A Slow Start to the NATL TC Season

Though the 2007 hurricane season in the North Atlantic officially began on June 1, the Atlantic basin has remained unusually silent in terms of tropical cyclones. This is welcome news for residents along the gulf coast still struggling to recover from powerful storms like Katrina and Rita that devastated coastal regions in 2005. So far during the 2007 season the Atlantic has seen three named tropical systems, none of which has reached hurricane strength.

The 2007 season initially looked like it would be quite active, with Subtropical Storm Andrea forming early in May off the eastern coast of Florida, well before the official start of the hurricane season. The official opening of the season was punctuated by Tropical Storm Barry, which formed in the Gulf of Mexico on June 1, later making landfall in the Tampa Bay region as a tropical depression. After this initial burst of activity, the Atlantic basin fell silent for nearly two months before Tropical Storm Chantal developed southeast of Nova Scotia on July 30. According to the NHC, it is unlikely to affect land and will quickly transition to an extratropical system.

So, why has there been such a lack of tropical activity in the Atlantic this season? It's likely a combination of several things--the formation and behavior of tropical cyclones are governed by many factors. Most tropical cyclones begin their lives as weak disturbances known as tropical waves. For a tropical wave to grow into a hurricane, many needs must be met. The ocean, the source of energy for hurricanes, must be deep and warm (at least 26 C) beneath the system. The atmosphere must be relatively free of vertical wind shear, which would rip the developing storm apart, and must be moist enough to support thunderstorms. Even then, the system cannot be too near the equator where the Coriolis effect is too weak to impart rotation.

Once a tropical cyclone forms, it is largely at the mercy of synoptic-scale weather patterns--the high and low pressure systems that affect our day to day weather. Most tropical cyclones are steered westward across the Atlantic by the Azores High, a semi-permanent high pressure system usually centered in the summer near the Azores islands. Once a hurricane encounters a trough or passes around the west side of the Azores High, it can then begin to recurve to the north, and eventually back to the northeast, either making landfall on the south or east coast of North America or traveling back out to sea to dissipate in the cold waters of the North Atlantic.

What can we expect for the rest of 2007? The hurricane season is far from over, lasting until November 31. Historically, peak activity occurs within the month of September, and storms have been known to form even through the end of December. NOAA's official forecast, released on May 22, 2007 predicts 13 to 17 named storms, with 7 to 10 becoming hurricanes, and 3 to 5 becoming major hurricanes (category 3 or higher on the Saffir-Simpson scale). So, though 2007 has been a quiet year thus far in the Atlantic, the action is likely far from over.

If you'd like to learn more about tropical cyclones, we recommend browsing the National Hurricane Center website.



Story is © Nate Snook, 2007