Just a couple of thoughts on TD 12…
Given the generally well agreed on large scale pattern shownon models in a week to 10 days, TD 12/Katia needs to gain as little latitude as possible before getting to 65-70W.
The PNA, which was positive while Irene was making a run at the east coast and contributed to shortwaves amplifying west of the east coast given short wave lengths, is expected to become neutral if not negative per the NAEFS during the coming days and is currently modeled to remain in that state through day 14, at least. While there is a chance this changes in the coming days, the GFS/ECM ensembles are both in relatively good agreement in heights becoming higher than normal over the western Aleutians, forcing a trough off the west coast that shifts ridging east well into the western CONUS/Canada. This in turn forces below normal heights near the east coast, as opposed to Irene when the ridging was along the west coast and forced shortwaves to amplify over the Great Lakes given the short late summer wavelengths.
The GFS, and ECM ensembles both reflect this well at D10:
12z Euro ensembles:
12z GFS ensembles:
In addition, there will not be higher than normal heights over the Davis Strait/Greenland to force a sharper trough over the eastern US/Great Lakes which would potentially draw a hurricane farther west if in place (like with Irene).
This all may change, but the ensembles are both in pretty strong agreement on these main upper level features at this time.
Given this, I believe the chances of recurve—likely between the US east coast and Bermuda given likely troughing along the east coast—are at around 85% if this system passes north of 25N, 70W. It’s not necessarily the normal benchmark (20N, 60W) but I believe it works better in this situation given where it looks like the troughing may be located and how amplified it might be in the 1-2 week timeframe. The other 15% would be something unforeseen like an unexpected weakening of the trough or a much weaker system.
Now, working backwards, what are the odds this system doesn’t gain too much latitude in the short term?
TD 12 is currently at a low latitude, south of 10N. However, there is a weakness in ridging between 40-50W that the system is approaching. This should result in a WNW over the next couple days as TD-12 approaches this weakness.
However, the global models agree on weakening this weakness and strengthening ridging over the subtropical Atlantic north of TD 12 over the next couple of days, and by day 4 the 12z Euro showed solid ridging to almost 70W. This should result in a continued WNW motion. However, note how the heart of the ridging is located to the east of the cyclone per the ECM.
This general trend is modeled to continue. The ridging will attempt to build west, however troughing will remain along the east coast with shortwaves continuing to move east (as discussed above) which will keep the western Atlantic ridging noticeably weaker than the eastern Atlantic ridging. This should continue to allow TD 12 to continue gaining latitude for the forseeable future. I’m by no means expecting an early recurve, however given what will likely be a strong system within a few days and weaker ridging over the west Atlantic, it will be hard for TD 12 to be far enough south when it gets to 60-70W to avoid being recurved by likely troughing along the east coast. A track between the goal posts seems to be a good first guess.