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NASA Sees Tropical Cyclone Evan Batter and Drench Samoan Islands

NASA's Tropical Rainfall Measuring Mission or TRMM satellite continues to provide rainfall and cloud height data on powerful Cyclone Evan as it crawls through the Samoan Islands with hurricane-force winds and heavy rains. NASA's TRMM satellite identified "hot towers" in the storm, hinting that it would continue to intensify.

On Dec. 14, American Samoa, Tonga and Fiji are all under warnings or alerts as Evan continues to move west. A gale warning is in effect for Tutuila and Aunuu. A high surf warning is in effect for all of American Samoa. A flash flood watch is in effect for Tutuila and Manua. A tropical cyclone alert is in force for Niuafo'ou and Fiji.

The TRMM satellite had an excellent view of tropical cyclone Evan on Dec. 12, 2012 at 1704 UTC when it was battering the Samoan Islands with hurricane force winds. Evan is predicted by the Joint Typhoon Warning Center (JTWC) to intensify and have winds of 130 knots (~150 mph) while remaining close to the islands. This wind speed would make it a strong Category 3 storm on the Saffir-Simpson Scale. A storm surge of 4.5 meters (14 feet) was already reported along the Samoan coast.

Evan's rainfall was analyzed using TRMM's Microwave Imager (TMI) and Precipitation Radar (PR) data. This analysis showed that the heaviest rainfall of over 80 mm (~3.1 inches) per hour was occurring in heavy rainfall within Evan's clear eye wall. Strong bands of thunderstorms were seen wrapping into the low level center of circulation.

TRMM's Precipitation Radar (PR) data sliced through Evan and were used to provide the 3-D cut-a-way view looking at Evan's northern side. The imagery clearly showed the vertical side surface of Evan's well-defined eye.

TRMM data revealed several "hot towers" or towering thunderstorms reaching heights of greater than 16.5 km (10.25 miles) within Evan's eye wall. A "hot tower" is a tall cumulonimbus cloud that reaches at least to the top of the troposphere, the lowest layer of the atmosphere which extends approximately nine miles (14.5 km) high in the tropics.

These towers are called "hot" because they rise to such altitude due to the large amount of latent heat. Water vapor releases this latent heat as it condenses into liquid. NASA research shows that a tropical cyclone with a hot tower in its eyewall was twice as likely to intensify within six or more hours, than a cyclone that lacked a hot tower.

On Dec. 14 at 1500 UTC (10 a.m. EST) Cyclone Evan had maximum sustained winds near 100 knots (115 mph/185 kph). Cyclone-force winds extend 35 nautical miles (40 miles/64.8 km) out from the center, while tropical-storm-force winds extend up to 105 miles (120.8 miles/194.5 km) from the center.

Evan was centered about 135 nautical miles (155.4 miles/250 km) northwest of Pago Pago, American Samoa, near 12.9 south latitude and 172.5 west longitude. Evan was moving slowly west at 5 knots. Evan is creating very rough seas with waves up to 32 feet (9.7 meters) high. Evan is a threat to American Samoa, Tonga and Fiji.

Evan is moving west away from American Samoa and will later turn southwest, away from American Samoa and is expected to continue to intensify as it moves just north-northwest of Fiji through Dec. 19.

For more information about the regional warnings, visit:

American Samoa: A gale warning is in force for Tutuila and Aunuu.
Tonga: A tropical cyclone alert is in force for Niuafo'ou.
NASA's TRMM satellite passed above an intensifying Tropical Cyclone Evan on Dec. 11 at 1759 UTC (12:59 p.m. EST/U.S.) and saw the tallest thunderstorms around Evan's center of circulation reached 16.5 km (10.25 miles) while other storms tops nearby were measured at 14.75 km (9.17 miles).

Credit: NASA/SSAI, Hal Pierce NASA Sees Intensifying Tropical Cyclone Moving Over Samoan Islands

NASA satellites have been monitoring Tropical Cyclone Evan and providing data to forecasters who expected the storm to intensify. On Dec. 13, Evan had grown from a tropical storm into a cyclone as NASA satellites observed cloud formation, height and temperature, and rainfall rates.

The Tropical Rainfall Measuring Mission (TRMM) satellite passed above intensifying tropical storm Evan in the South Pacific Ocean on Dec. 11, 2012 at 1759 UTC (12:59 p.m. EST/U.S.). An analysis of Evan's rainfall from TRMM's Precipitation Radar (PR) and Microwave Imager (TMI) showed that Evan already had an eye-like structure at the time of that TRMM orbit. Evan would later develop an eye on Dec. 13.

TRMM's 3-D Precipitation Radar (PR) data captured on Dec. 11 were used to measure the heights of Evan's storm tops. It found that the tallest thunderstorms shown around Evan's center of circulation reached 16.5 km (10.25 miles) indicating powerful storms and heavy rainmakers. Other thunderstorm cloud tops nearby were measured at 14.75 km (9.17 miles).

NASA's Aqua satellite passed over Tropical Cyclone Evan after it had attained cyclone status on Dec. 13 and two instruments provided insight into what was happening with the storm.

The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard NASA's Aqua satellite captured a visible image of Tropical Cyclone Evan when it was directly over the Samoa Islands on Dec. 13 at 0105 UTC. Evan's maximum sustained winds had increased to 90 knots (103 mph/166.7 kph).

The other instrument aboard Aqua that captured data from Evan was the Atmospheric Infrared Sounder (AIRS) instrument. AIRS captured an infrared image of Tropical Cyclone Evan at 0059 UTC. The infrared image showed a compact, circular area of strong thunderstorms around Evan's center that reached high into the troposphere where temperatures are as cold as -63 Fahrenheit (-52 Celsius). Those areas also indicated heavy rainfall. Infrared imagery also showed that Evan's eye was about 6 nautical miles wide. Imagery also showed tightly-curved deep convective (rising air that creates the storms that make up the cyclone) banding of thunderstorms were wrapping into the center.

By 1500 UTC (10 a.m. EST) on Dec. 13, Evan's maximum sustained winds had increased to 90 knots (103 mph/166.7 kph). Evan was centered just 65 nautical miles (74.8 miles/120.4 km) west-northwest of Pago Pago, American Samoa, near 13.7 south latitude and 171.7 west longitude. Evan was crawling to the northwest at 2 knots (2.3 mph/3.7 kph).

Evan is expected to track to the west and continue strengthening over the next couple of days.

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

Rob Gutro | EurekAlert!
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