What is likely to develop into Tropical Storm Isaac was designated as Tropical Depression 9 at 5 AM this morning. The storm rolled off of Africa a few days ago and has been steadily marching westward -- we've already mentioned it a few times as "potential" down the line. Well, that potential is developing and has the potential to be a pretty potent tropical cyclone in a couple of days time as it continues to organize.
It is in a relatively low shear environment, has decent atmospheric moisture to work with and should develop slowly, steadily over the next few days. There are a few areas of less-than-desirable wind shear in its path between now and 48 hours from now which may slow its development but model guidance weakens those wind shear areas near Puerto Rico by the time the storm passes just to its south on Thursday morning.
There are three big questions with the storm...and the answers to a couple of them determine the third.
1) Track. An upper level ridge extends to the north of what will be Isaac and will help to push this storm along to the west. Past performance this year with the subtropical ridge has been to keep the ridge a bit stronger than advertised in most modeling (Ernesto tracked a bit south of where modeling took it). The strength of the subtropical ridge as the storm moves through the Caribbean will be key in determining when/where the storm begins to tug north and then perhaps northeast. In terms of model guidance, there's not a lot of difference in track as we get out to seven to ten days out...the guidance "uprights" if you will are generally spread between 75 and 85 degrees W...taking the storm either over the eastern Bahamas or into the Gulf of Mexico. This far out -- that's not a huge spread but it does mean a huge difference in terms of impact since the eastern most spread of modeling would be a miss for Florida while the western most brings it into the Gulf.
2) When does this storm turn north and what will the result be? That's the $64 million question. You can tell by the model "spread" below that Hispaniola and Cuba lie near or close to the path of this storm this weekend -- given both islands are riddled with areas of higher terrain there will likely be some impact on Isaac's intensity as it pushes through the islands unless the ridge is strong enough to keep the storm just south of the storm on a Charley (2004) or Dennis-like track from 2005. The Euro produces such a scenario, bringing Isaac over Western Cuba (the least mountainous part) before pushing it up the eastern Gulf of Mexico. Such a scenario is probably the strongest-case scenario for the storm...and would result in a Gulf "hit", likely over Florida's Big Bend or the peninsula somewhere. The storm would arguably need to travel slower and farther south for it to avoid any weakness in the subtropical ridge pulling it northwestward over the islands. The Euro does that...the GFS is faster and slightly north.
A GFS-like track takes the storm over central and eastern Cuba and then up the eastern side of Florida, into Georgia before gradually pushing the storm's remnants northeast and off of the Outer Banks. The GFS track is also much faster than the Euro (see the graphic below showing the comparison between the GFS and the last two forecasts of the Euro) and has the storm's remnants pushing northeast out to sea the middle of next week.
3) Does it hit the US? Odds are leaning strongly towards at least some sort of US hit. If the storm crosses over Haiti and the Dominican Republic, the storm may not be as strong as it would be with just a brush of Western Cuba. A Fay-like track (from 2008) would bring the storm over both islands before it turned north and went over Florida. In Fay's case the storm was impacted by three landmasses...that scenario is on the table but Isaac would be a stronger storm when it traversed over the Caribbean islands and thus a bit stronger than Fay. While it would be a bad scenario from those who want bad storms, a Fay-like track probably is one of less impacts for the US than the scenario the Euro is outlining in this morning's computer model run. The question regarding US hit is whether it's East Coast or Gulf...that part is too early to pin down.
More: Tropics Tracker