NASA's Aqua satellite passed over a developing tropical low pressure area in the Northwestern Pacific Ocean on September 21 when the latest depression was coming together. The AIRS instrument aboard Aqua used infrared light to determine temperatures of cloud tops and where the strongest storms were located.
At NASA's Jet Propulsion Laboratory in Pasadena, California, data from the Atmospheric Infrared Sounder or AIRS instrument aboard Aqua was used to create a false-colored infrared image of developing Tropical Depression 21W. The data was taken on September 21 at 16:17 UTC (12:17 p.m. EDT) and showed cold cloud top temperatures as cold as or colder than -63F/-53C circling the consolidating center of circulation. Cloud top temperatures that cold have been shown to generate heavy rainfall.
By September 21 at 2100 UTC (5 p.m. EDT the low pressure system organized enough to be classified as a tropical depression.
At 1500 UTC (11 a.m. EDT) on September 22, Tropical Depression 21W (TD21W) had maximum sustained winds near 30 knots (34.5 mph/55.5 kph). It was centered near 17.8 North latitude and 139.2 East longitude, about 845 nautical miles (972 miles/1,565 miles) southeast of Kadena Air Base, Iwo To, Japan.TD21W was moving to the northwest at 11 knots (12.6 mph/20.3 kph) and is expected to turn to the north-northwest. This depression is currently no threat to land areas.
The Joint Typhoon Warning Center expects 21W to strengthen into a tropical storm and to typhoon-strength by September 24 on its journey northward.