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Tropical Storm Evan (South Pacific Ocean)
12.20.12
 
GOES image of Evan› Larger image
This infrared image from NOAA's GOES-15 satellite on Dec. 20 at 1500 UTC (10 a.m. EST) shows the remnants of former Tropical Cyclone Evan blown apart by very strong wind shear, northeast of New Zealand (lower left). The low pressure center is northwest of all of Evan's associated remnant clouds. Credit: NASA GOES Project
NASA Sees Cyclone Evan Blown Apart by Wind Shear

Cyclone Evan is no more than a remnant low pressure area in the South Pacific Ocean now. NOAA's GOES-15 satellite captured an image of the remnants from its fixed orbit in space on Dec. 20 that showed strong wind shear had basically blown the storm apart.

The last official bulletin by the Joint Typhoon Warning Center was issued on Dec. 19 at 2100 UTC (4 p.m. EST/U.S. or 12:56 a.m. Fiji local time on Dec. 20). At that time, Evan's maximum sustained winds were still near 35 knots (40 mph/64.8 kph) and it had transitioned into an extra-tropical storm. It was located 400 nautical miles south of Nadi, Fiji, near 24.3 south latitude and 178.5 east longitude. Evan was moving to the south-southeast at 4 knots (4.6 mph/7.4 kph). Evan has since become a remnant low pressure system.

NOAA's GOES-15 satellite captured an infrared image of Evan's remnants on Dec. 20 at 1500 UTC (10 a.m. EST) showed the remnants of former Tropical Cyclone Evan blown apart by very strong wind shear, northeast of New Zealand. The low pressure center appears northwest of the plume of clouds associated Evan's remnants. The image was created by NASA's GOES Project, located at NASA's Goddard Space Flight Center in Greenbelt, Md.

Northwesterly wind shear continued to be very strong, blowing between 40 and 50 knots (46 and 57.5 mph/74 and 92.6 kph) and cooler ocean waters were weakening Evan quickly.

Evan has now gone into the history books as one of the strongest cyclones to hit Fiji and American Samoa in recent memory.

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




Update Two:

AIRS image of Evan› Larger image
This time series of infrared images from the AIRS instrument aboard NASA's Aqua satellite show the changes in intense thunderstorms (purple) within Cyclone Evan as it weakened from Cyclone to Tropical Storm strength in 36 hours. The left image is from Dec. 18, center from Dec. 19 at 0159 UTC, and right from Dec. 19 at 1259 UTC, showing a smaller area of intense storms, and the cyclone appears more disorganized. Credit: NASA JPL, Ed Olsen
Time Series of Infrared NASA Images Show Cyclone Evan's Decline

Cyclone Evan is now far south of Fiji and wind shear and cooler sea surface temperatures have been taking their toll on the storm and weakening it. Infrared data from NASA's Aqua satellite has shown a quick decline in the storm's structure over one day.

A time series of infrared images from the Atmospheric Infrared Sounder (AIRS) instrument that flies aboard NASA's Aqua satellite showed changes in intense thunderstorms within Cyclone Evan between Dec. 18 and Dec. 19. Over a time period of 36 hours, Evan weakened from Cyclone strength to Tropical Storm strength. In an AIRS image captured on Dec. 18 there were two large areas of strong thunderstorms with very cold cloud top temperatures colder than -63 Fahrenheit (-52 Celsius).

By Dec. 19 at 0159 UTC (Dec. 18 at 8:59 p.m. EST/U.S.) the area of strong thunderstorms had become smaller, and the storm appeared less organized. In the AIRS infrared image from Dec. 19 at 1259 UTC (7:59 a.m. EST), the area of strongest thunderstorms had been reduced further and cloud top temperatures throughout the storm were warming, indicating cloud heights were falling because of less evaporation. Evan had moved over sea surface temperatures below the 80 degree Fahrenheit (26.6 degree Celsius) threshold, so evaporation and thunderstorm development had waned.

Wind shear had increased as well, pushing the bulk of the thunderstorm activity about 65 nautical miles (74.8 miles/120.4 km) to the southeast, according to the Joint Typhoon Warning Center. Northwesterly wind shear was very strong, blowing between 40 and 50 knots (46 and 57.5 mph/74 and 92.6 kph). Animated multi-spectral satellite imagery also showed the low-level circulation center remains fully exposed.

