November 10, 2010
System 93L Not Yet Tropical, But NASA Sees it as a Rainmaker in Puerto Rico
Hurricane Season 2010: System 93L (Caribbean Sea)
A low pressure area doesn't have to be a tropical cyclone to produce heavy rainfall, and that's what is happening with "System 93L." NASA's Aqua satellite captured an infrared image of some cold, high thunderstorms within the low pressure area that are expected to bring some heavy rain to Puerto Rico and the Northern Leeward Islands.
Cold cloud temperatures in infrared satellite data indicate strong, high, cold thunderstorm cloud tops within a low pressure area, and that's a sign of strength. When NASA's Aqua satellite flew over System 93L in the Caribbean Sea on Nov. 10 at 06:23 UTC (1:23 a.m. EST) it showed the strongest convection, highest thunderstorms and strongest rainfall southwest of Puerto Rico, over open ocean.
The colder the cloud top temperatures, the stronger the convection (rapidly rising air that forms the thunderstorms that power a tropical cyclone) and uplift. The Atmospheric Infrared Sounder (AIRS) instrument showed some cloud top temperatures in System 93L as cold as or colder than -63 degrees Fahrenheit, indicating strong uplift in the low pressure area.
System 93L is considered a weak surface trough, or an elongated area of low pressure. It is producing thunderstorms over parts of Puerto Rico and the Northern Leeward Islands today. Upper-level winds continue to remain moderate, so it will be unable to develop any further today or tomorrow.
That low continues to remain south of Puerto Rico and moist, humid air is moving around it over Puerto Rico and the U.S. Virgin Islands. In addition to System 93L, there is also a nearly stationary front that is expected to move slowly northeast to southwest across the U.S. Virgin Islands today. In addition, the National Weather Service in San Juan noted that another boundary has formed to the north of the area and is expected to move southeast across Puerto Rico on Thursday. All of these things spell heavy rainfall for Puerto Rico, so the National Weather Service (NWS) has issued a Flash Flood Watch until Friday morning.
The NWS said today, Nov. 10 that "Rain has also fallen over the extreme northwest portion of Puerto Rico and some isolated flooding may already be in progress. Isolated areas of 1 to 3 inches may fall in the mountain areas and on the southern coastal plains this afternoon and tonight."
Because the ground is already saturated, and river levels are above normal it would not take much more rainfall to cause flash flooding, river flooding and mudslides in any areas of high terrain.
The National Hurricane Center noted that as a result of System 93L, locally heavy rainfall is expected over portions of the Northern Leeward Islands and Puerto Rico over the next day or so, as the low moves northeastward.
NASA Goddard Space Flight Center
, Greenbelt, MD