Tenacious Irene has finally started to turn more towards the north, heading off into the open seas, likely only to impact shipping lanes and, down the line, Greenland in the next five days. Sounds kinda dull, but as always with this tropical season, there's a caveat.
The majority of tropical cyclones that recurve out to sea undergo transition, where they morph from a tropical system into a weather system that resembles the fronts that we see come across the United States every few days. There are many changes during this process -- maximum winds move further away from the center, the system becomes asymmetric, the energetics of the storm change, and precipitation and convective structures change, among other things -- some of which are pretty apparent just from satellite imagery. The has been forecasting Irene to undergo ET for some time now, ultimately becoming a vigorous cyclone. Most of these estimates have brought the storm to winds of about 50kt after transition -- not a lot to write home about with a tropical storm, but pretty intense for an storm in August. Model guidance is actually forecasting the storm to potentially deepen even further, with pressure forecasts into the 960mb range in about 3 days, as it moves north near Greenland.
Given a narrowing upper-level trough to the NW of the storm, warmer-than-normal surface temperatures over water (helping to enhance the temperature gradient between ocean and land), and a quick transition from favorable tropical development conditions to favorable development conditions, it is likely that we will see warm-seclusion development out of Irene after it undergoes transition. What is this development, and why is it important? Warm-seclusion development is where the storm reacquires a low-level warm-core after transition, often with markers in the pre-transition environment (all mentioned previously; the paper with this info is scheduled for publication late 2005/early 2006). These storms often are much stronger than their cold-core counterparts and are potentially more damaging, as they both have more intense winds nearer the center and a wide expanse of very strong winds from the center -- features of warm-core and cyclones combined into one. Thankfully, Greenland isn't very populated. Nevertheless, Irene looks poised to follow in the footsteps of Irene from 1999, albeit not as strong with the development, but still stronger-than-normal -- especially for this time of year. You can follow it's evolution from the Cyclone Phase Space product at http://moe.met.fsu.edu/cyclonephase/.
After looking more impressive this afternoon in terms of convection, ex-TD 10 has been looking a little sickly of late near the diurnal convective minimum. Every so often, a new convective burst tries to take hold near the center, but none have yet to maintain themselves since the main area of convection died off earlier this afternoon. Microwave satellite imagery confirms that the low-level organization of the storm isn't what it was, say, over the past couple of days, though a weak low-level center is still present with the storm. Wind shear values are still improving in the area and while the upper-level flow is still largely cyclonic, conditions are improving for this disturbance. In fact, with divergent upper-level winds near the disturbance, it is kind of surprising that convection has not maintained itself better than it has this evening. As HF mentioned in a post on the main board, the current state of the storm suggests it is no longer as likely to redevelop than before and needs to show something overnight or else the circulation may be in danger of opening up. We are heading towards the diurnal max, and all it can take is one convective burst to get things going again, so it bears watching overnight.
The weakness in the subtropical ridge that has been steering this storm largely towards the west continues to progress westward itself, now located near the eastern Bahamas (where it was near the Lesser Antilles yesterday evening). Thus, the disturbance should continue largely towards the west-northwest over the next couple of days, with the upper-low out ahead of it moving in the same general direction at the same time. Where the disturbance -- whatever comes of it -- goes from there will be determined by where the weakness of the ridge lies in 2-3 days' time. Models are still forecasting there to potentially be a coastal low -- or at least a broad surface trough -- along the East coast of the US in the coming days. If this bears out, the storm will likely tend to recurve towards the north...ditto if it starts to develop into a substantial system. The weaker it remains and the weaker the troughing is along the coast, the further west the disturbance will get. Most dynamical model guidance suggests that the disturbance is more likely to recurve than not for now, so that will be the current call -- subject to change given that this is a low-confidence forecast given a weak disturbance.
Further east in the basin, a couple of waves are showing themselves at times, but there's not a lot of organization with any of them. They may continue to perk up from time to time, but I would not put a lot of stock in them developing. Model guidance still suggests a very strong wave coming off of the coast of Africa in about 3 days now, overtaking and assimilating the energy along the coast of Africa now as it does so, developing it and tracking it west along 14-16N through the end of the period. Given no change in the call on the feature and strong correlation between the models on this occurring, it's a good bet we'll have something else to watch early in the weekend. Now there are hints at another disturbance behind it as well in 5-6 days over W. Africa...it's that time of year, so we'll watch it as well. Ridging and SSTs are favorable along the coast, moisture is increasing (and projected to continue to do so), but those pesky troughs are still there along 40W. If those can move north or out of the picture, conditions will become a lot more favorable for something to develop. Maybe later this month.
Back west, a large conglomoration of energy is situated in the central Caribbean to extreme eastern Pacific Ocean. The western extent of all of this is the home of the next E. Pacific invest -- and the subject of some vigorous development by the model in particular. Given it's organization and very favorable upper-level conditions -- and largely untapped waters -- it should get going over the next few days. The Caribbean feature has a shot as it pivots NW around this feature towards the Bay of Campeche. Not a strong bet yet, but worth watching. This would be another Mexican/maybe S. Texas impact. Further west in the Pacific, the old invest near Baja California probably was classifiable earlier today, but given the current trends in organization, probably isn't there now. It's heading for cooler waters, so a slow spin-down is likely with this one. The remnants of Greg are essentially done, while the remnants of Fernanda continue west in the Central Pacific under southerly shear and in a stable environment...not likely to regenerate.
There's a good bet there's something classifiable in the Atlantic in the next 5 days, with an outside shot at two somethings. We'll watch all of the areas noted above for any development and I'll update as conditions warrant. If ex-TD 10 gets going again tonight, that might be sometime tomorrow...otherwise, likely Friday afternoon or late Friday night.
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