On Dec. 19 at 0900 UTC (4 a.m. EST/U.S.), Evan was a tropical storm with maximum sustained winds near 45 knots (51.7 mph/83.3 kph). It was located about 335 nautical miles (385.5 miles/620.4 km) south of Nadi, Fiji, near 23.7 south latitude and 178.3 east longitude. Evan was moving to the south-southeast at 7 knots (8 mph/13 kph).

Forecasters at the Joint Typhoon Warning Center noted that because of the strong wind shear and cooler sea surface temperatures, Evan may dissipate sometime on Dec. 20.

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




Dec. 19, 2012

Update One:

TRMM image of Evan› Larger image
TRMM captured data on Cyclone Evan's cloud heights and rainfall on Dec. 16 and spotted an extremely tall storm-cell in the north side of the eyewall. The updrafts in this tower extended high enough to lift precipitation-size ice 17 km above the ocean surface (red in the image). The insert shows the infrared cloud top temperatures. The clockwise arrow shows the direction of the winds circling the eyewall. At the north side of the eyewall, the explosive circular shape is the upper-level outflow from the extremely tall tower, Credit: NASA/Owen Kelley
NASA Satellite Finds an Unusually Tall Storm-cell in Cyclone Evan

NASA's Tropical Rainfall Measuring Mission or TRMM satellite found an unusually tall towering thunderstorm in Cyclone Evan.

According to Owen Kelley of the TRMM satellite team at NASA's Goddard Space Flight Center in Greenbelt, Md, the most startling feature of the December 16 overflight of Tropical Cyclone Evan was the extremely tall storm-cell in the north side of the eyewall. At the time TRMM passed overhead and captured an image of the storm, Evan was about to rake across the northern coast of the islands of Fiji.

The updrafts in this tower extended high enough to lift precipitation-size ice 17 km (10.5 miles) above the ocean surface. Tall precipitation cells are generally taken to be anything at least 14.5 km (9 miles) high and are nicknamed "hot towers," but what was seen in Evan's eyewall was a different category of storm cell.

Storm-cells as tall as the one in the eyewall of Evan have been long known to occur occasionally over land, but before the TRMM satellite, there were not thought to occur over ocean far from land. While field campaigns have periodically studied one location or other over the ocean, what TRMM has taught us is that such sporadic observations are insufficient if you want catch rare events. After 15 years of continuous operation, TRMM satellite reveals the rare features and challenges our understanding of how the weather works. The ocean is an unlikely place to find extremely tall oceanic cells because the ocean surface stays roughly constant in temperature, unlike the land which quickly heats up over the course of a day, increasing low-level instability, and encouraging tall cells to form.

During the first 10 years of the TRMM mission, only 5 thunderstorm cells as tall as the one seen in cyclone Evan were observed in South Pacific tropical cyclones. Due to their rarity, perhaps these 17-km-tall (10.5 mile) cells deserve their own nickname. To distinguish them from run-of-the-mill hot towers, one can call these cells "titans," "super towers," or just extremely tall.

Over all of the tropical oceans, only 174 such extremely tall cells were observed during the first 10 years of TRMM (1). That's 174 extremely tall cells out of the approximately 9 million oceanic storms that TRMM saw during that time. It is worth noting however, that even TRMM has its limitations. It does not observe the whole earth continuously and frequently misses short-lived events. With this in mind, these extremely tall cells most likely occur more often than TRMM observes them although they do make up a very small fraction of the ocean's weather.

TRMM also observed cloud top temperatures. At the north side of the eyewall, was the upper-level outflow from the extremely tall tower, i.e. the tower's "exhaust fumes." The exhaust moves outward horizontally in every direction, including toward the eye at the center of the tropical cyclone. It is through a process called "forced subsidence" that the exhaust from eyewall towers may warm the air in the tropical cyclone's eye. Warming the air in the eye lowers the surface pressure and encourages intensification of the winds circling the eye.

The TRMM satellite is a joint mission between the United States and Japan.

