Atlantic Hurricane Season Springs to Life: A Recap of the 2009 Season So Far

After a rather slow start, the 2009 Atlantic hurricane season has seen a sudden surge in activity with the rapid emergence of three named storms: Tropical Storms Ana and Claudette and Hurricane Bill, the first hurricane of the season. After the first tropical depression (TD) of the season, called Tropical Depression One (TD #1) formed back in late May, the Atlantic was quiet for two and a half months.

Since 2001, there have been nearly 5 named storms on average by the middle of August.

Sea surface temperatures have been slightly above normal over nearly all of the tropical North Atlantic since mid-July. A strong tropical wave that moved off of the west coast of Africa on Wednesday, August 12 was able to take advantage of the warm water and formed into a tropical depression (TD #3) on the morning of August 15 as it was moving westward about 700 miles west-southwest of the Cape Verde Islands. TD #2, which would later become Tropical Storm Ana, had also formed in this same general area four days prior. Storms that form in this region are known as "Cape Verde" storms. Cape Verde storms most often occur in August and September during the height of the season.

TD #3 was upgraded to a tropical storm and named Bill on the afternoon of August 15. Bill slowly intensified but remained at tropical storm intensity on the 16th as it made its way through the central Atlantic. On the morning of August 17, Bill continued to intensify and was upgraded to hurricane intensity.

The Tropical Rainfall Measuring Mission satellite (known as TRMM) was placed into service in November of 1997. From its low-earth orbit, TRMM has been providing valuable images and information on tropical cyclones around the Tropics using a combination of passive microwave and active radar sensors, including the first precipitation radar in space.

TRMM was able to capture an image of Bill at 11:33 UTC (7:33 am EDT) on August 17, 2009 just after Bill was upgraded to a hurricane. The image shows the horizontal pattern of rain intensity within the storm. Rain rates in the center swath are based on the TRMM Precipitation Radar (PR), and those in the outer swath on the TRMM Microwave Imager (TMI). The rain rates are overlaid on infrared (IR) data from the TRMM Visible Infrared Scanner (VIRS). TRMM reveals that although Bill does not yet have an eye or eyewall, it does have a well-developed cyclonic circulation as evidenced by the curvature in the surrounding rain bands (green and blue areas indicating moderate to light rain, respectively) spiraling in towards the center. Within these rain bands are embedded areas of intense rain (shown in red). At the time of this image, Bill's maximum sustained winds were estimated at 65 knots (75 mph) by the National Hurricane Center (NHC).

Bill is expected to continue to intensify and could become a major hurricane. It is forecast by the National Hurricane Center to recurve to the northwest and be in the vicinity of Bermuda by Saturday.

TRMM is a joint mission between NASA and the Japanese space agency JAXA.