Brazilian tropical disturbance, and tornado damage surveys

The season's second South Atlantic tropical/sub-tropical disturbance has formed off of the coast of Brazil today. The disturbance formed from the remains of a cold-core low, which sat over warm waters of 27 degrees C long enough to start acquiring tropical characteristics. We saw this same behavior this past hurricane season with the Greek storms Delta, Epsilon, and Zeta. The disturbance is mostly just a swirl of low clouds, but has seen two bursts of deep convection today. The most recent burst of convection, seen in the satellite photo below, formed in a spiral band well removed from the center. Early this morning, a more impressive burst of deep convection formed near the storm's center, but was quickly ripped away by strong westerly upper-level winds. These strong winds are expected to continue to bring high levels of wind shear over the disturbance over the next few days, and likely keep it from forming into a tropical depression. The system is expected to move slowly southwest, parallel to the Brazilian coast, and get absorbed into a frontal system to the south by Friday. No threat to land is likely, and this storm is mostly just of academic interest.


Figure 1. Tropical/subtropical disturbance off the coast of Brazil. Image credit: NASA Global Hydrology and Climate Center.

So, the academic question to ask is, does this second tropical system of the year off the coast of Brazil show that climate change is affecting the Atlantic? Only one hurricane and two tropical depressions have been observed in the South Atlantic since 1970, when accurate tracking methods became available with the advent of weather satellites. There is usually too much wind shear to allow a tropical cyclone to form, and the South Atlantic lacks an active "Intertropical Convergence Zone" (ITCZ)--that stormy band of weather that stretches along the Equator and acts as a source region for many of the disturbances that grow into Northern Atlantic hurricanes. With Hurricane Catarina of March 2004, another tropical depression in January 2004, a "near miss" tropical cyclone February 24 of this year, and now another tropical or subtropical system trying to form in the South Atlantic today, it is quite possible that climate change might be to blame. It may also be that we are seeing an active period in the South Atlantic that has a long cycle, and last repeated itself before satellites were around. Given the Atlantic Multi-decadal Oscillation (AMO) that affects hurricane activity in the North Atlantic, it is reasonable to think we might see a similar pattern in the South Atlantic.

In either case, I believe is it time that the NHC considered adopting a naming system for the South Atlantic. Had today's system intensified into a tropical storm, it would not have been given a name, since there is no naming system in place for the South Atlantic Ocean. Given the current trends we're seeing, it would be no surprise if we saw more tropical systems here in the next few years.

Tornado damage surveys from the weekend's outbreak
More damage surveys are complete from the weekend's major tornado outbreak, and it now appears that the strongest tornadoes were of F3 intensity on the Fujita scale. There were at least four F3 tornadoes in the outbreak, and these tornadoes had winds in the 158 - 206 mph range (roughly the same winds as found in a Category 5 hurricane). The Springfield, MO NWS office has posted a detailed summary of the the three F3 tornadoes that affected the southwestern portion of Missouri over the weekend, and the Central Illinois NWS office has posted a nice summary of the two F2 tornadoes that slammed Springfield, IL on Sunday, March 12. Included are zoom radar animations of the impressive hook echo, plus many damage photos. Not all the tornado-like damage from the weekend's wild weather was due to tornadoes, though--a storm survey done in along the west side of the Quad City Airport in Moline from Sunday concluded that the damage from a 107 mph wind gust done to homes, trees, and power lines on the west side of the airport was due to a severe thunderstorm downdraft (microburst).

Jeff Masters