Following the pattern established by Florence, Gordon and Helene, yet another Atlantic storm, Isaac, has formed in the Central Atlantic, intensified into a hurricane, and recurved well before approaching the East Coast. The 2006 Atlantic hurricane season has continued to be a welcome respite from last year's onslaught with not a single hurricane yet to make landfall in the US. So far, including Isaac, there have been 9 named storms, 5 hurricanes, and 2 major hurricanes in the Atlantic--numbers very typical of an average season.

Armed with its array of passive and active sensors, the Tropical Rainfall Measuring Mission satellite, known as "TRMM", has proven itself to be a valuable platform for monitoring Atlantic hurricanes. This series of images was obtained by TRMM and provides a unique look at Isaac in the Atlantic.

Tropical depression #9 formed on the 27th of September 2006 in the middle of the Atlantic well away from any land areas. As with the preceding depressions, TD #9 formed from an African easterly wave that was propagating westward across the Atlantic. The first image shows TD #9 as it was moving northwest through the Central Atlantic about 680 miles east-southeast of Bermuda. The image was taken at 10:40 UTC (6:40 am EDT) on the 28th of September 2006 and shows a top-down-view of the rain intensity within TD #9. Rain rates in the center of the swath are from the TRMM Precipitation Radar (PR), a one-of-a-kind space-borne precipitation radar. Rain rates in the outer swath are from the TRMM Microwave Imager (TMI). The rain rates are overlaid on infrared (IR) data from the TRMM Visible Infrared Scanner (VIRS). TRMM reveals a system in the early stages of development. TD #9 does not have an eye or eyewall, deep convection is lacking, and the system is very compact. There is, however, a coherent area of light (blue) to moderate (green) rain north and west of the center with some evidence of banding (curvature), which indicates the presence of a low pressure circulation. At the time of this image, TD #9 had sustained winds estimated at 30 knots (35 mph) by the National Hurricane Center (NHC). A few hours after this image was taken, TD #9 was upgraded to a minimal tropical storm and given the name Isaac.

	See a 3-D FLYBY QUICKTIME animation of the image above.

Initially, Isaac changed little in intensity, being hampered by moderate wind shear, dry air and cool waters left in the wake of Hurricanes Gordon and Helene. The storm also had some subtropical characteristics. On the afternoon of the 29th, however, conditions for development became more favorable as the wind shear relaxed and the storm began to pass over warmer water. Isaac began to strengthen. By the morning of the 30th, Isaac had become a strong tropical storm. This next image on the left above from TRMM shows Tropical Storm Isaac at 10:24 UTC (6:24 am EDT) on the 30th. Although the storm is still quite small and has an asymmetric structure (note the bulk of the rain is east of the center), Isaac now has a ragged eye. Furthermore, a prominent area of intense convection is present just to the east of the center as evidenced by the area of intense rainfall (the darker red area).

The image on the right above, taken concurrently with the previous image, provides a unique 3D perspective of Isaac courtesy of the TRMM PR. The view is to the north. TRMM reveals that the areas of intense rain visible in the previous image are associated with deep convective towers with tops reaching up to 15 km (shown in red). Such towers can be an indication of future intensification especially when they are located near the core of the storm as is the case here with Isaac. This area of deep convection is responsible for releasing heat into the storm. This heating, known as latent heating, is what lowers the storm's central pressure and drives the storm's circulation. At the time of these last two images, NHC estimated Isaac's sustained winds to be 60 knots (69 mph). Isaac became a hurricane soon afterward.




As Isaac moved closer to Bermuda on the night of the 30th, it began to feel the effects of a deep-layer trough and took a turn towards the north, keeping it well to the east of Bermuda. The final image shows Isaac at 12:44 UTC (8:44 am EDT) on the 1st of October as it was passing east-northeast of Bermuda. The eye is now less distinct with the southwestern portion now exposed as a result of increasing southwesterly wind shear. Isaac remains a small storm.

Isaac maintained Category 1 hurricane intensity throughout the evening of the 1st and the morning of the 2nd. By the afternoon of October 2nd, Isaac had weakened to a tropical storm and was moving quickly to the north-northeast with the center passing just southeast of Newfoundland.

TRMM is a joint mission between NASA and the Japanese space agency JAXA. Images produced by Hal Pierce (SSAI/NASA GSFC) and caption by Steve Lang (SSAI/NASA GSFC).