First off, a quick question for everyone: do you like these daily updates when there are active storm events, or prefer fewer posts on the whole that are edited within threads on the main page? Send me a PM if you have a preference either way.

Irene is getting ready to head on out to sea, much to the relief of the Mid-Atlantic and New England coastline. Initial thoughts of it heading towards land have not borne out, so before going on to the current state of the storm, let's take a look at why this occurred. My initial thoughts on the storm were for an impact to the SE coast, somewhere from south Georgia to the Outer Banks. This was based on the upper-low to the southwest of the storm weakening, ridging building in, and the storm responding to this and moving more towards the west. There didn't appear to be a lot upstream to change this pattern -- and really, it hasn't changed that much. However, that upper-low did not weaken or move out -- in fact, it is now located in the Florida Straits -- which did not allow for the ridge to build in as expected. The upper-low located over the Gulf of Mexico stuck around but, as it moved slowly northward, started to shift the stream of shortwaves to the north of Irene even further north. These gave Irene the initial nudge northward, leaving the storm in no-man's land, where it sits right now. Ridging did build in north of Irene, but it is a progressive ridge, with a clear channel for the storm to move north through approaching from the west as I type. The slow motion around 3 days out that was initially forecast bore out, just in a much different location than predicted. It all goes to show you how the various evolution of particular features can result in quite different end-game solutions with tropical cyclones.

That said, as the weakness approaches Irene from the west, the storm should gradually move north and then northeastward around the western periphery of the ridge, ultimately recurving out into the North Atlantic. The NHC path looks good for the evolution of this one, maybe shifted just a little right after day 3. Nevertheless, at that point, it'll be undergoing extratropical transition and should be one for the shipping lanes, leaving us waiting for the next one (more on that in a minute). In the interim, Irene is starting to and should continue to become better organized, achieving hurricane strength sometime on Sunday. As noted by Stacy Stewart in the 11p Sat discussion, it's possible the storm could become stronger than the official forecast indicates, a possibility I believe to be likely. It will be passing over waters that range between 27-27.5C over the coming days, a narrow swath of "cooler" waters in fact, before reaching some warmer eddies along the northern edge of the Gulf Stream. At that point, it should be accelerating to the north and east, minimizing the impact of shear, allowing it to reach its maximum intensity at that point -- somewhere in the middle end of category 1, around 85mph give-or-take. After that, extratropical transition and off into the netherlands of the north Atlantic.

Edit -- TD 10 was de-classified on Sunday morning by the NHC. As long as the low-level center remains, it has a shot at perking up once it nears the islands. Thus, the intensity forecast below needs to be revised downward, but the track thinking of the feature remains -- if not trending a tad more westward.
Next up is TD 10. Currently feeling the effects of some southwesterly shear, the depression is showing signs of being located further south than the NHC estimate by about 1 deg of latitude, as alluded to in the 11p discussion. Two concurrent microwave imager passes (SSM/I and TMI) suggest that the low-level center may be located near 14N/46W, but also giving hints of it being merely a local vortex rotating around a broader center of rotation near 13.3N/46W. Interestingly, despite the current state of the depression, it is still kicking up high surf along its path, as evidenced by readings of nearly 10ft waves from buoy 41041 located just north of the depression (note: the wind obs from this buoy are evidence used in the above location of the center as well). Most of the convection associated with the depression is on the NE side of the storm, a feature likely to continue for the next day or so. It is worth noting that the convective "blob" visible on this evening's satellite imagery SE of the depression is largely a response to convergence at low levels along the ITCZ and is not a candidate for development.

The track forecast is problematic for the depression at this time. Most of the global models do not capture the depression all that well -- or at all, or lose it quickly -- so we've gotta look at water vapor imagery and the general flow pattern it suggests and as projected by the models to really track this one. The GFS has the best depiction of the storm, in the 850mb vorticity product, and suggests that it should ultimately start to recurve out to sea in about five days in response to a trough in the west-central Atlantic. The NOGAPS does not really capture the storm at all and keeps what little signature is there weak and heading generally to the west. It does not develop the aforementioned trough that the GFS does, nor does the GFDL. The UKMET is somewhere between the GFS and NOGAPS. All three (GFS, UKMET, NOGAPS) differ as to whether or not the upper-low and block pattern between 40-45 W north of the storm will leave behind a weak mid- to upper-low over the central Atlantic, and thus all three differ in how TD 10 reacts to that as well. NOGAPS is again weaker with this feature, while the UKMET is a bit stronger, drawing the storm a bit further northward in about 5 days...but still further south than the current NHC track.

