17P is now a fading depression

On February 22 at 6 p.m. ET (21 UTC), the Joint Typhoon Warning Center issued their final advisory on 17P. At that time, its maximum sustained winds were near 34 mph (30 knots) and it continued to weaken. It was still 615 nautical miles east-northeast of the island of Pago Pago, near 12.2 degrees South latitude and 160.8 West longitude.

17P isn't close enough to impact the island of Pago Pago and doesn't appear that it is ever going to get that close. Although the forecast for Pago Pago through the week calls for scattered showers with temperatures in the mid-80s (Fahrenheit), those are convective or pop-up thunderstorms created from daytime heating, and are not associated with Tropical Depression 17P.

NASA's Aqua satellite flew over 17P as it continued to become more disorganized today, February 23 at 1158 UTC (6:58 a.m. ET). The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard Aqua captured in infrared image of the storm. In the satellite image, it was difficult to find a center of the storm.

Animated infrared satellite imagery, such as that from another instrument on Aqua called the Atmospheric Infrared Sounder, or AIRS noticed last night, that the low level circulation of the storm had been separated or “decoupled” from most of the showers and thunderstorms. That's an indication that the storm is becoming less organized and weakening.

Tropical Depression 17P is now in an area of moderate to strong westerly vertical wind shear and that's bad news for any tropical cyclone, because wind shear can tear those storms apart. Wind shear means that the speed or direction of wind changes over a relatively short period of time, or a short distance.

Tropical cyclones develop vertically as rapidly rising air creates thunderstorms. Whenever there's a higher wind shear, the storm is spread over a larger area, and that limits the storm's ability to produce those thunderstorms.

That wind shear is caused by an upper level low pressure area to the southwest of 17P's center. As 17P continues moving south-southwest, atmospheric conditions are going help weaken the storm even more. There's always a chance 17P may redevelop so forecasters will continue watching it, even though the final bulletin has been issued on the storm.

More information about Tropical Cyclone 17P: http://www.nasa.gov/mission_pages/hurricanes/archives/2010/h2010_17P.html