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A Brazilian tropical disturbance to watch

By: Dr. Jeff Masters, 3:15 PM GMT on March 05, 2010

An area of disturbed weather has formed off the coast of Brazil, near 18S 38W. This disturbance has the potential to develop into subtropical or tropical depression early next week. Satellite winds estimates from the WindSat instrument show an elongated area of converging winds, but no organized surface circulation. Satellite loops show little organization to the cloud pattern, and only limited heavy thunderstorm activity. Wind shear over the region is about 20 knots, which is rather high, and should keep any development slow. Sea surface temperatures are about 28°C, about 1°C above average, and plenty warm enough to support a tropical storm.

Figure 1. Morning visible satellite image of the Brazilian disturbance.

Several global models, such as the ECMWF, UKMET, and NOGAPS models have been developing this system in recent runs. Phase space diagrams form Florida State University confirm that this storm is expected to primarily be a warm-cored system, meaning it will probably be classifiable as a subtropical or tropical storm if it attains surface wind speeds of at least 39 mph. The system is capable of bringing heavy rains to the Brazilian coast while it is in its formative stages over the next few days, but I doubt that these rains would be heavy enough to cause flooding concerns. By Monday, the storm should be headed southwards or south-eastwards out to sea, and it appears unlikely that Brazil would see tropical storm-force winds of 39+ mph from this system. I give this storm a low (< 30% chance) of developing into a tropical or subtropical depression by Sunday.

Figure 2. The MODIS instrument on-board NASA's Aqua satellite captured this true-color image of a rare tropical cyclone in the South Atlantic ocean just off the coast of Santa Catarina and Rio Grande do Sul, Brazil's southernmost states. The National Hurricane Center in Miami estimated the storm was a full-fledged, Category I hurricane with central winds between 75 mph and 80 mph (121 kph to 129 kph), making it the first hurricane in the South Atlantic in recorded history.

Comparisons to Cyclone Catarina
Brazil has had only one landfalling tropical cyclone in its history, Cyclone Catarina of March 2004. Catarina is one of only six known tropical or subtropical cyclones to form in the South Atlantic, and the only one to reach hurricane strength. Tropical cyclones rarely form in the South Atlantic Ocean, due to strong upper-level wind shear, cool water temperatures, and the lack of an initial disturbance to get things spinning (no African waves or Intertropical Convergence Zone exist in the proper locations in the South Atlantic to help spawn tropical storms). Today's disturbance is located much closer to the Equator than where Catarina formed. Thus, it has warmer waters to work with, and potentially less wind shear.

I'll probably do a quick update this weekend.
Jeff Masters


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