My feeling, as I mentioned yesterday, is that unless the storm starts to move northwest almost immediately, it will head more towards the west and west-northwest. The upper-low is showing signs of starting to move northward out of the region, with a weak upper-low near 45N/40W showing signs of moving north over the Rex block along 40W and not leaving behind any remnants to the south. The ridge that is currently steering Irene looks to be building in, with a southward reflection of the ridge starting to show up on water vapor imagery, particularly around 60W. As this moves a bit further east and the trough starts to move out, I expect the depression to be steered on a general west-northwest track through 5 days. This would bring it towards the northern Lesser Antilles in about 4 days. My track is thus similar to how the 12z UKMET tracks the 850mb vorticity center of the storm and further west of the 18z GFS, 18z GFDL, and 11p NHC official track, but east of the weak signal that the 12z NOGAPS model tracks into the central islands. I will say that it is too early to be confident on any path with this storm...this is a low confidence forecast, unfortunately, meaning those in the islands should just watch this one, just in case, for now.

A potential complicating factor in this is if the storm continues to try to reform to the NE nearer the deeper convection, as Irene did. However, given the current trend of the center to be further south than previously thought and the lessening effects of the trough and southwesterly shear upon the storm, I don't view this as likely in the long-term. Intensity in the short-term should be kept in check by this feature, with the storm gradually strengthening after about 2 days under a more favorable upper-level environment -- and more favorable SSTs & oceanic heat content. I do feel that the intensity of the storm will thus be a bit above NHC projections, but not by much -- and certainly not to the GFDL's standards, which bombs the storm into a category 2 hurricane from almost nothing within the span of 24hr at the end of the period.

Nevertheless, this will be one to watch for some time to come, as the track should bend a little more towards the west later in the period as another ridge builds in north of the storm. Any US impact, however, is over a week away and it is terribly imprudent to suggest that any area will be affected by this storm right now, particularly given the uncertainty amongst the model guidance and steering currents out to 3-5 days. Interests in the northern and central Lesser Antilles should watch this one; unless the NOGAPS verifies, however, the storm should pass north of Puerto Rico and the Greater Antilles.

Looking at the rest of the basin...models still suggest some lowering of pressures in the Bay of Campeche/NW Caribbean over the next 4-5 days, perhaps in response to the tropical wave in the central Caribbean at this time. It's not a candidate for development now -- the convection is largely being forced by divergent flow aloft to the east of a broad upper trough extending from the upper-low over the Florida Straits -- but we've seen little features perk up in this region twice already this season, so it bears watching. The African wave train is still lining up over the continent, but the feature near the coast right now isn't looking all that impressive. A broad area of low pressure came off the coast along with the wave -- it actually is a pretty deep area of low pressure, with a signature evident on water vapor imagery -- but shows no signs of organization at this time. Conditions are projected to become more favorable for development in the next few days, and considering the model guidance and the waves lined up over Africa now, we'll probably see something start to perk up near the end of the period.

To the East Pacific -- Fernanda, holding tough as a hurricane longer than predicted, is finally feeling the effects of the cooler waters and stabler environment. The storm is still well-organized, but convective tops have been warming over the past 24hr. As it weakens and moves further west, it should begin to accelerate a bit to the west and west-southwest with the low-level flow. The current forecast keeps it as a minimal tropical storm for quite some time, but considering climatology, the stable environment, marginal SSTs, and that the central Pacific is generally a tropical cyclone graveyard, I feel this might be a little aggressive. Hawaii should watch it, but I don't think it's a threat. As Fernanda weakens, the environment around Greg should become more favorable for it to develop, as it is already showing signs of doing. The initial forecasts taking it to hurricane strength look good, just a couple of days later than projected, largely due to the impacts of Fernanda. Model guidance is split on this one as to where it goes -- an upper-low well north of the storm seems to be the culprit -- but this feature appears to be rather weak and too far north to really grab the storm. Figure a west to west-northwest motion of the storm, similar to Fernanda's footsteps, is most likely through the period. Still hints at features perking up further east, but none have yet to maintain themselves. Good bet that one does in the next couple of days, which we'll watch for potential development.

Back tomorrow if things change with TD 10 (or Irene), otherwise back next week.