The Atlantic hurricane season is not over by a long shot. In fact, we’re in the middle of the most active few weeks of the season historically. However, the tropics seem poised to enter a temporary quiet period once tropical storm Maria becomes part of an extratropical rainstorm over the North Atlantic in a couple days. A spell of inhospitable conditions that will shut down storm development is beginning to temporarily settle over Africa and the Atlantic Basin.
To date, this season has produced a lot of hype and very few hurricanes. In all, we’ve seen one tropical depression, 12 tropical storms, and two hurricanes. Though the total storm count is atypically high, only Irene made a North American landfall at hurricane strength - and did so just barely along a small section of North Carolina’s Outer Banks. All other members of the 2011 class so far either made landfall at tropical storm strength or curved out to sea a long way from our shores.
During the next week or so, mechanisms that partly govern the dryness, wind shear, and vertical stability (i.e., resistance to thunderstorm development) over the tropics will conspire to suppress the likelihood of tropical cyclone genesis in the Atlantic Basin. The image below offers a simplistic overview of what’s currently going on.
The areas contoured in green roughly outline regions where the development of large clusters of thunderstorms (shaded in blue and red) is encouraged. It is no coincidence that the blue/red shading and the green lines match up pretty well. The areas marked by brown lines are places where the atmosphere limits storms – as noted by the clearer skies. Notice that between West Africa and Mexico, the green contouring is minimal. And as one moves from there towards North America, brown contours start to show up. Clearly, thunderstorm activity is right now much more pronounced over Indonesia and the West Pacific than over the tropical Atlantic. Minor exceptions lie over some small areas right atop tropical storm Maria and over northern South America.
This picture is telling us that the locales we monitor for hurricane development in our part of the world, all the way from the Caribbean to west Africa, are probably not going to deliver much soon. In fact, satellite imagery zoomed in over this very region reveals a particularly dry atmosphere aloft (dark and bronze shading) with relatively sparse thunderstorm activity accompanying the parade of easterly waves over Africa.
Indeed, our next named storm may not come until those green lines become more prominently clustered over Africa and/or the Caribbean.
Meteorological guidance from the global weather models and from statistical forecasts of the tropical atmosphere suggests that this general pattern will continue for the next 10 days or so.
Soon thereafter, there are some indications that the pattern will shift in a way that re-activates our hurricane season, possibly in the Caribbean first. The ocean heat content is still sufficiently high in the development regions (including the Gulf of Mexico) to support major hurricanes. And history says we have a long way to go before we can put the season to bed. But until our current pattern flips, easterly waves rolling off of Africa just might travel harmlessly across the Atlantic, and any developments in the Caribbean may very well be squashed by the unfriendly surroundings.
Related links: Hurricane Tracking Center