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Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.Brazilian tropical/subtropical storm deserves a name; tornadoes rip Arkansas
The unnamed South Atlantic tropical/subtropical cyclone (90Q) off the coast of Brazil continues to spin slowly out to sea, and is not a threat to any land areas. The storm, just the 7th tropical or subtropical cyclone on record in the South Atlantic, has a well-defined surface circulation, top wind speeds near minimum tropical storm force (40 mph), a surface pressure near 1004 mb, and a modest amount of heavy thunderstorm activity on its south side. Strong upper-level winds from the northeast are creating about 20 knots of wind shear over the storm, keeping the northeast side of the circulation exposed to view. Sea surface temperatures are about 25.5°C, which is about 1°C below what is typically needed to support a tropical storm. The storm is headed southeastward out to sea at about 10 mph, and will lose its tropical characteristics and get absorbed by a frontal system by Saturday.
Figure 1. Morning visible satellite image of the Brazilian unnamed tropical/subtropical storm.
When the storm formed two days ago, it began as a cold-cored system aloft, with a warm core at the surface, making it a hybrid subtropical storm. Yesterday, the entire core of the storm warmed enough for it to be worthy of consideration for tropical storm status. Tropical and subtropical storms are so rare in the South Atlantic that there is no official naming of depressions or storms done. The cyclone had top winds of at least 35 mph this morning at 7am EST as seen on an ASCAT pass, and satellite estimates of the storm's intensity have held steady at 40 mph (minimum subtropical storms strength) for the past 12 hours. ASCAT winds tend to be biased a little low at this speed range, so 40 mph--minimum tropical storm strength--is probably the best estimate of the winds. Phase space diagrams from Florida State University are showing that the storm has grown less tropical (warm-cored) in nature over the past 12 hours. When compared to similar systems that have developed in the North Atlantic that have been named, I definitely think today's storm deserves a name. The World Meteorological Organization should act to come up with a list of names for the South Atlantic, since these storms have the potential to cause considerable death and destruction along the South American coast, and deserve to receive the extra attention naming would provide.
South Atlantic tropical storm history
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. Today's storm is probably the fourth strongest tropical/subtropical storm on record in the South Atlantic, behind Hurricane Catarina and an unnamed February 2006 storm that may have attained wind speeds of 65 mph, and a subtropical storm that brought heavy flooding to the coast of Uruguay in January 2009.. 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 storm is located close to where Catarina formed.
Climate change and South Atlantic storms
It is uncertain whether climate change may cause an increase in South Atlantic tropical storms in the future. While today's storm formed over waters that were about 1°C above average in temperature, Catarina in 2004 formed over waters that were 0.5°C cooler than average. Sea surface temperature is not the main limiting factor inhibiting these storms, wind shear is. How climate change might change wind shear over South America has not been well-studied.
Tornadoes rip Arkansas, Louisiana
It's March, and that means we can expect severe weather. Last night, one tornado was reported in Louisiana and four in Arkansas, in association with a powerful low pressure system tracking slowly across the nation's midsection. The most damaging tornado hit Cleburne, Arkansas, destroying several houses and blowing down trees and power lines. Three serious injuries were reported. The storms also dropped baseball-sized hail near Plain Dealing and Texarkana, Arkansas. The severe weather has pushed east, and NOAA's Storm Prediction Center is forecasting a "slight" chance of severe weather over Florida today, plus portions of Louisiana, Mississippi, and Alabama.
Links to follow:
Interactive tornado map
Severe weather page
Figure 2. Severe weather forecast for today from the NOAA Storm Prediction Center.
Jeff Masters
Figure 1. Morning visible satellite image of the Brazilian unnamed tropical/subtropical storm.
When the storm formed two days ago, it began as a cold-cored system aloft, with a warm core at the surface, making it a hybrid subtropical storm. Yesterday, the entire core of the storm warmed enough for it to be worthy of consideration for tropical storm status. Tropical and subtropical storms are so rare in the South Atlantic that there is no official naming of depressions or storms done. The cyclone had top winds of at least 35 mph this morning at 7am EST as seen on an ASCAT pass, and satellite estimates of the storm's intensity have held steady at 40 mph (minimum subtropical storms strength) for the past 12 hours. ASCAT winds tend to be biased a little low at this speed range, so 40 mph--minimum tropical storm strength--is probably the best estimate of the winds. Phase space diagrams from Florida State University are showing that the storm has grown less tropical (warm-cored) in nature over the past 12 hours. When compared to similar systems that have developed in the North Atlantic that have been named, I definitely think today's storm deserves a name. The World Meteorological Organization should act to come up with a list of names for the South Atlantic, since these storms have the potential to cause considerable death and destruction along the South American coast, and deserve to receive the extra attention naming would provide.
South Atlantic tropical storm history
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. Today's storm is probably the fourth strongest tropical/subtropical storm on record in the South Atlantic, behind Hurricane Catarina and an unnamed February 2006 storm that may have attained wind speeds of 65 mph, and a subtropical storm that brought heavy flooding to the coast of Uruguay in January 2009.. 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 storm is located close to where Catarina formed.
Climate change and South Atlantic storms
It is uncertain whether climate change may cause an increase in South Atlantic tropical storms in the future. While today's storm formed over waters that were about 1°C above average in temperature, Catarina in 2004 formed over waters that were 0.5°C cooler than average. Sea surface temperature is not the main limiting factor inhibiting these storms, wind shear is. How climate change might change wind shear over South America has not been well-studied.
Tornadoes rip Arkansas, Louisiana
It's March, and that means we can expect severe weather. Last night, one tornado was reported in Louisiana and four in Arkansas, in association with a powerful low pressure system tracking slowly across the nation's midsection. The most damaging tornado hit Cleburne, Arkansas, destroying several houses and blowing down trees and power lines. Three serious injuries were reported. The storms also dropped baseball-sized hail near Plain Dealing and Texarkana, Arkansas. The severe weather has pushed east, and NOAA's Storm Prediction Center is forecasting a "slight" chance of severe weather over Florida today, plus portions of Louisiana, Mississippi, and Alabama.
Links to follow:
Interactive tornado map
Severe weather page
Figure 2. Severe weather forecast for today from the NOAA Storm Prediction Center.
Jeff Masters
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.