Two tropical depressions formed 450 miles apart from each other in the last 24 hours in the Western Pacific Ocean. Tropical Depression 18W (TD18W) is expected to strengthen as it moves towards Guam. Tropical Depression 19W (TD19W) is expected to move through Micronesia, then move northwest toward Taiwan.

Because both storms are close together, forecasting their paths is a challenge because they may interact.

TD 19W was located 235 miles south of Guam, near 9.3 north and 144.1 east during the morning hours (Eastern Daylight Time) on September 28. TD18W was located 630 miles east-southeast of Guam near 9.1 north and 154.8 east. Both had maximum sustained winds near 35 mph.

NASA’s Aqua satellite flew over the TD18W on September 28 at 3:11 UTC (September 27 at 11:11 p.m. EDT) and captured an image of its cloud cover, which appeared disorganized.

Tropical Depression Eight didn’t have much of a lifetime in the far eastern Atlantic Ocean. In fact, it formed after working hours (Eastern Time) on Friday September 25 and faded away about 24 hours later. Tropical depression eight (TD#8) formed on Friday, September 25 at 5 p.m. EDT near latitude 17.4 north and longitude 32.3 west or about 560 miles west of the Cape Verde Islands. TD#8 had maximum sustained winds near 35 mph, and was moving northwest near 14 mph.

The depression experienced 11-17 mph (10-15 knots) of southwesterly vertical wind shear late on Friday night, and those winds increased over the weekend, keeping the storm below tropical storm strength. Adding more challenge to the storm’s development were cooler sea surface temperatures (cooler than the 80F needed to keep it going). Those two factors led to TD#8’s short lifetime.

NASA’s Aqua satellite flew over TD#8 shortly after midnight on September 26, and its Atmospheric Infrared Sounder (AIRS) instrument captured an image of the storm’s cold clouds. The system was not well organized, and didn’t show much strong convection.

By 5 p.m. EDT on Sunday, September 26, TD#8 became a remnant low pressure area near 18.9 north and 35.7 west. Its maximum sustained winds were down to 25 knots (28 mph), and its minimum central pressure was near 1008 millibars. The depression has since officially dissipated.

Since last night, Tropical Storm Nora has gained and lost strength and NASA’s Aqua and Tropical Rainfall Measuring Mission (TRMM) satellites have seen that fluctuation.

Last night, September 23, by 8 p.m. EDT Nora had maximum sustained winds up to 60 mph. This morning by 11 a.m. EDT, Nora’s sustained winds are back down to 50 mph. Nora was located about 760 miles west-southwest of the southern tip of Baja California, near 17.3 north and 120.0 west. She’s moving west-northwest near 5mph, and is expected to move on a more westerly route in the next couple of days bringing her farther out to sea. Nora’s minimum central pressure is near 1000 millibars.

NASA’s TRMM satellite has been eyeing Nora’s rainfall as a way to help determine fluctuations in her strength. In a satellite image captured on September 23, Nora had four areas of moderate rainfall around her center. The image was made at NASA’s Goddard Space Flight Center in Greenbelt, Md. and it takes some ingenuity to create. The image combines the infrared and visible (VIRS) channels overlaid with a precipitation analysis from the TRMM Microwave Imager (TMI) and Precipitation Radar (PR) instruments.

Forecasters at the National Hurricane Center continually use TRMM data in their forecasting. At 11 a.m. EDT (8 a.m. PDT) on September 24, the National Hurricane Center discussion said “Nora is beginning to be affected by westerly vertical wind shear. Recent TRMM data indicated that the low-level center was near the western edge of the convective mass…which has increased over the last few hours.”

NASA’s Aqua satellite flew over Tropical Storm Nora and the Atmospheric Infrared Sounder (AIRS) instrument onboard captured an infrared image on September 24 at 5:17 a.m. EDT. Nora appeared as a small circular area of cold clouds on the image. Tropical storm-force winds in Nora only extend up to 50 miles from the center and the AIRS image of Nora’s cold clouds reflect that distance. In AIRS infrared imagery, the colder the clouds, the higher they are, and the stronger the thunderstorms. There were some high, strong thunderstorm tops still apparent in Nora, with cloud top temperatures as cold as minus 63 Fahrenheit.

The National Hurricane Center expects Nora to weaken over the next couple of days because of wind shear in the upper-levels of the atmosphere that will tear into her circulation. Nora has likely peaked in intensity and is forecast to weaken to a tropical depression in 36 hours and even further into a remnant low in a few days.

So far at least 9 people are dead and others are missing across the southeastern United States as a result of severe flooding brought about by several days worth of heavy showers and thundershowers. Normally at this time of year, slow moving or stalled out tropical storms or hurricanes bring the amounts of rainfall and flooding seen in this week’s event, but not this time.

Atlanta, Georgia was especially hard hit with reports of over 15 inches of rain in the metro region. The culprit was a persistent area of low pressure located over the lower Mississippi River Valley that pumped copious amounts of moisture up from the Gulf of Mexico northeastward over the Southeast, providing fuel and a trigger for the numerous showers and thundershowers. A stagnant upper-air pattern allowed the area of low pressure to persist for several days in the same location.

Armed with both a passive microwave sensor and a space-borne precipitation radar, the primary objective of the Tropical Rainfall Measuring Mission satellite (better known as TRMM) is to measure rainfall from space. For increased coverage, TRMM can be used to calibrate rainfall estimates from other additional satellites.

The TRMM-based, near-real time Multi-satellite Precipitation Analysis (TMPA) at the NASA Goddard Space Flight Center, Greenbelt, Md. is used to monitor rainfall over the global Tropics. TMPA rainfall totals for the 8-day period from September 14 to 22, 2009 for the southeastern U.S. and the surrounding region show the highest rainfall amounts in central Tennessee, central Alabama, north central Mississippi, and north central Georgia, around the Atlanta metropolitan area.

Nearly the entire southeastern U.S. from the Gulf Coast to the Ohio Valley and from the southern Mississippi Valley to the southern Appalachians received at least 50 mm of rain (~2 inches, medium green) with a good portion receiving at least 100 to 150 mm (4 to 6 inches, shown in light green and yellow areas, respectively). Embedded within these regions are locally higher amounts exceeding 250 to 300 mm of rain (~10 to 12 inches, shown in orange and red, respectively). The highest TMPA rainfall totals for the Atlanta region are on the order of 350 mm (~14 inches).

Tropical depression seventeen-e formed around 11 p.m. EDT Tuesday night and within twelve hours it strengthened into Tropical Storm Nora.

By 11 a.m. EDT today, Wednesday, September 23, Nora had maximum sustained winds near 45 mph, in the open waters of the eastern Pacific Ocean. Nora’s center was about 665 miles southwest of the southern tip of Baja California, near latitude 16.5 north and 117.5 west. Nora is moving west-northwest near 9 mph. Nora’s estimated minimum central pressure is 1002 millibars.

The Atmospheric Infrared Sounder (AIRS), instrument that flies aboard NASA’s Aqua satellite, captured an image of Tropical Storm Nora yesterday at 5:29 a.m. EDT, that showed she was still coming together. Eighteen hours after the image was taken, Nora formed as tropical depression 17-E.

She’s currently a small storm, as tropical storm-force winds only extend out to 40 miles from the center. Some strengthening is expected over the next 24 hours