As of 11pm this evening, Irene is back to tropical storm status, with winds of 40mph. Earlier this evening, some of the data (namely QuikSCAT) were inconclusive as to whether or not there was a low-level center. However, the storm was right on the edge of the pass and, as we've seen, the technology isn't always perfect. It does a good job, particularly with wind speeds, but with small systems and circulations, it does have trouble from time to time. Nevertheless, visible imagery was absolutely conclusive -- there was and is a low-level center with the storm, with good banding features to the south of the center of circulation. Microwave imagery (SSM/I pass this evening, available from the website) confirmed these banding features, both at the low and mid levels, suggesting a system with the building blocks firmly in place. The deepest convection is building closer to where the center is located on the NW side of the overall cloud pattern, shear values are down to around 10kt, and waters are progressively warmer to the west.
This storm should intensify, albeit at a modest rate given the dry air in the environment...though I refer you all back to the 11a discussion about the impacts of the dry air keeping Irene as a smaller storm. While it seems counterintuitive, I've heard that idea thrown about from another scientist in the past; we'll see how well it holds up. Nevertheless, keeping the energy bundled in one area would serve to help provide a focus for intensification. Given all of that and the trends we are seeing with the storm, I feel the need to bump up my intensity forecast from earlier this afternoon. Modest intensification is likely early, with the storm likely to peak near cat 2 intensity before landfall. It has an outside shot at major hurricane status, but I'm not willing to go there right now.
Track forecast is more complicated, I feel. There is a building ridge near and over the system that is projected to build west with time. How far west will it build? It is a pretty strong ridge right now -- 595dm at 12Z at 500mb in Bermuda, 593dm heights over Florida at 00Z -- but will that hold? The upper-low that we've been tracking for a week now has begun to open up to the west of Irene, but with nothing to really kick it out, what is left of it is likely to slowly spin down over the western Atlantic. There is another upper-low over the Gulf of Mexico that is slowly moving NE; here too, however, there is little to move it out of the picture. South of Wilmington, the flow is largely to the west in the mid- to upper-levels -- but we're more interested in 4 days from now as opposed to right now. But, the flow pattern is largely zonal (west-east) across the eastern US. Impulses continue to move across the northern tier of states, but are having a hard time getting further south than southern Virginia and Kentucky. There's nothing in the immediate future to suggest that this will change in the next 3 days.
The culprit behind all of this is the happenings in the eastern Pacific. A vast upper-low is located between 40-50N along 150W south of Alaska with a ridge of high pressure to its north. This is a classic blocking scenario -- a Rex block, high-over-low, as we call it -- and there isn't a whole lot to change this, either. It would take a change in this to substantially change the pattern across the eastern US at this point, failing to see anything more immediate to the region that could effect a change, and I don't see that happening. An impulse is butting up against this block at 50N/170W, but it is showing signs of slowing down and I believe it will weaken and ultimately undergo trough fracture, heading to the north around the blocking ridge (with some of the energy potentially feeding the larger upper-low). Further southwest, an upper-low SW of California is helping to keep the pattern stagnant across the western US right now as well. Simply put, there's not going to be a lot of major changes over the next few days.
That said, given the building ridge in the Atlantic, there is likely to be a weakness along the coast -- give or take some distance, which could be key in determining the ultimate path of the storm -- as Irene approaches. But, whether or not there will be an impulse/shortwave trough to slide into this weakness and capture the storm is another story entirely, and I'm not convinced there will be one at this point in time. This leads me to believe that the storm will slow down as it approaches the coast. What Irene does from there depends upon how strong it is and the exact placement of the ridge and weakness between it and another ridge forecast to the west and southwest.
From my experience, when we see a pattern similar to this one evolve, a weaker system tends to move slowly towards the west and northwest, while a stronger storm tends to drift more towards the northwest and later gets caught by something down the line. This is partially due to the natural beta drift enhancement for a larger/stronger storm and partially due to a stronger storm feeling more impact from the upper-level steering pattern. For a storm of Irene's projected intensity, a west-northwest path at a slow rate of speed is likely. My current thinking is to the left of the track and left of most of the global model guidance for the reasons noted above. This brings landfall to around 6 days from now between St. Simons Island, GA and Wilmington, NC. However, the entire coastline from S. Florida to the Mid-Atlantic needs to watch this storm. While landfall may occur in the southeast, as the storm moves further inland, it will ultimately get caught up in the midlatitude pattern and directed towards the north and east -- i.e. the Northeast US -- in the next 7-10 days. As always, this thinking is subject to change, but represents my best thinking at this point in time.
We're in a pattern where the model guidance has been too far to the right for quite some time with this storm, and given the strength of the ridge out there now, it is tempting to jump to the conclusion that a similar pattern to last year's storms may unfold with this one. I don't have the full suite of data to make that claim, however, but note that like all model trends, it must be watched and accounted for in analysis and forecasting. As an aside, for a good breakdown of the available model products and their uses, see the posts by Random Chaos and myself about 30 posts down the thread entitled "Irene Becomes Tropical Storm Again ... Threat to Carolinas Increasing."
Brief future note -- some models are still picking up on a possible development in 3-4 days in the east-central Atlantic, though all of them show just a very weak feature. Worth watching for, but not too worried about this one yet. Of more interest is a massive surge projected to exit the coast of Africa in 5-6 days, picked up by most of the major models. It is projected to come off at a pretty high latitude, but given its vast expanse could help spin up something along the . At the very least, it is likely to be the precursor wave that starts things going in the eastern Atlantic, right about the time as conditions (synoptically & climatologically) get going. Like HF said, the flurry of activity is just about to hit us.
Brief E. Pacific note -- Fernanda has peaked in intensity for now, but I'd be willing to bank that it peaks over hurricane intensity in the next day or so. These E. Pacific storms, just from following them over the past 10 years, seem to be a lot tougher to nail intensity-wise than one might think. Commonly, you'll see a lot of storms in a "pristine" environment (which I'll define as just one storm heading away from Mexico with sufficient SSTs, low wind shear, and sufficient moisture) strengthen much more than the initial forecasts, often to minimal hurricane intensity, but then not get as strong as the later forecasts would suggest. It's not a data problem, as we have good SST data across the oceans, but it just seems like the intensities are tougher than one might think. The system behind Fernanda -- invest 98E -- has a good shot at developing, but its intensity will likely be kept in check due to its proximity to a rather large sized Fernanda ahead of it. Nevertheless, slow development is likely there over the next 2-3 days before it meets colder waters as well.
Next major update projected for sometime Friday or Saturday, subject to change if conditions warrant.
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