Another Labor Day, another tropical system. Late this morning, TD #6 is slowly becoming better organized out in the central Atlantic, despite the presence of some moderate southwesterly shear and a weak Saharan dust layer to its west. The five day forecast shows an initially northwestward track bending back to the west at later time periods, and by all accounts this looks to be a very reasonable forecast through the work week. Let's delve a little into the details, however.
The deep trough of low pressure currently located to the north of TD 6 is unanimously projected to lift out over the next couple of days, allowing for ridging to build in to the north of the cyclone and turn it back toward the west. There are indications that this is starting to occur this morning. Looking at water vapor satellite imagery (e.g. http://www.ssd.noaa.gov/goes/east/catl/loop-wv.html), note the anticyclonic turning in the moisture fields around 55W as well as the lack of southward motion of the trough itself. There is nothing upstream or at higher latitudes to suggest that these trends will not continue over the next two days. In the interim, however, the proximity of the cyclone to the trough should keep intensification and organization in check.
But, at the end of that two day period, the available guidance is also fairly unanimous in showing a setup ideal for trough-related intensification (or constructive trough interaction). I refer you to my Learning Series topic on trough interaction for more of the nitty-gritty behind the process (see http://flhurricane.com/cyclone/showflat.php?Cat=0&Number=64429&an=0&page=0#64429). As an example, take a look at the 325K potential vorticity loop of the from 06Z this morning, available for the next few days at http://moe.met.fsu.edu/cgi-bin/gfstc2.cg...;hour=Animation. Potential vorticity is a unique field that combines the effects of vorticity, stability, and vertical structure into a single tool, making it ideal to use to diagnose this sort of event. Of particular interest in that loop is the evolution from 60-72h into the future. Note the narrow tail of higher PV values extending west-southwestward from about 30N/30W to the cyclone. The scale and intensity of this feature matches that of the cyclone fairly well, an ideal scenario for trough interaction. An examination of other models (Canadian and UKMet; is too coarse to show such detail) shows much the same evolution as well.
What this means is that in the 2-4 day time period, we will need to watch TD 6 (future Florence?) closely for the potential for some steady to rapid intensification as shear values quickly plummet underneath the building ridge and the storm potentially interacts with the departing trough.
As an aside, the reason we are getting this sort of pattern evolution is due to the development of a block in the Gulf of Alaska, thanks to a developing cut-off low in the region. This setup tends to promote ridging over the intermountain west, a trough in the eastern US, and a ridge over the Western Atlantic and is a common scenario for this time of year. The same 325K PV loop referenced above is a good tool for showing this as well.
In the longer term, the big question is going to be how far the cyclone continues westward underneath the developing western Atlantic ridge. The model guidance (including now) again is unanimous in centering this ridge somewhere near Bermuda, but with slightly different locations of its western periphery. On the whole, though, the western periphery is located along the immediate southeast US coastline. This would tend to favor a sharp recurvature along the northern Bahamas and out to sea. Two complicating factors arise, however: one, the block over the Gulf of Alaska is projected to break down in the 4/5 day time period, allowing the flow to become progressive again across North America and into the Atlantic, and two, what downstream effects -- if any at all -- will be felt in the cross-Pacific pattern thanks to the recurvature of Typhoon Ioke into the midlatitudes. Models tend to handle such events rather poorly in terms of the downstream features, so I'm going to keep a little bit of skepticism for now in the evolution of the overall pattern. My belief is that the effects from Ioke will be minimal and not felt this far east, however. Needless to say, the potential is there for a recurvature well east of the Bahamas...or a threat to the US in the 8-10 day period. It's way too early to tell, however, and we have plenty of time to watch the pattern evolve.
Needless to say, we'll be watching TD 6 through the course of the week.
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