Text Credit: Owen Kelley
NASA's Goddard Space Flight Center




Dec. 18, 2012

MODIS image of Evan› Larger image
NASA's Aqua satellite flew over Tropical Cyclone Evan at 2240 UTC on Dec. 17 (5:40 p.m. EST/U.S.) after it had passed the Fiji Islands. Credit: NASA Goddard MODIS Rapid Response Team
NASA Sees Powerful Cyclone Evan Move Past Fiji

Northern and western Fiji is under a state of emergency after Cyclone Evan ravaged the island nation. NASA's Aqua satellite captured a view of Evan as it was leaving Fiji and heading south in the South Pacific Ocean.

Evan was a category 4 cyclone (hurricane) when it hit Fiji's main island of Viti Levu. Evan's maximum sustained winds at the time of landfall were near 135 mph (217 kph).

What Does the Satellite Imagery Show?

NASA's Aqua satellite flew over Tropical Cyclone Evan at 2240 UTC on Dec. 17 (5:40 p.m. EST/U.S.) after it had passed the Fiji Islands and the Moderate Resolution Imaging Spectoradiometer or MODIS instrument captured a visible image of Evan's clouds. The image was created by the NASA MODIS Rapid Response Team located at NASA's Goddard Space Flight Center in Greenbelt, Md.

Satellite imagery reveals that Evan is starting to elongate, which is a sign of weakening. An analysis of the upper level atmosphere shows that strong westerly wind shear is now battering the upper levels of Evan, according to the Joint Typhoon Warning Center. That wind shear is causing the storm to elongate.

Weather Bulletins Active from Fiji on Dec. 18

The Fiji Meteorological Service (FMS) issued a Special Weather Bulletin on Dec. 18 at 3:44 p.m. local time with a strong wind warning in effect for Kadavu. The FMS bulletin says "Strong west to northwest winds with average speeds of 45 to 55 km/hr and momentary gusts to 65 km/hr. Occasional rain, heavy at times with squally thunderstorms. Rain easing tomorrow."

The FMS noted that the rest of Fiji will experience occasional rain and west to northwesterly winds, gusty at times. Rains will ease on Wed., Dec. 19. FMS cautions that flooding is possible in low-lying areas especially over eastern Viti Levu. For more warnings and watches, visit the FMS site: http://www.met.gov.fj/current_warnings.php.

What are Evan's Current Conditions?

On Dec. 18 at 0900 UTC (4 a.m. EST/U.S.), Evan's maximum sustained winds were near 95 knots (109.3 mph/175.9 kph). Evan's cyclone (hurricane) force winds extend up to 45 nautical miles (51.7 miles/83.3 km)from the center, while tropical-storm-force winds reach as far out as 145 nautical miles (166.9 miles/268.5 km) from the center. That means that Evan is about 300 nautical miles (345.2 miles/555.6 km) wide. As Evan weakens the wind field is expected to grow, and the storm is expected to cover a larger area.

Evan's center had passed Fiji and continues moving south at 7 knots (8 mph/13 kph). Evan's center was about 115 nautical miles (132.3 miles/213 km) south of Nadi, Fiji near 21.0 south latitude and 177.1 east longitude. Evan was still generating very rough seas with wave heights to 35 feet (10.6 meters), so mariners should take note.

Post-Cyclone Evan Conditions in Fiji

According to the Sydney (Australia) Telegraph, resident of Fiji have begun cleaning up after Cyclone Evan passed leaving power outages, downed trees, flooding, and damage to many structures. About 8,500 residents remained in shelters across the island on Dec. 17. The government declared a state of natural disaster for the north and western parts of Fiji. Euronews reported the airport was closed and 2,000 tourists were stranded.

Evan is moving south and is weakening under unfavorable atmospheric conditions and cooler ocean temperature. The sea surface temperatures around Evan are near 25 degrees Celsius (77 degrees Fahrenheit), and the Joint Typhoon Warning Center noted that those temperatures will drop sharply as the system passes 25 degrees south latitude. Tropical cyclones need a sea surface temperature as warm as 26.6C (80F) to maintain strength, and anything colder saps the storm's strength quickly.

Evan is expected to become extra-tropical before it approaches North Island of New Zealand.

