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).