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Hurricane Season 2010: System 91S (Southern Indian Ocean)
12.17.10
 
December 17, 2010

NASA Satellite Tracks Soaking System 91S in Western Australia

AIRS showed System 91S on Dec. 17 at 0547 UTC inland over in Western Australia and dumping heavy rainfall. › View larger image
NASA's AIRS infrared instrument showed System 91S on Dec. 17 at 0547 UTC (12:47 a.m. EST) inland over in Western Australia and dumping heavy rainfall. Powerful thunderstorms with heavy rainfall appear in purple.
Credit: NASA/JPL, Ed Olsen
System 91s making landfall on Dec. 16 (left) and moving inland later (right). › View larger image
NASA's AIRS showed System 91S's center making landfall on Dec. 16 at 0647 UTC (1:47 a.m. EST) near the town of Carnavon in Western Australia (left). A large area of strong thunderstorms surrounded the center (purple) and cloud tops were as cold as or colder than -63F. Accompanying those thunderstorms was heavy rainfall. Aqua again passed over System 91S at 17:41 UTC (12:41 p.m. EST) that day and System 91S's center had already made landfall and was moving inland.
Credit: NASA/JPL, Ed Olsen
NASA's Aqua satellite captured a series of images from its Atmospheric Infrared Sounder (AIRS) instrument over the last two days and saw the low pressure area known as System 91S make landfall in Australia. System 91S may not have become a tropical depression, but it's dropping heavy rainfall in Western Australia.

NASA's AIRS infrared instrument showed System 91S's center making landfall on Dec. 16 at 0647 UTC (1:47 a.m. EST) near the town of Carnavon in Western Australia. At that time, there was a large area of strong thunderstorms around the center of circulation, and the cloud tops appeared on the infrared imagery to be as cold as or colder than -63 degrees Fahrenheit. Accompanying those thunderstorms was heavy rainfall. When NASA's Aqua satellite passed over Western Australia again at 17:41 UTC (12:41 p.m. EST) on Dec. 16, the center of System 91S had already made landfall and was moving inland. It still contained a large area of strong thunderstorms and heavy rainfall, although most were in the western and southern side of the low.

On Friday, December 17 (Eastern Time) local radar in Western Australia showed that the remnants of the low were bringing rainfall to inland areas of the region. At 11:05 a.m. EST (12:05 a.m. on Saturday, December 18 WST local time) rain was falling in an oval shaped-area over the towns of Meekatharra, Peak Hill, Reedy, Wiluna and Cue along route 95, also known as the Great Northern Highway. North of those towns, rain was also falling in Collier Range National Park.

At 12:05 a.m. on Saturday, December 18 WST local time, the Meekatharra Airport also reported a temperature of 63 degrees Fahrenheit with winds from the east-southeast at 20 mph, and a minimum central pressure of 29.60 inches.

The Australian Government Bureau of Meteorology (AGBM) in Western Australia issued a severe weather warning about the remnants of System 91S. From 9am Thursday (local time) to 9 a.m. local time Friday, Carnarvon Airport reported 8.01 inches (205mm) and Shark Bay Airport 4.2 inches (107 mm) of rain.

The AGBM noted that System 91S's center was near 23 South latitude and 112 East longitude at 7 p.m. WST (6 a.m. EST), which is about 125 miles (200 kilometers) southwest of Exmouth. Exmouth is a town on the tip of the North West Cape in Western Australia.

At that time, heavy rain was occurring in a band southeast of the low's center, and AGBM forecasters expect significant rainfall over parts of the Gascoyne, extending east into the northern Goldfields and south to far northern parts of the Central West during the overnight hours (local time). That heavy rainfall is likely to cause flash flooding and rising streams. As a result, a Flood Watch was issued for streams between Carnarvon and Kalbarri, including the Murchison River Catchment.

System 91S's remnants were moving slowly south-southeast and are expected to move toward the southwest this weekend.

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



December 16, 2010

Stronger Wind Shear Spells Trouble for System 91S

This AIRS infrared image shows a large area of strong convection around the center of the system's circulation. › View larger image
When NASA's Aqua satellite passed over System 91S on Dec. 15 at 06:05 UTC (1:05 a.m. EST), 2010, the AIRS instrument infrared image showed a large area of strong convection around the center of the system's circulation. The cloud tops were as cold or colder than -63 Fahrenheit (purple), indicating high, strong thunderstorms.
Credit: NASA/JPL, Ed Olsen
Wind shear is a bad thing when it comes to keeping tropical cyclones or low pressure areas together, and that's what System 91S is dealing with, and its stronger than it was yesterday. Satellite data shows that the strongest thunderstorms have been blown off center because of wind shear.

Yesterday, the wind shear affecting System 91S wasn't strong enough to weaken it. In fact, satellite data showed some strong convection and the low appeared to be getting organized. Twenty-four hours later System 91S has moved into an area of stronger wind shear and it's tearing the storm apart. The wind shear is so strong that infrared satellite data shows it has pushed that deep convection (strongest thunderstorms) about 70 to 80 nautical miles away from the center of circulation!

Today, Dec. 16, the center of System 91S was located about 235 nautical miles west-northwest of Learmonth, Australia near 19.0 South and 109.5 East. As the system continues to move southward, it's moving into even stronger vertical wind shear. That spells trouble for System 91S and gives a bleak or poor outlook for the system to develop into a tropical depression.

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



December 15, 2010

NASA Sees System 91S Ripe for Development Northwest of Western Australia

This AIRS infrared image shows a large area of strong convection around the center of the system's circulation. › View larger image
When NASA's Aqua satellite passed over System 91S on Dec. 15 at 06:05 UTC (1:05 a.m. EST), 2010, the AIRS instrument infrared image showed a large area of strong convection around the center of the system's circulation. The cloud tops were as cold or colder than -63 Fahrenheit (purple), indicating high, strong thunderstorms.
Credit: NASA/JPL, Ed Olsen
This visible AIRS image clearly shows the circulation within 91S. › View larger image

NASA's Aqua satellite flew over System 91S on Dec. 15 at 06:05 UTC and the AIRS instrument captured this visible image. The image clearly shows the circulation within the storm. Australia can be seen to the bottom right in the image.
Credit: NASA/JPL, Ed Olsen
A low pressure system designated System 91S is spinning in the Southern Indian Ocean, and NASA's Aqua satellite captured infrared and visible images of it earlier today. Satellite imagery indicates that the system is getting organized and may soon develop into a tropical depression.

At 0600 UTC (1 a.m. EST) on December 15, System 91S was located about 310 miles northwest of Learmonth, Australia near 18.1 South latitude and 110.6 East longitude. Learmonth is located in the extreme western coast of Australia.

When NASA's Aqua satellite passed over System 91S on Dec. 15 at 06:05 UTC (1:05 a.m. EST) the Atmospheric Infrared Sounder (AIRS) Instrument captured an infrared image that showed a large area of strong convection around the center of the system's circulation. The cloud tops were as cold as or colder than -63 Fahrenheit, indicating high, strong thunderstorms. The circulation of the low was clearly evident in the visible image from the AIRS instrument.

The strongest surface winds appear to be on the northeastern side of the storm between 25-30 knots (28-34 mph or 46-55 km/hr) where the strong convection is occurring. Minimum estimated pressure is 1000 millibars. System 91S is moving southwest near 5 mph.

Because System 91S continues to intensify and organize it has a good potential for developing into a tropical depression in the next 24 hours.

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