Hurricane Season 2011: Tropical Depression 91S (South Pacific)

NASA's TRMM Satellite Sees Tropical Depression 91S Soaking Northern Australian Coastline

Australia's Northern Territory has been experiencing rainfall and winds from the low pressure system called "System 91S" for several days this week. Today, NASA's Tropical Rainfall Measuring Mission satellite spotted light to moderate rainfall in the system as continues tracking southwest and bringing rains and winds to the northern coast of Western Australia this weekend.

System 91S was located in the Timor Sea, west-southwest of Darwin, Australia (Northern Territory) and was moving in a west-southwesterly direction. System 91S is forecast to continue traveling in that direction and its center is expected to remain at sea over the next several days as it heads toward the Southern Indian Ocean.

As it continues to track along coastal areas of the Northern Territory and Western Australia, cyclone warnings are in effect. A Cyclone Warning is in effect for Western Australian coastal and island areas from Kuri Bay to the Western Australia/Northern Territory border, including Kalumburu and Wyndham. In addition, a cyclone watch is in effect from Kuri Bay to Cape Leveque, not including Derby.

The TRMM satellite, operated by both NASA and the Japanese Space Agency, passed over the system at 1451 UTC on April 1. The U.S. Navy and Naval Research Laboratory's Monterey Marine Meteorology Division, Calif. overlaid TRMM rainfall rate imagery on top of Japan's METSAT-2 infrared imagery to provide a complete picture of the low pressure areas cloud extent and rainfall rates. TRMM's precipitation radar instrument measured rainfall rates close to 1 inch (25 mm) per hour.

Surface winds on April 1 are estimated between 25 and 30 knots (29 to 34 mph / 46 to 55 kmh). Satellite imagery indicated that the center of Depression 91S' circulation was located near 13.2 South latitude and 128.9 East longitude. It is moving in a southwesterly direction near 4 knots (5 mph/7 kmh).

Infrared satellite imagery shows that the low-level center of circulation appears to be consolidating, and there is improved banding of thunderstorms wrapping into the center of the storm. Both of those factors hint at a storm organizing and strengthening. The only challenge in the forecast is that there is a moderate wind shear battering the storm which is inhibiting further strengthening into a tropical storm today. That vertical wind shear is expected to decrease as System 91S tracks along the northern coast this weekend.

There is a good chance that System 91S could become a tropical storm over the weekend. Regardless, residents of the northern coastal areas of Western Australia should expect some heavy rainfall, gusty winds and rough surf at the beaches this weekend.

Text Credit: Rob Gutro, NASA's Goddard Space Flight Center, Greenbelt, MD