Featured Images

Hurricane Season 2010: Tropical Storm Sarah (Southern Pacific)
03.01.10
 
March 1, 2010

NASA's Terra satellite captured a visible image of 17P on Feb. 25 at 2:09 p.m. ET > View larger image
NASA's Terra satellite captured a visible image of 17P on Feb. 25 at 2:09 p.m. ET.
Credit: NOAA/JTWC
Cyclone 17P Briefly Named Sarah, Then Fizzles

Cyclone 17P finally strengthened enough to get named Tropical Storm Sarah over the weekend, as it continued on a southern track toward the South Cook Islands. It didn't hold together long however, as by Monday, March 1, the storm has dissipated.

Tropical Storm Sarah's winds peaked around 39 mph (35 knots) on Saturday, February 27. Thirty-nine mph is the lowest maximum sustained wind speed a cyclone can have to be classified as a tropical storm. At 10 a.m. ET on Saturday, the Joint Typhoon Warning Center issued its final warning on the system. It was about 555 miles east-southeast of Pago Pago, American Samoa at that time, near 17.4 South latitude and 162.5 East longitude. Sarah/17P was still moving at the slow pace it maintained the previous week, at 4 mph (3 knots). Sarah was generating 8-foot high waves on Saturday that affected the coasts of the South Cook Islands.

As Sarah neared on Saturday, the Cook Islands Meteorological Service posted a strong wind warning for the southern Cook Islands. At about 9 a.m. local time, Sarah was about 93 miles (150 kilometers) north-west of Palmerston and about 298 miles (480 kilometers) northwest of Aitutaki. In February, Cyclone Pat lashed parts of Aitutaki with heavy rain and high winds that damaged houses and buildings, so residents were paying close attention to Sarah's approach.

The islands experienced scattered heavy showers, ocean swells and coastal flooding in some low lying areas. By March 1, the forecasts for both Aitutaki and Palmerston called for widely scattered pop-up thunderstorms and light winds

Text credit: Rob Gutro, NASA's Goddard Space Flight Center



February 26, 2010

NASA's Terra satellite captured a visible image of 17P on Feb. 25 at 2:09 p.m. ET > View larger image
NASA's Terra satellite captured a visible image of 17P on Feb. 25 at 2:09 p.m. ET.
Credit: NOAA/JTWC
Low 17P Has a Good Chance for Tropical Reformation This Weekend

The Joint Typhoon Warning Center is the forecast center that issues advisories for tropical cyclones in the Southern Pacific Ocean, and they noted on February 26, "Available data does not justify issuance of numbered tropical cyclone warnings at this time" That may change over the weekend, as maximum sustained winds are near tropical depression strength, 34 mph (30 knots) and environmental factors are looking more favorably for further development.

17P's center is near 16.3 degrees South latitude and 163.6 West longitude, that’s about 425 nautical miles east-southeast of Pago Pago. The system is crawling south-southwestward at 2 mph (2 knots).

NASA's Terra satellite captured a visible image of 17P on Feb. 25 at 2:09 p.m. ET, that showed an organized rounded circulation, with what appears to be higher thunderstorms (more powerful) around the center. Meanwhile, animated infrared satellite imagery shows the system is slowly consolidating as bands or arms of convective thunderstorms wrap from northeast to southeast around the storm.

Forecasters also took a look at the upper level of the troposphere (the layer of atmosphere closest to earth, where weather happens), and analysis indicated some moderate vertical wind shear (winds that can weaken or tear storms apart). Water vapor imagery showed improved outflow of the storm and that means the storm has an improved chance for development. In fact, forecasters give 17P a good chance to become a tropical cyclone again over the weekend.

Text credit: Rob Gutro, NASA's Goddard Space Flight Center



February 25, 2010

The GOES-11 satellite captured in infrared image of 17P's clouds and the storm appears to be getting re-organized. > View larger image
The GOES-11 satellite captured in infrared image of 17P's clouds on Feb. 25 at 1622 UTC (11:22 a.m. ET), and the storm appears to be getting re-organized (note its circular shape).
Credit: NOAA/JTWC
Tropical Low 17P May Get a Second Chance

Tropical cyclone 17P may be a low pressure area right now, but environmental conditions have become more favorable to give it a likely comeback as a tropical storm. Forecasters are using satellite imagery and observing various factors to see if 17P may be reborn.

