At 11 a.m. EDT on October first, the eighteenth tropical depression of the Eastern Pacific hurricane season was born. He’s a little guy, but is likely going to grow up to be a tropical storm and get the name Olaf later today or tomorrow. He’s not, however, expected to reach hurricane strength.

At the time of his birth, Tropical Depression 18-E (TD18E) had maximum sustained winds near 35 mph. He was located 580 miles west-southwest of the southern tip of Baja California, near 19.0 North and 117.9 West. TD18-E was moving west-northwest at 8 mph and is expected to turn toward the northwest later today or tonight. His estimated minimum central pressure is 1004 millibars.

Forecasters at the National Hurricane Center confirmed TD18E’s birth using data from NASA’s Atmospheric Infrared Sounder (AIRS) instrument combined with data from the Advanced Microwave Sounding Unit (AMSU) data to create a microwave of the storm. Both AIRS and AMSU are instruments that fly on NASA’s Aqua satellite.

NASA’s Aqua satellite flew Tropical Depression 18E five and a half hours before it was born. Aqua’s instruments were used to create a microwave image from 5:23 a.m. EDT on October 1 that showed high thunderstorms, a sign that the storm was intensifying. The imagery revealed cold areas in the storm that indicate ice in cloud tops, and light to moderate precipitation. The National Hurricane Center reported ” overnight microwave (and scatterometer data from NASA’s QuikScat satellite) data indicate that the circulation associated with the broad low pressure area southwest of the southern tip of Baja California has become better defined.

The Hurricane Center also said that “The depression does not appear to have much of an opportunity to strengthen. Southerly to southwesterly vertical shear is forecast to increase later today and become stronger thereafter.” In addition, TD18E is going to start moving into cooler waters on its northward track.

The Tropical Rainfall Measuring Mission or TRMM satellite, orbits the Earth and measures the amount of rainfall created by a tropical cyclone. When Typhoon Ketsana (known in the Phillippines as “Ondoy”) made landfall early this past weekend TRMM was monitoring its rainfall. That data was used to create a 3-D map of rainfall over the Philippines from September 21-28.

According to the Philippine Atmospheric Geophysical Astronomical Services Administration (PAGASA), Ketsana dropped 17.9 inches (455 mm) of rain in Manila in just 24 hours on Saturday, September 26.

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 7-day period 21 to 28 September 2009 for the northern Philippines and the surrounding region showed that the highest rainfall totals occurred south of the storm’s track in an east-west band over central Luzon that includes Manila. Amounts in this region are on the order of 375 mm (~15 inches) to over 475 mm (~19 inches). The highest recorded amount from the TMPA near Manila was 585.5 mm (almost 24 inches).

Ketsana maintained minimal tropical storm intensity as it crossed central Luzon on the afternoon of September 26 (local time). The main deluge in the Manila area, located on the western side of Luzon, began around 8:00 a.m. local time even though the center of Ketsana had yet to make landfall on the eastern side of the island.

A record 13.43 inches of rain fell in Manila in the six hours between 8 a.m. and 2 p.m. local time, which is equivalent to about a month’s worth of rain for the area. The enhanced rainfall over on the Manila-side of the island as the storm approached was because of an interaction between Ketsana’s circulation and the seasonal southwest monsoon.