It appears the quiet end of week may be ending, as mid August starts to roll in, the activity hinted at earlier for this week is already starting to show up in the form of two waves in the central and eastern Atlantic. Now starts the time of year where the models can be used a bit more, and many show strong development later in the week.

One of the waves already has the invest designation (92L) the other not yet (But may soon)., and these both are worth watching over the next few days, and indeed it is possible to have one or more named systems in the Atlantic sometimes this week.

More to come as it happens... Those in the Lesser Antilles, especially the Leewards, will want to keep a close eye on 92L. Longer range models suggest it may be something the mainland US has to watch later on as well.

What do you think this system will do? Take a guess in the 92L Lounge.

- Mike C

There are some intriguing changes suggested by the balance of the Global numerical models, regarding the orientation of the long wave features from the Pacific, across north America, and into the western-central Atlantic at mid latitudes.

Currently, the North Atlantic Oscillation is rising very fast; it has recovered nearly 1 Standard Deviation in just less than a week, and continues to rise. The 7 and 7-10 day outlooks regarding this index suggests that it will rise to approximately +.5 SD, or a total recovery of nearly 1.5! Normally in the summer time, subtle changes in this index will have less correlation to patterns at middle latitudes, do to a typically more nebular flow (short wave lengths among a weaker gradient field). However, we have been running along with anomalously strong gradients this summer, which has kept both the wave lengths a bit longer than usual, but also has provided for steeper gradients in the overall geopotential medium. If you understand the NAO (read primer above if you do not), a positive NAO correlates to a strengthening of the subtropical ridge in the western/central Atlantic Basin.

In simpler terms, the flow thus far this season has been more amplified than normal, and that opens the suggestion to being more sensitive to changes in the NAO. That said, a 1.5 modality in the index is fairly significant during any time so one would expect a flow morphology away from the recent eastern N/A trough plague, to take place at middle latitudes. What we are seeing is, thus, a retrograde in the trough axis longitude, to roughly the Great Lakes and Mississippi Valley areas, for the middle and extended range - again, a pleasing fit for a rising NAO. This opens a lot of questions and concerns regarding longer term steering fields, to put it conservatively.

This is important because as we head into what appears more and more likely to be an active period in the 2008 hurricane season, we would like to have some intuitive idea on what that may mean, and for whom. The strengthening plausibility in the subtropical ridge nearing mid month is physically being modeled by most operational Global numerical models, but also in their ensemble means (which makes sense in the latter for the GFS, considering that the NAO index is computed off the ensemble mean). That is why seeing these waves develop and push mightily in to the Puerto Rican archipelago and/or Caribbean and Bahamas should be raising eyebrows.

Another clue as to the potentially active interval nearing in this season is the MJO related 200mb level vertical velocity anomalies. We can see that the entire Basin has lost the DVM (Downward Vertical Motion) anomalies, and has replaced the region by neutral to UVM (Upward Vertical Motion) potential. This upward motion is a reflection of strengthening anticyclonic motions in the upper levels, and the restoring force for outward motion at those levels is upwelling as a mean tendency - hence the UVM anomaly potential.

What all this means in totality is that, yes...INDEED, things should be getting active in a hurry. It would not shock this Meteorologist to see several concurrent cyclones generated during the next 2 week interval of time. And, it wouldn't be too absurd to at least consider threats to the Islands and Mainland of the U.S., given the fairly obvious paradigm shift in the pattern as discussed above.

John (TT)

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