T.D. 12 forms, Gulf of Mexico development possible mid-week
Tropical Depression 12 formed early this morning in the Eastern Atlantic from the African wave that had been tagged 92L the past couple of days. It is the main feature in the Eastern half of the Atlantic. The Western Atlantic is very active convection-wise today. The convection is being aided by a lingering boundary left behind by Irene, a surface trough of low pressure moving across the Western Caribbean, and an upper level low pressure system near Puerto Rico. The upper low is causing divergence aloft as it interacts with the surface features to the west, forming the convection near Bermuda as well as aiding in convective development with the Western Caribbean surface trough. The Gulf of Mexico is quiet for now, dominated by upper ridging and NW flow aloft.
Atlantic Wide View
TD 12 Discussion
Tropical Depression 12 had its first advisory issued at 5AM this morning. The NHC has discussed the current issues with easterly and northeasterly shear which is causing the system to be heavily weighted to the west. I discussed in my last blog that I believed TD 12 would strengthen gradually, rather than quickly until it reached 50W. I am still sticking by that statement and the NHC agrees as they have the wind speed strengthening 15mph after TD12 moves past 50W. The reasoning for that is the sudden jump in tropical cyclone heat potential as well as a less sheared environment.
TCHP: Notice the spike at 50W
We also discussed in last night's blog that the models had TD12 moving too far north and I am still staying with that statement. The reason for my track goes hand in hand with the strength forecast. The strength is going to have a lot to do with how far north TD12 goes before reaching the longitude of the Antilles. With a more gradual intensification, I don't expect 92L to gain much latitude until after it reaches 50W. I believe you can put the pieces of the puzzle together and figure out why I believe that. After 50W, a more true WNW motion should begin and this should put TD12 (Katia) dangerously close to the Northern part of the island chain, as well as Puerto Rico. It remains to be seen just how much of weakness forms from a cut-off low to mid-level low pressure system, which would be located to the northwest of then Katia. The strength of the low will determine the size of the weakness. It is interesting to note that the 12z run of the European is much weaker with that low, and thus sends Katia on a more westerly course after missing the islands to the north. The pattern is favorable for landfalling U.S. tropical cyclones, with a neutral to weakly positive NAO. However, we must not forget about our friends in the islands as they may have to deal with Katia before us, just like with Irene. I'm not going to speculate on a path after the islands as that is nothing more than a guess at this point. However, I feel fairly confident in a track directly at or just north of Puerto Rico at this time.
Elsewhere, we may need to watch closer to home before we even worry about Katia. The models are starting to latch onto the surface trough of low pressure that is moving northwest in the Western Caribbean. The trough is helping to ignite a good deal of convection today and it should be closely monitored the next few days. Until today, the NAM was the only model being aggressive with development. However, the Euro jumped aboard at the 12z run, which is enough for me to really respect this wave's potential. The GFS had hinted at this possibility several runs back, but dropped it and is possibly picking back up on it during the 12z run. We will need to pay attention to this wave as the Texas ridge lifts out to the north and northwest and opens the Gulf just enough for development to occur. It should be easier for any disturbance to develop now than it was with Don when the Texas ridge was firmly in place, causing shear and subsidence across the Gulf. Upper level conditions may not be ideal, however, so I don't expect anything stronger than a moderate tropical storm for now. The water temperatures in the Gulf are warmer than any other ocean basin in the world, so we must respect what can happen if upper level support is adequate. At this time anywhere from Central Louisiana to South Texas looks to be at risk from the possible tropical cyclone. We still have a few days to watch, so stay tuned.
Gulf of Mexico SSTs