This is the MSLP forecast from the 6z GFS at hour 384- the longest this model goes to. That's roughly 14+ days out. What you are seeing is a pair of tropical cyclones going down two different paths. The one on the left would most likely take a path similar to Isaac, while the one on the right would go out to sea. While this forecast has the slimmest of chances to verify, the idea that this is even a possibility to the GFS is quite alarming. In today's Tropical Corner, we'll investigate why a late season tropical burst of activity is possible.
This is the 500mb height anomaly chart for hour 168, meaning this forecast is valid 168 hours from now. The GFS Ensembles that made this chart are showing a deep trough in the West and a strong ridge in the East, typical of a strong negative PNA pattern. It is common for tropical systems to gain an advantage when a pattern like this is present, as the ridge in the East diverts the jet stream north. Because the jet stream is diverted north, the upper level winds in the Atlantic are fairly low, enabling thunderstorms to strengthen and thus enhance potential for tropical cyclone formation.
I find a few problems with the GFS solution, and even the ECMWF, which is also showing tropical troubles by the end of the next 10 days. This ridge should die off by the time 8 days is up from now. After that 8 days, the negative PNA pattern will collapse and the trough will progress east into the central and eastern parts of the nation. This trough's movement east will push the jet stream south and thus drop tropical cyclone formation potential over the Caribbean and Atlantic areas. The timing is not matching up.
Climate models are projecting unfavorable conditions for tropical cyclone formation as well, with the CFS long range model hinting at strengthening upper level winds in the 10-15 day time frame, which would drastically hurt tropical cyclone formation chances.
Andrew
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