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




Dec. 17, 2012

Update Two: MODIS image of Evan› Full MODIS image
NASA's Aqua satellite flew over Tropical Cyclone Evan at 0135 UTC on Dec. 16 (8:35 p.m. EST/U.S., Dec. 15) when it was over the Fiji Islands. Credit: NASA Goddard MODIS Rapid Response Team

AIRS image of Evan› Larger image
NASA's Aqua satellite passed over Cyclone Evan at 9:11 p.m. EST, Dec. 16 and the AIRS instrument captured an infrared image of the cloud tops. AIRS data showed that temperatures (purple) were as cold as -81F (-63C) over Fiji, indicating very strong thunderstorms wrapping around Evan's center. Those thunderstorms were also generating heavy rainfall. Credit: NASA JPL, ED Olsen
NASA Sees Dangerous Category 4 Cyclone Evan Lashing Fiji

Cyclone Evan is one of the strongest cyclones to affect Fiji in almost two decades, and NASA satellites are analyzing the storm and providing data on rainfall, cloud height, temperature data and more to forecasters.

According to NBC News, over 3,500 people in Fiji went to emergency shelters. Today, Dec. 17, Evan is lashing Fiji, just days after battering Samoa where it killed at least three people and left thousands homeless.

On Monday, Dec. 17 many warnings and watches were in effect. A tropical cyclone warning is in effect for Fiji. A Hurricane warning is in effect for the Mamanuca island group, west of the line from Lautoka to Sigatoka and nearby smaller islands. A storm warning is in effect for the Yasawa group, Vatulele and the interior of Viti Levu. A gale warning is posted for the Lomaiviti group, rest of Viti Levu and nearby smaller islands, Kadavu, Beqa, and nearby smaller islands. There are also other local warnings in effect for Fiji.

The Joint Typhoon Warning Center (JTWC) is issuing forecasts for Evan and is using NASA and other satellite data to provide warnings and status updates on the storm. On Dec. 17 at 0900 UTC (4 a.m. EST), Cyclone Evan had maximum sustained winds near 115 knots (132 mph/213 kph). Evan is a Category 4 cyclone on the Saffir-Simpson Scale.

According to the National Oceanic and Atmospheric Administration, a Category 4 means " Catastrophic damage will occur: Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months."

At 0900 UTC (4 a.m. EST) on Dec. 17, Evan's center was only 15 nautical miles southwest of Nadi, Fiji, near 18.3 south latitude and 177.2 east longitude. Evan was moving to the south-southwest at 10 knots.

NASA satellite imagery has helped forecasters at JTWC that Evan is moving along the coast of Vitu Levu, Fiji. When NASA's Aqua satellite passed over Cyclone Evan at 0211 UTC on Dec. 17 (9:11 p.m. EST, Dec. 16) the Atmospheric Infrared Sounder (AIRS) instrument captured an infrared image of the cloud top temperatures. The colder the temperatures, the higher and stronger the thunderstorms are that make up a tropical cyclone. AIRS data showed that temperatures were as cold as -81F (-63C) over Fiji, indicating very strong thunderstorms wrapping around Evan's center. Those thunderstorms were also generating heavy rainfall. JTWC noted that in addition to the heavy rainfall, Evan is generating very high surf that could also create major flooding.

Forecasters at the JTWC also noted that in the upper levels of the atmosphere, above Evan lies a strong anti-cyclone (high pressure area) that is providing good outflow to the system being enhanced by a subtropical jet stream to the south of Cyclone Evan. That means that the vertical wind shear over Evan is low, which has allowed the storm to become so strong.

According to Weather Underground, a weather reporting station in Nadi, Fiji reported a maximum sustained wind of 69 mph (111 kph) with a gust as high as 104 mph (167.4 kph) on Dec. 17.

Evan may be the strongest storm to affect Fiji since 1993, when Cyclone Kina killed 23 and left thousands without homes.

Cyclone Evan is now moving south and will continue weakening as vertical wind shear increases and sea surface temperatures drop. As the storm weakens and becomes extra-tropical, it is expected to speed up. The JTWC expects Evan to move toward North Island, New Zealand and begin affecting the region by Dec. 21 or 22.

