Today marks the end of the first week of August.   So far this season, we’ve seen only one relatively weak tropical depression form back on May 28th.    There were no significant tropical threats in the Atlantic Basin in June or July.  It’s not so unusual to reach the first week of August without a named storm.  That’s what I was saying back in late July of 2004.  Shortly afterward, the tropics sprang to life with 8 named storms in August and 4 in September. Four hurricanes, three of them major, hit Florida that year.  But that was 2004. The general atmospheric and oceanic conditions present in 2004 are not present today.  As of today, moderate to strong wind shear dominates much of the tropics, signaling a quieter August and September than 2004.

One of the factors which can result in increased shear across the deep tropics is El Niño.  Last May, we predicted that El Niño would arrive sometime during the month of June.  El Niño arrived on schedule, but we think that this year’s El Niño is going to be a weak to moderate event, unlike the very strong El Niños of 1997 or 1983.   But El Niño should be strong enough this month and next to result in increased wind shear in the deep tropics (south of 20N latitude), as indicated in the graphic below.

While El Niño typically results in increased upper-level wind shear, we’ve also seen increased lower-level shear, particularly in the Caribbean Sea, this summer.  The reason for this increased lower-level shear is a stronger Azores-Bermuda High in 2009.  Long-range models had been predicting that the Bermuda High would be stronger this summer, and they were right.  As an example, the graphic below is a plot of the projected winds at 850 millibars (5000 ft) across the Caribbean Sea for this weekend.

Any tropical wave reaching the eastern Caribbean would run into rapidly-accelerating winds in the lower levels, this low-level divergence (spreading apart of the air) causes the air above to sink, knocking out the thunderstorms and preventing tropical development.  However, we are seeing some signs that the Bermuda High may weaken a bit over the next week or two.  This could render the deep tropics at least a little less hostile toward development by mid month.Another factor that governs the potential intensity of a developing tropical cyclone more so than development chances is sea surface temperature (SST).

The image below represents the current (August 6th) SST anomaly for both the Eastern Pacific and the Atlantic Basins.  First of all, one can see the anomalously warm temperatures across the Tropical Pacific associated with the current El Niño.  Looking across the Tropical Atlantic, SSTs are cooler than normal south of 10N between Africa and South America.  This cooler water is the result of upwelling caused by stronger easterly trade winds south of the Bermuda High.  There is even a smaller pocket of cooler water across the central Caribbean.  Now, SSTs aren’t cool in these regions by any means, they’re just a little cooler than normal.  Theses SSTs may not support a large number of major hurricanes this season, but they’re warm enough to at least allow for tropical cyclone development.

While the SSTs in the deep tropics are a little cooler than normal this season, the same cannot be said for the Gulf of Mexico.  Let’s compare today’s Gulf of Mexico SSTs to the temperatures of 2005.   In 2005, the warm waters of the Gulf of Mexico fueled two Category 5 hurricanes.  How do the Gulf SSTs of 2009 compare?

Gulf of Mexico SSTs for 2009 (bottom image) are even warmer than in 2005. So there is plenty of heat available for tropical cyclone intensification in the Gulf of Mexico.

El Niño seasons generally have fewer named storms.  Combine an El Niño with a stronger-than-normal Bermuda High and we have a recipe for a relatively quiet remainder of the 2009 hurricane season.  And that’s what our August seasonal update reflects.  For the remainder of 2009, our forecast remains unchanged from June.  We’re predicting 10 named storms, 4 of them will become hurricanes and 2 of them major Category 3-4-5 hurricanes.  We think that at least one of the major hurricanes will have a Hurricane Severity
Index (HSI) of between 30-35.

In the graphic above, I’ve also included the August updates from Dr. Klotzbach and Dr. Gray of Colorado State University and Dr. Mark Saunders of Tropical Storm Risk in the U.K..   I also included the June forecast from the United Kingdom’s Meteorological office (UKMet). El Niño.  As you can see, Gray/Klotzbach have reduced their numbers slightly, down from 11/4/2 to 10/4/2, same as we’ve been saying.  In contrast, Dr. Saunders at TSR raised his numbers from 11/6/2 to 13/7/3 on August 4th.  Dr. Saunders cites warmer than expected SSTs and predicted weaker trade winds in the Tropical Atlantic for the increase.  While we agree that SSTs will be warmer than expected, we think that wind shear will continue to inhibit development across most of the Atlantic Basin in August and September.  The U.K. Meteorological office’s prediction of 6 named storms, 3 hurricanes and 1 major hurricane seems a bit low.

But such numbers are not uncommon in past El Niño seasons.Where will the storms track this season?  While we don’t think there’s an increased risk (over climatology) for a land falling hurricane in 2009, we can identify some regions that are a little more likely to be impacted with the current pattern.  One such area would be the islands of the northeast Caribbean.  We think that the central Caribbean should remain relatively free of storms this season, but storms may form a bit farther to the north east of the Caribbean, resulting in a track toward the northern Caribbean islands.  Along the U.S. Coast, the area more likely to see an impact would be the northeast Gulf Coast from Tampa to Mississippi.  In addition, the area from southeast Florida to the Carolinas may be impacted this year.  Finally, there’s a chance of an August impact in northeast Mexico or perhaps the lower Texas coast.

I’ll leave you with one cautionary note regarding El Niño seasons.  Although conditions across the deep tropics may be generally unfavorable for development during an El Niño season, the atmosphere north of the tropics will often be unaffected by El Niño.  This means that the Gulf of Mexico and the region near and east of the Bahamas may remain favorable for development.   It’s possible that a storm could spin up in the Gulf of Mexico rather than the Caribbean Sea, giving coastal residents less time to prepare.  Remember that Gulf SST chart above?  Should something get into the Gulf and develop, there may be a chance of rapid intensification.  So we’ll have to be wary of any close-in development this season.

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