Epsilon formed into a tropical storm on the 29th of November 2005 from a midlatitude low pressure center in the central Atlantic well east of Bermuda. Over the next couple of days, the storm, which was initially moving westward, made a counterclockwise loop ahead of an advancing deep-layer trough. During this time, Epsilon remained a tropical storm. Around midday (local time) on the 2nd of December as it was being steered northeastward ahead of the trough, Epsilon strengthened into a minimal hurricane with maximum sustained winds estimated at 65 knots (75 mph) by the National Hurricane Center (NHC), becoming the record 14th hurricane of the season. The old record of 12 hurricanes was set back in 1969.

In November of 1997, the Tropical Rainfall Measuring Mission (TRMM) satellite was launched with the primary mission of measuring rainfall over the global Tropics. However, TRMM with its array of passive and active sensors has turned out to be a valuable platform for monitoring and studying tropical cyclones (e.g., tropical storms and hurricanes), especially over remote parts of the open ocean as is the case with Epsilon in the central Atlantic.

The first image was taken by TRMM at 19:30 UTC (2:30 pm EDT) on 4 December 2005 and shows a top-down view of the horizontal rain intensity associated with Epsilon as the system was moving east across the central Atlantic. Rain rates in the central part of the swath are from the TRMM Precipitation Radar (PR), the only radar that can measure rainfall from space. 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 shows that Epsilon has a large closed eye (clear center surrounded by the ring of moderate [green] to light [blue] rain). This feature along with the excellent banding in the surrounding rain field (blue and green arcs) indicates that Epsilon has a well-developed circulation. The next image provides a 3D perspective of Epsilon, which was contructed using information on the depth of the precipitation particles from the TRMM PR, at the same time. This image shows a healthy eyewall (green annulus) structure, appearing solid and uniform. At the time of these images, Epsilon was a Category 1 hurricane with sustained winds estimated at 75 knots (86 mph) by NHC.

For the next two days, Epsilon maintained hurricane strength despite a potentially unfavorable environment as it continued to track eastward across the Atlantic. The final set of images were taken at 16:01 UTC (11:01 am EDT) on December 6th. The top-down view still shows a fairly well-defined eye, but the eye is becoming elongated (more oval) and the eyewall is eroded on the southwest side. These indicate that wind shear and dry air are starting to take a toll on Epsilon. This is even more evident in the final 3D perspective image, which shows that the southwestern eyewall is nearly gone as dry air penetrates close to the center. At the time of these last images, Epsilon was a minimal hurricane with sustained winds of 65 knots (75 mph). The system was moving south due to strengthening high pressure north and west of the hurricane. Epsilon is expected to continue southwestward and weaken due to increasing wind shear.

TRMM is a joint mission between NASA and the Japan Aerospace Exploration Agency (JAXA).

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