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




Update One for Evan on 12/17/12

TRMM image of Evan
Image Credit: NASA/Owen Kelley

A Need to Look Again: TRMM Satellite Observations of Tropical Cyclone Evan

The radar on NASA's Tropical Rainfall Measuring Mission (TRMM) satellite had observed Tropical Cyclone Evan four times as of Sunday, Dec. 16, and two of those overflights merit a closer examination.

On Tuesday, Dec. 11, the TRMM satellite saw Evan about 24 hours before the storm struck American Samoa, and the radar data at first seem incongruous for such a weak system. At the time, Evan was estimated to be less than tropical "cyclone" strength, and had 35 knot (40.2 mph/64.8 kph) surface winds, making it a tropical storm. More specifically, the TRMM radar saw a "complete eyewall," i.e. an eyewall that circled all the way around the eye.

Either the TRMM radar was showing that Evan was much stronger than 35 knots (40.2 mph/64.8 kph) on Dec. 11 or perhaps Evan was a member of a rare breed of 35-knot systems with well-formed eyewalls that are known to rapidly intensify (1). After the South Pacific tropical cyclone season is over, researchers and operational agencies alike should take a closer look at Evan on Dec. 11. Through informal channels, an unofficial debate on this question has already begun.

The infrared cloud top imagery collected by the TRMM satellite isn't much help on this question (as seen in the background of the TRMM image). The infrared instrument finds the eyewall obscured by upper-level outflow.

Meanwhile, the TRMM radar observed light precipitation under the cloud tops (as shown in the image by the gray-green-yellow volume) of 20 dBZ radar reflectivity.

The term "dBZ" means "decibels relative to Z." Basically, dBZ is a measure of equivalent reflectivity (Z) of a radar signal bouncing off an object. "Reflectivity" of a cloud depends on the number and rain, snow, and hail, and their size. In respect to size, a large number of small raindrops will reflect the same as one large raindrop.

A respectable 30-dBZ radar-reflectivity signal reached most of the way around the eye. A close look at the radar data reveals that, with increasing altitude, the inner edge of the volume of precipitation slopes outward from the eye, like seating in a football stadium.

Launched in 1997 by NASA and the Japanese Space Agency known as JAXA, the TRMM satellite has observed enough systems of this strength to suggest that it is rare, but not unheard of, for them to have complete eyewalls. Most new systems, upon reaching tropical-storm strength, still look like poorly organized "blobs" in TRMM radar overflights not football stadiums, like Evan did.

The (insert, lower left) passive microwave observations were collected by the TRMM Microwave Imager (TMI) at 85 GHz. Colder temperatures at 85 Ghz locates where storm cells have more ice-phase precipitation because these upper-level ice chunks are scattering out radiation coming up from the ocean's surface. TMI's coarser horizontal resolution and limited information in the vertical, causes the compact eye of the storm (labeled "E" in the image) to be harder to distinguish from the large region of less-organized rain cells to the southeast (labeled "R") in the image. More specifically, the microwave observations are spaced 5 by 14 km (3.1 by 8.6 miles) apart compared the TRMM radar's 5 by 5 km (3.1 by 3.1 mile) horizontal resolution and 250 meter (0.15 mile) vertical resolution.

While there were overcast skies at Wallis and Futuna Island within a couple of hundred kilometers (miles) to the north and west of Evan's eyewall on Dec. 11, the TRMM radar shows a complete absence of precipitation anywhere near these islands and the approximately 15,000 people who live there. Having spared them on Dec. 11, Cyclone Evan would ironically loop back and strike these two islands directly on its return trip west, four days later.

TRMM image of Evan
Image Credit: NASA/Owen Kelley

On Friday, Dec. 14, NASA's TRMM satellite saw Evan after it had hovered near Samoa for about a day. At the time of the Dec. 14 overflight, Evan was one day from striking the islands of Wallis and Futuna and two days from striking the larger island nation of Fiji. There was an unusually strong radar signal on the northwest side of Evan's eyewall, a few kilometers (miles) above the ocean surface.

