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A break in the tropics? Not so fast my friend ....
Submitted by Tim Buckley on Tue, 09/21/2010 - 8:25am.
A look at this morning's satellite picture across the Atlantic might lead you to believe that the tropics are packing up and ready to call it a year. As our football friend Lee Corso would say, "Not so fast my friend!" There's plenty of season left and plenty of reasons to think that we're still bound to see a good deal of development in the near future.
Of course, Tropical Storm Lisa came into the picture earlier this morning. Lisa is another one of the "Cape Verde" type storms that we've seen so often here in the last month, starting with Danielle. A quick fact, it's been so active that we've had at least one named storm in the Tropical Atlantic since Danielle formed on August 22nd almost one month ago. Eight storms later, we get to Lisa - which appears may be the last in the line of storms to form directly off the African coast.
While it's not out of the question that we see a few more Cape Verde storms, there are several reasons that point toward storm activity shifting toward the Caribbean instead of the open Atlantic. The first, is climatology. We're getting to a point here in late September and early October where historically storms tend to favor the warmer Caribbean basin as opposed to the waters off of Africa. The two maps below show all the storms to have formed in the Atlantic over the past 150 years in two week time frames:
The first shows the storms forming in early September. You can clearly see that the area between the African Coast and the Lesser Antilles is a highly favored area for development during this time frame. Also notice, that the Caribbean Sea is nearly free of any tropical systems developing at this time.
Now fast forward the map into early October. Historically, the chances for "Cape Verde" type of hurricanes decreases significantly. Instead, the area most heavily favored for development overwhelmingly becomes the northwest Caribbean Sea.
The second reason a pattern shift is expected involves sea-surface temperatures. Now, a wise professor back in college once told me that once you get to this point in the hurricane season, "The water is warm everywhere." While that's been true for the last month, we're beginning to see a greater spread in water temperatures come into view.
Looking at the sea-surface temperature anomalies this week we can see that the bout of so many storms passing over the same waters in the Atlantic has cooled down the waters some along the East Coast and north of the Greater Antilles. At the same time, look at the Caribbean. This area has remained virtually untouched by tropical activity so far this year - and it's water temperatures remain above normal. Now, this is not to say storms cannot form or cannot be supported along track that we've seen from storms like Earl and Igor. It's just becoming less likely, and the pattern for development is going to favor the area of warmest waters - which is now the Caribbean.
Not only is this becoming a historically favorable pattern, but the models are really beginning to jump on board with this idea of development within the Caribbean over the next two weeks into the month of October.
We're about to see a large ridge of high pressure build in across the East and connect with the subtropical ridge across the Atlantic, the feature that plays such a large role in steering storms. You can think of this ridge as a 'cap' - essentially shutting down development in this area. To balance out such a large area of high pressure across the basin, there need to be relatively lower pressures elsewhere. The place to do it? The Caribbean Sea. That's just what the GFS computer model is showing below over the next several days.
Looking at the map not only do you see the massive ridge (indicated by the oranges and reds) setting up across the Atlantic and the East, but you also see the much lower pressures (indicated by the greens and the blues) setting up over the Caribbean Sea. This is a trend echoed in several of the other models we look at to make up our forecasts as well. Not only are lower pressures forecasts, but many storms are being spun up on model runs as well.
The problem into the future for the US, is once this ridge lifts out -- it allows the storms a place to go, and the most logical place is to the north and west. This could spell trouble for the Gulf Coast states, and even into the Southeast in our backyard heading into the month of October. Looking ahead several weeks out is not helpful to find an accurate forecast, but it can often be helpful to find an accurate trend - and I think that's what we're seeing here.
I won't post the images here for fear of spreading mis-information, but the GFS and others have not one, but several landfalling hurricanes coming north into the Southeast ones we head into the month of October. Even though we've already had a "September to Remember", it is definitely too early to let your guard down as October could be another month that will keep us on our toes.
That's all for now.
By: Tim Buckley