When the Geostationary Operational Environmental Satellite known as GOES-11 flew over 17P's center on Feb. 25 at 1622 UTC (11:22 a.m. ET), it captured in infrared image of its clouds. The satellite image showed that the storm appears to be getting re-organized, as seen in the circular shape of its clouds. At the time GOES-11 passed overhead, maximum sustained winds were between 25 and 30 knots (28-34 mph) and the minimum central pressure was 1000 millibars. The center of 17P was near 16.2 South latitude and 164.6 West longitude, approximately 370 nm east-southeast of Pago Pago, American Samoa. 17P is moving west-southwest at 6 knots (7 mph) and keeping to open ocean.

Satellite imagery revealed that there is improved banding of thunderstorms around 17P. Basically that means the "arms" that wrap around the center of the tropical cyclone are getting more developed (more thunderstorms are being created in them) and more organized indicating a strengthening tropical cyclone. Meanwhile, an upper-level analysis indicated that vertical wind shear was decreasing, so the storm will be further allowed to strengthen.

The Joint Typhoon Warning Center noted that as a result of the stronger "banding" of thunderstorms around 17P's center, and lower wind shear, there is a good chance that 17P will re-develop into a tropical storm in the next 24 hours

Text credit: Rob Gutro, NASA's Goddard Space Flight Center



February 24, 2010

TRMM captured the rainfall rates of the 17P low on Feb. 24 at 0709 UTC (2:09 a.m. ET). > View larger image
TRMM captured the rainfall rates of the 17P low on Feb. 24 at 0709 UTC (2:09 a.m. ET). The rainfall was scattered around the low's center. The yellow and green areas indicate moderate rainfall between .78 to 1.57 inches per hour. The small red area indicates heavy rainfall at almost 2 inches per hour.
Credit: NASA/SSAI, Hal Pierce
17P Now a Low with a Fair Chance for a Comeback

The low pressure area that was formerly known as Tropical Cyclone 17P is still moving south in the Southern Pacific Ocean, and forecasters give it a fair chance of making a comeback into a tropical cyclone. NASA satellite data shows that 17P's rainfall is scattered. Sometimes tropical cyclones will reach tropical storm or hurricane strength then weaken into a depression then further degrade into a low pressure area, and re-generate into a tropical depression. It all depends on what's happening with the atmosphere and ocean temperatures.

If vertical wind shear is moderate to strong, and/or ocean temperatures are colder than 80 degrees Fahrenheit, then a once- tropical cyclone doesn't stand much of a chance of being reborn. In the case of ex-tropical cyclone 17P, however, the system is still maintaining its circulation despite being in an area of strong vertical westerly wind shear. What's keeping the system alive is an elongated area of low pressure in the upper troposphere, that's located southwest of the low's center. That low pressure is enhancing the low's outflow (air that flows outwards from a thunderstorm).

Today, February 24, the low formerly known as 17P is centered near 13.1 degrees South latitude and 161.4 degrees West longitude, approximately 560 nautical miles east of Pago Pago.

The Tropical Rainfall Measuring Mission (TRMM) satellite, managed by NASA and the Japanese Space Agency flew over 17P's low on Feb. 24 at 0709 UTC (2:09 a.m. ET). TRMM data showed that the rainfall was scattered around the low's center at that time. Most of the rain was light to moderate, falling at rates between 20 and 40 millimeters (.78 to 1.57 inches) per hour. Later in the morning, the rainfall shifted mostly to the southwest of the center.

TRMM images are pretty complicated to create. They're made at NASA's Goddard Space Flight Center in Greenbelt, Md. At Goddard, rain rates in the center of the swath (the satellite's orbit path over the storm) are created from the TRMM Precipitation Radar (PR) instrument. The TRMM PR is the only space borne radar of its kind. The rain rates in the outer portion of the storm are created from a different instrument on the satellite, called the TRMM Microwave Imager (TMI). The rain rates are then overlaid on infrared (IR) data from the TRMM Visible Infrared Scanner (VIRS).

Animated multispectral imagery showed that the main area of convection (rapidly rising air that produces thunderstorms) is located to the southwest of the center of the low. The system has maximum sustained winds between 22 and 27 knots (25-31 mph), and has a minimum central pressure of 1004 millibars.