An initial interpretation might be that such a strong signal would indicate that the eyewall was working particularly vigorously to inject energy into the tropical-cyclone heat engine that converts latent heat of vaporization into increased tropical cyclone wind intensity. One might think this low altitude, strong radar signal would indicate intensification, but think again.

A more careful analysis would suggest that the strong radar signal at the base of the eyewall on December 14 did not signal intensification. In the Dec. 14 TRMM overflight of Evan, the feature in question (shown in the image as the purple volume) locates the where the radar signal equals or exceeds 50 dBZ.

Since reflectivity is reported on a logarithmic scale, the 50 dBZ signal in the Dec. 14 overflight suggests that a factor of 10 more scattering from precipitation is occurring on Dec. 14 than the 40 dBZ signal in the Dec. 11 overflight would imply. Other factors being equal, the stronger the radar signal, the more water vapor is condensing into liquid or ice precipitation in the eyewall. Condensation releases latent heat, the fuel of tropical cyclones. However, the multiple energy transformations that occur within the inner core of a tropical cyclone contain subtleties that decades of research have yet to fully describe.

A statistical analysis of years of TRMM satellite overflights of tropical cyclone's world-wide found that high-altitude light precipitation (a 20 dBZ radar reflectivity above approximately 14.5 km (9.0 miles) altitude) is associated with tropical cyclone intensification. In contrast, the same statistical analysis found that very intense precipitation (50 dBZ) occurs rarely in tropical cyclones, and close to equally rarely regardless of whether or not the tropical cyclone is intensifying (2).

Tropical cyclone simulations suggest that strong precipitation fixed on one side of the eyewall can be caused by persistent large-scale wind-shear (3). Such wind shear threatens to tear the tropical cyclone apart and can make it difficult, though not impossible, for a tropical cyclone to maintain, let alone increase, its intensity. Consistent with this interpretation, passive microwave observations of Evan from multiple satellites showed a persistent precipitation maximum on the north side of Evan's eyewall during several days centered on the Dec. 14 TRMM overflight.

Furthermore, operational analyses suggested Evan was holding steady or weakening slightly at the time the TRMM saw radar reflectivity in excess of 50 dBZ at the base of the eyewall. In short, the strong radar signal at the base of the eyewall on Dec. 14 is noteworthy but not the sign of a tropical cyclone that is about to become a monster storm.

Text Credit: Owen Kelley
NASA's Goddard Space Flight Center

References:

(3) Braun, S. A., M. T. Montgomery, Z. Pu, 2006: High-resolution simulation of Hurricane Bonnie (1998). Part I: The organization of eyewall vertical motion, Journal of the Atmospheric Sciences, 63, 20-42.

(2) Kelley, O. A., 2008: The association of eyewall convection with tropical cyclone intensification, Ph.D. dissertation, George Mason University, Fairfax, Virginia.

(1) Kieper, M., and H. Jiang, 2012: Predicting tropical cyclone rapid intensification using the 37 GHz ring pattern identified from passive microwave measurements. Geophys. Res. Lett., 39, L13804, doi:10.1029/2012GL052115.


Dec. 14, 2012

TRMM revealed several › View larger image
TRMM data on Dec. 12, 2012 at 1704 UTC revealed several "hot towers" or towering thunderstorms in Cyclone Evan reaching heights of greater than 16.5 km (10.25 miles) within Evan's eye wall. The heaviest rainfall (dark red) of over 80 mm (~3.1 inches) per hour was occurring in heavy rainfall within Evan's clear eye wall. Credit: NASA/SSAI, Hal Pierce
TRMM revealed several › View TRMM Animation
TRMM data on Dec. 12, 2012 at 1704 UTC revealed several "hot towers" or towering thunderstorms in Cyclone Evan reaching heights of greater than 16.5 km (10.25 miles) within Evan's eye wall.
Credit: NASA/SSAI, Hal Pierce
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.
http://www.nws.noaa.gov/view/validProds.php?prod=HLS&node=NSTU

Tonga: A tropical cyclone alert is in force for Niuafo'ou.
http://www.met.gov.fj/aifs_prods/20027.txt

Fiji: A tropical cyclone alert is in force for Fiji.
http://www.met.gov.fj/aifs_prods/20020.txt

