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Early thoughts on TD 7
      #73616 - Sun Sep 10 2006 10:36 PM

Late this evening, Tropical Depression #7 has formed in the central Atlantic, located about 10 degrees south and east of Hurricane Florence, ready to bear down on Bermuda sometime early Monday. While the end game solution for Florence is fairly well set in stone, with a swipe at Bermuda on Monday followed by transition into a significant extratropical cyclone and potential impact to Newfoundland into mid-week, such is not as clear for the newly formed TD 7.

The initial pattern is somewhat complex. Hurricane Florence is a large system, with impacts felt well away from its center, including a significant moisture connection that extends to the south all the way to the Virgin Islands. This moisture channel is immediately ahead of TD 7 and is fairly stationary at this time. Complicating factors is a weak upper-level low located just northwest of TD 7 at this time, elongated north-south along 56W and centered about 25N. This upper low is shielding the formative cyclone from the negative impacts of Florence's inflow channel and possibly serving to fracture the trough that is starting to extend to the south from the hurricane.

Models are in good agreement that as Florence enters the midlatitudes, it will turn toward the east and cause the flow to become relatively progressive in the North Atlantic. What this means is that the ridge that TD 7 is on the southwest side of -- and the one that has largely steered Florence to this point -- will begin to break down and slide eastward. As an aside, Florence & the amplification of the midlatitude pattern should help kick out the upper trough over the eastern Atlantic, setting up a favorable regime for a wave near the coast of Africa in the next few days.

In response to the midlatitude ridge moving out, models show two smaller ridges building in across the subtropical Atlantic -- one centered north of Puerto Rico and another centered just west of the Cape Verde Islands. This latter one should have no impact upon TD 7, but things get tricky when it gets to the other one. How fast TD 7 moves along in the next few days will be key. Most of the models do not have a good handle on (or really even a signature of) TD 7 right now and show what is there slowly sliding toward the west with time. If this is the case, then TD 7 could enter a region favorable for development as well as allow it to slide westward with time. This is contingent on the weak upper low to TD 7's north being significant enough to fracture the southern end of the trough left behind by Florence's departure into the mid-latitudes.

Now, the alternate -- and more likely -- scenario. History has shown that when one tropical cyclone undergoes extratropical transition, any that trail it tend to recurve in much a same manner to that initial storm and occasionally as a direct result of the prior storm. With TD 7 actually developing, it will be a more vertically coherent cyclone than depicted in the models and more likely to feel the midlatitude westerlies, which are fairly significant at about 30N from Florence on westward. The upper low near TD 7 is showing signs of elongating itself to death and may well not be enough to fracture the developing trough.

All of these latter factors plus climatology suggest that TD 7 will be a fish storm. But, there is more than enough uncertainty in what will evolve over the next few days in terms of what TD 7 does development-wise as well as what the extratropical transition of Florence does to the midlatitude pattern to leave open the other possibilities mentioned. Note that extratropical transition events generally compromise model skill across an entire basin for several days after the event, so any model solutions must be taken with a grain of salt for now. Watch what Florence does and how the upper low northwest of TD 7 evolves over the next two days, as the evolution of those two features will provide insight into how TD 7 is going to evolve. But, for right now, TD 7 has an uphill battle with some shear before it can be deemed any threat to anyone.

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