Although the 2006 hurricane season has now seen the formation of three Atlantic hurricanes in the month of September, the latest being Hurricane Helene, all of them have recurved well before approaching the US East Coast. With high pressure anchored over the Azores in the eastern Atlanic and generally troughy conditions in the western Atlantic, storms that form in the Central Atlantic are quickly drawn northward into the prevailing westerlies and forced back out into the Atlantic. This was the pattern for Hurricanes Florence and Gordon and now too for Hurricane Helene.

The following series of images from the Tropical Rainfall Measuring Mission (or TRMM) satellite provides a unique look at the life cycle of Hurricane Helene in the Atlantic.

The eigth tropical depression of the season (TD #8) formed on the 12th of September 2006 (local time) just south of the Cape Verde Islands from an African easterly wave that had emerged off the coast of Africa. The first image shows TD #8 south of the Cape Verde Islands. The image was taken at 2359 UTC (7:59 pm EDT) 12 September 2006. It shows the instantaneous rain rates associated with areas of convection within the depression. Rain rates in the center swath are from the TRMM Precipitation Radar (PR), while those 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 that is still in the very early stages of development. No eye is present, there is little evidence of banding (curvature) in the rain field, and convection is limited as indicated by the relatively isolated areas of heavy (red) to moderate (green) rain.




The next image shows what is now Tropical Storm Helene two days later. Although there is still no eye visible and only weak banding features, the amount of convection within the system has increased significantly as indicated by the more widespread areas of heavy (shown in red) to moderate (in green) rain. Convection is responsible for releasing heat into the storm, which fuels it and drives its circulation. The image was taken at 2343 UTC (7:43 pm EDT) on September 14th.

As Helene tracked westward across the Atlantic, it continued to become better organized. This next image from TRMM shows Tropical Storm Helene at 1434 UTC (10:34 am EDT) on the 15th. A large, coherent rainband is now apparent wrapping around the center on the south and east. The curvature in this and other rainbands surrounding the center indicate that Helene now has a fairly well-developed, closed circulation.

On the 16th, Helene developed into a hurricane and took a more northerly motion as it passed through the Central Atlantic. With conditions continuing to remain favorable for development, Helene intensified, first into a Category 2 storm on the 17th and then into a Category 3 storm during the night of the 17th. The next image in the series was captured by TRMM at 2134 UTC (5:34 pm EDT) on the 18th of September as Category 3 Helene was moving northwest through the Central Atlantic. An eye is now readily visible in the IR cloud image (dark whole in the center of the white cloudy area). Surrounding the eye is a well-defined eyewall in the northern semicircle (dark red arc north of the center). There are several rainbands tightly wrapping around the center (green arcs). All of these features reveal that Helene is now a major hurricane with a strong circulation. At the time of this image, Helene's maximum sustained winds were estimated to be at 100 knots (115 mph) by the National Hurricane Center.

Helene then slowly began to weaken, becoming a Category 2 storm on the 19th. Helene maintained Category 2 intensity on the 20th but began to recurve around the western periphery of the subtropical ridge. As the storm moved northward on the 21st, it weakened further down to Category 1 intensity.

The final image shows Helene at 1614 UTC (12:14 pm EDT) on the 22nd of September as it is beginning to track off to the northeast through the Central Atlantic and move farther away from the US East Coast. The center is now cloud filled and appears open, devoid of an eyewall along the southern edge of the center. A mass of moderate (green) to heavy (red) rain wraps around north of the center and still exhibits good banding. However, as the system moves over cooler waters, the circulation will slowly begin to spin down in the absence of deep convection near the center. Helene is expected to accelerate off to the northeast into the north-central Atlantic and transition into an extra-tropical low.

Images produced by Hal Pierce (SSAI/NASA GSFC) and caption by Steve Lang (SSAI/NASA GSFC).