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




Dec. 13, 2012

AIRS captured this infrared image of Tropical Cyclone Evan over the Samoa Islands on Dec. 13 › View larger image
The AIRS instrument aboard NASA's Aqua satellite captured this infrared image of Tropical Cyclone Evan over the Samoa Islands on Dec. 13 at 0059 UTC. Evan's maximum sustained winds had increased to 90 knots (103 mph/166.7 kph) at the time of this image. The purple rounded area is Evan's center of circulation and is populated by strong thunderstorms that reach high into the troposphere where temperatures are as cold as -63 Fahrenheit (-52 Celsius). Those areas shaded in purple also indicate heavy rainfall.
Credit: NASA/JPL, Ed Olsen
TRMM passed above Evan on Dec. 11 and saw the tallest thunderstorms around Evan's center of circulation › View TRMM Animation
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








Dec. 12, 2012

AIRS captured an infrared image of newborn Tropical Storm Evan over American Samoa on Dec. 12, 2012. › View larger image
The AIRS instrument aboard NASA's Aqua satellite captured an infrared image of newborn Tropical Storm Evan over American Samoa on Dec. 12, 2012 at 12:47 UTC (7:47 a.m. EST). The strongest thunderstorms (purple) appear around the center of circulation and in a band of thunderstorms north and east of the center.
Credit: NASA/JPL, Ed Olsen
GOES-15 captured this visible image of newborn Tropical Storm Evan in the South Pacific on Dec. 12, 2012. › View larger image
NOAA's GOES-15 satellite captured this visible image of newborn Tropical Storm Evan in the South Pacific on Dec. 12, 2012 at 1500 UTC (10 a.m. EST).
Credit: NASA GOES Project
NASA Sees Newborn Tropical Storm Evan Causing Trouble for American Samoa

The date 12-12-12 may be numerically significant, but for the residents of American Samoa and Fiji in the South Pacific Ocean, it means a newborn tropical storm named Evan is causing problems. NASA's Aqua satellite and NOAA's GOES-15 satellite both captured imagery of the newborn storm's cloud cover.

Tropical Storm Evan caused regional warnings to be posted on Dec. 12. In American Samoa, a gale warning and storm watch are in effect for Tutuila, Aunuu, Manua and Swains Island.

The Atmospheric Infrared Sounder (AIRS) instrument aboard NASA's Aqua satellite captured an infrared image of newborn Tropical Storm Evan over American Samoa on Dec. 12, 2012 at 12:47 UTC (7:47 a.m. EST). The strongest thunderstorms with cloud top temperatures exceeding -63F (-52C) appeared around the center of circulation and in a band of thunderstorms north and east of the center. Cloud top temperatures that cold indicate the highest, strongest storms with the heaviest rainfall within the tropical cyclone.

In addition to NASA's Aqua satellite, NOAA's GOES-15 satellite captured a visible image of newborn Tropical Storm Evan on Dec. 12, 2012 at 1500 UTC (10 a.m. EST) that showed a rounded storm.

At 1500 UTC (10 a.m. EST), Tropical Storm Evan had maximum sustained winds near 50 knots (57.5 mph/92.6 kph). Those tropical-storm-force winds extended 45 nautical miles (51.7 miles/83.3 km) from the center. The center of Evan was located near 13.9 south latitude and 173.2 west longitude, about 145 nautical miles (170 miles/268.5 km) west of Pago Pago, American Samoa. Pago Pago is the capital of American Samoa.

The National Weather Service (NWS) in Pago Pago, American Samoa noted in their forecast for Tutuila and Aunuu that the strongest winds are likely to occur there today, Dec. 12, and Thursday, Dec. 13. The NWS forecast calls for northwest winds between 35 and 45 mph (56.3 and 72.4 kph) tonight, and between 40 and 55 mph (64.3 and 88.5 kph) tomorrow, Dec. 14. Sustained winds on Dec. 15 are expected between 30 and 35 mph (48.2 and 56.3 kph). For more updated forecasts, visit: http://forecast.weather.gov/MapClick.php?zoneid=ASZ001.

Evan continues to move to the east and expected to strengthen. Evan is expected to move to the southwest and towards Fiji.

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