurricane season, about half the tropical storms and hurricanes form after September 10th. So far this season, we have seen 6 named storms form, two of them hurricanes, and both hurricanes reached major hurricane strength. A closer look at the graphic below reveals that there are typically more tropical storms which form after September 10th as opposed to hurricanes. That is, the hurricane to tropical storm ration is typically higher prior to September 10th. So much for climatology, what’s happening out across the tropics now?
We’ve had a burst of tropical development over the past 3-4 weeks, starting with Ana in mid August and ending with Fred, currently way out in the East Atlantic. A look at the tracks of the 2009 storms as of today (below) reveals that our seasonal prediction has proven to be quite good. Back in May, we thought that a combination of analog years and the currently developing El Niño would limit activity in the Caribbean Sea this season. That has certainly proven to be the case. Only two storms entered the Caribbean this year, Ana and Erika, and both storms were torn apart by a combination of wind shear and dry air soon after entering the Caribbean Sea. We thought that about half of the storms would form and track farther to the north between Africa and the East Caribbean, turning northward east of the Caribbean. Three of the six storms took such a track (Bill, Danny and Fred). We did warn about the possible formation of a storm in the Gulf vs. a long-term track toward the Gulf. Claudette developed right in the eastern Gulf of Mexico with very little warning. So far, most of our predictions have come true. But what about the rest of the season?
As I look out across the tropics today, I see a pattern that is more typical of late in September or October in terms of the amount of wind shear present. Atmospheric conditions across most of the tropics are simply hostile toward development. Long-range projections are for a considerable amount of sinking air (no thunderstorms) across the Caribbean Sea and the Main Development Region east of the Caribbean through the rest of the month. All the wind shear and dry air won’t necessarily prevent development completely. Additional strong disturbances will occasionally move off the west coast of Africa for another 2-4 weeks. Wind shear across the western Caribbean Sea and Gulf of Mexico could relax at times over the next month, allowing for a few more storms to develop.
I think that the pattern we’ve seen all season will continue through October. That pattern would be storms forming a bit farther north, skirting near or north of the eastern Caribbean Sea, then turning northward - somewhere. So far that “somewhere” has been east of the U.S. But with the pattern across the U.S. forecast to change over the next few weeks, it’s a little more likely that a developing storm could impact Florida northward through the Carolinas during the last half of September and the first half of October. So don’t let your guard down yet. And I’m still not ready to rule out another developing storm in the Gulf of Mexico this season. Sure, wind shear is currently too high there. But there are now thunderstorms in the Gulf, a vital ingredient for hurricane development. If the wind shear drops off for a few days, then we have to watch the Gulf for close-in development.
As for totals by the end of season, our May/June prediction of 10 named storms, 4 hurricanes (2 of them major hurricanes) looks pretty good. But if I were to make any changes, I’d say that we may well see less than 10 named storms by the end of November. We’ll need to keep a close eye on the tropics for about another month. After that, I think it’ll be all over - at least for the Caribbean Sea, the Gulf of Mexico, and the East U.S. Coast.
