As I look out across the tropics today, I see what appears to be a transition to what I’ll call a winter pattern developing. By that I mean dry air and increasing wind shear are evident across the Atlantic Basin. Tropical cyclones will have a very difficult time developing in such a pattern. But since the NHC is now naming what some might argue as questionable low centers over cold water in the far north Atlantic, I won’t absolutely rule out the “development” of perhaps one or two more named storms this season.
Take a look at the animation below from the University of Wisconsin. It’ s a special microwave image that measure the amount of water in the air. Red areas represent high water content, blue areas are relatively dry. The dry air is taking over the tropics now:
The Intertropical Convergence Zone, the band of storms north of the Equator from which tropical cyclones often develop, is starting to shut down between Africa and the eastern Caribbean. There are still some waves moving off the west coast of Africa, but they’re weak compared to the waves that moved off the west coast of Africa a few weeks ago. And they’re now surrounded by lots and lots of dry air. Add to that increasing wind shear across the Tropical Atlantic and I think it’ll be hard for a system to develop east of the Caribbean for the rest of the season. And that dry air is moving across the Caribbean Sea, too. The only area with a considerable amount of moisture is the western Gulf of Mexico, but wind shear is on the increase in the Gulf, so development is unlikely. That strong cold front which will push out across the Gulf this coming weekend may be the signal that development in the Gulf may be shut down for 2009.
Besides the lack of moisture, the jet stream is becoming more and more active, digging farther south with each weak. Cold fronts are becoming stronger and pushing farther out across the Gulf and off the East U.S. Coast. This means that wind shear will be on the increase this week. Wind shear has been an issue across just about all of the Atlantic Basin in 2009. Even though the analysis charts say that the wind shear in 2009 wasn’t far from average, I think that we really don’t have a good handle on how strong the wind shear really is out over the oceans most of the time. It’s clear that strong wind shear caused the demise of most of the storms (and disturbances) in 2009. Here’s a montage of 2009 storms/disturbances from Chris Velden at the University of Wisconsin. Note how many of this year’s storms were fighting strong wind shear - their centers exposed with thunderstorms way off to one side or the other.
Click to Enlarge
We still have about 6 weeks remaining in the 2009 hurricane season. I think there’s a very good chance that we won’t see any more named storms this year. Except, perhaps, another stray storm out in the middle of the Atlantic - far from land. The Caribbean Sea is climatologically the place to look for late season development (Omar and Paloma in October and November of 2008), but the Caribbean has been a hostile environment in 2009, and I see no signs of that changing. Just to be safe, give it another 2-3 weeks then we can all breathe a huge sigh of relief that the 2009 season is over, and we escaped the death and destruction that has been so common in recent years.