Forecasters continue to keep an eye on the 17P low, and give it a fair shot at redeveloping as it continues on its southward journey in open waters.

For more information about the TRMM satellite visit: http://www.trmm.gsfc.nasa.gov/.

Text credit: Rob Gutro, NASA's Goddard Space Flight Center



February 23, 2010

The MODIS instrument captured in infrared image of the storm. > View larger image
NASA's Aqua satellite flew over 17P on February 23 at 6:58 a.m. ET and the MODIS instrument captured in infrared image of the storm. It was difficult to find a center of the storm.
Credit: NASA/JTWC
17P is Now a Fading Depression

Tropical cyclone 17P has had a brief life. After becoming a tropical storm yesterday, atmospheric conditions have weakened the cyclone back down to tropical depression status today, February 23 and it is expected to dissipate in the next couple of days.

On February 22 at 6 p.m. ET (21 UTC), the Joint Typhoon Warning Center issued their final advisory on 17P. At that time, its maximum sustained winds were near 34 mph (30 knots) and it continued to weaken. It was still 615 nautical miles east-northeast of the island of Pago Pago, near 12.2 degrees South latitude and 160.8 West longitude.

17P isn't close enough to impact the island of Pago Pago and doesn't appear that it is ever going to get that close. Although the forecast for Pago Pago through the week calls for scattered showers with temperatures in the mid-80s (Fahrenheit), those are convective or pop-up thunderstorms created from daytime heating, and are not associated with Tropical Depression 17P.

NASA's Aqua satellite flew over 17P as it continued to become more disorganized today, February 23 at 1158 UTC (6:58 a.m. ET). The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard Aqua captured in infrared image of the storm. In the satellite image, it was difficult to find a center of the storm.

Animated infrared satellite imagery, such as that from another instrument on Aqua called the Atmospheric Infrared Sounder, or AIRS noticed last night, that the low level circulation of the storm had been separated or "decoupled" from most of the showers and thunderstorms. That's an indication that the storm is becoming less organized and weakening.

Tropical Depression 17P is now in an area of moderate to strong westerly vertical wind shear and that's bad news for any tropical cyclone, because wind shear can tear those storms apart. Wind shear means that the speed or direction of wind changes over a relatively short period of time, or a short distance.

Tropical cyclones develop vertically as rapidly rising air creates thunderstorms. Whenever there's a higher wind shear, the storm is spread over a larger area, and that limits the storm's ability to produce those thunderstorms.

That wind shear is caused by an upper level low pressure area to the southwest of 17P's center. As 17P continues moving south-southwest, atmospheric conditions are going help weaken the storm even more. There's always a chance 17P may redevelop so forecasters will continue watching it, even though the final bulletin has been issued on the storm.

Text credit: Rob Gutro, NASA's Goddard Space Flight Center



February 22, 2010

Seventeenth South Pacific Tropical Cyclone Forms

GOES-11 captured a visible image of the Tropical Storm 17P at 1800 UTC (1 p.m. ET) February 22. > View larger image
GOES-11 captured a visible image of the Tropical Storm 17P at 1800 UTC (1 p.m. ET) February 22. Credit: NOAA/JTWC
On February 21, the seventeenth tropical depression formed in the South Pacific Ocean. Today, February 22, the storm has strengthened into Tropical Storm 17P (TS 17P) with maximum sustained winds near 39 mph, and it was about 740 miles east-northeast of Pago Pago.

The Geostationary Operational Environmental Satellite, GOES-11 captured a visible image of the storm at 1800 UTC (1 p.m. ET) February 22. The storm does not appear well organized. TD 17P was located near 9.6 South latitude and 159.0 East longitude, and was moving south-southwest near 4 mph (3 knots). TS 17P was creating 15 foot-high waves in open waters.

Although TS 17P is expected to continue tracking in open waters its winds and surf may impact some land areas. So, regional warnings have been posted for the Northern Cook Islands. Currently, a gale wind warning is in effect for Penrhyn and an alert is in effect for Rakahanga, Manihiki and nearby islands.

TS 17P is in an area of wind shear, and that's limiting any intensification of the storm. It is expected to strengthen a little more over the next couple of days however, before it dissipates later this week.

Text credit: Rob Gutro, NASA's Goddard Space Flight Center