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July 13, 2010 / Dave Gorham

Quiet Mid July . . . Enjoy It While It Lasts

ImpactWeather Lead Hurricane Forecaster Chris Hebert guest-blogs this afternoon.

After a rather active period covering the last week of June and the first week of July, the tropics have certainly quieted down this week.    A significant factor responsible for the lack of activity in the past week is the current outbreak of dry, dusty Saharan air.  The Azores-Bermuda High strengthened temporarily about a week ago, driving a large mass of dry air off the west coast of Africa.  This dry air can be seen in the image below.  The dry air is represented by the yellow and orange colors on the image.

Image: University of Wisconsin CIMSS

But it looks as though this dry air intrusion into the tropics will be short-lived.   Computer model guidance indicates that the Azores-Bermuda High will continue to weaken and shift eastward over the coming week.  This should reduce the amount of dry dusty air being driven off the west coast of Africa.   In addition, several very large tropical waves with plenty of moisture are poised to slip off the west coast of Africa over next 3-5 days.  Such bursts of dry Saharan air are not uncommon in June and July.  It’s one reason why most seasons never really get started until the first week of August, as can be seen in the image below.

Image: ImpactWeather, Inc.

When looking at the frequency of Atlantic Basin tropical cyclones over the past 100 years, one can see that there is typically a sharp ramp-up in the number of storms right at the end of July into early August.   Around that time, the last remnants of moderate wind shear and dry Saharan air depart the deep tropics, leaving a very favorable environment for tropical cyclone development.  That’s what we’re expecting to happen in a few weeks.  However, some models are suggesting that development could occur even sooner.

Over the past few days, several models have been indicating that something might flare up in the western Caribbean by late this coming weekend or early next week.  Now it’s not uncommon for the models to predict such development on a regular basis during the hurricane season.  Fortunately, the models are in error in predicting such development much of the time.  But with the Saharan air thinning out by this weekend, and with a moderate tropical wave reaching the western Caribbean around that time, it’s an area that we’ll be keeping a close eye on.

As for the rest of the season, there is one thing that’s bothering me today.  June and the first two weeks of July have been unseasonably warm across the northeastern U.S.    This indicates that the high pressure we were expecting to remain over the central U.S. may be taking up residence a little farther to the east this summer.  If such a pattern continues, then that may increase the risk of a major hurricane landfall farther west along the northern Gulf Coast – all the way to Texas.  Previously, our outlook had indicated the highest risk from southeast Louisiana east through Florida.  We may need to indicate a significantly increased risk of a landfalling hurricane farther west to Texas on our August update.  Stay tuned…

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