Tropical weather analysis - June 21, 2010

Began writing at approximately 8:25 PM eastern

Almost immediately after persistent 92L dissipated, we have a new invest -- Invest 93L. This system has much more favorable atmospheric conditions with which to develop than 92L ever did, even when it looked to be consolidating in the far eastern tropical Atlantic. According to the last of the visible satellite animations, along with buoy observations, strongly suggest 93L does not appear to have a surface circulation at this time, and certainly not a closed one. CIMSS 850 mb vorticity data does show, however, a small area of more concentrated rotation to the south of eastern Hispaniola.

Satellite animations also confirm that there exists some weak rotation within this area. However, I do not believe that this is the surface center, because again, not only do I believe that isn't one yet, it's also because this area of vorticity has been gradually weakening in recent hours. A recent ASCAT pass indicates a sharp wind shift near 16N 70W, though satellite imagery is in conflict with this. Surface pressures are not falling in the vicinity, so no rapid organization will occur tonight and into this morning.

Upper-level shear is only 10 to 15 kt over the system at the moment, and has been steadily decreasing over the last 12 hours. This, along with some decent upper-level divergence, low-level convergence, and finally, a very moist environment, supports continued convective development. Remember though, this system is disorganized at present, with no signs of a surface circulation or falling pressures. Hence, I do not believe that any rapid organization is in order in the near-term (the next 12 to 18 hours).

This system is currently moving W at 5-10 kt, and I expect this motion to continue for the next 12 hours or so, after which point a steady WNW movement, with some occasional oscillations to the W, should occur. I base this on the 18z GFDL, which I feel has the most realistic representation of 93L, in terms of depth. A more shallow system, as the CMC, NOGAPS, and GFS are predicting, would tend to follow the low-level flow and move more southerly, which I do not buy at this time, based on current steering analysis from CIMSS, as well as due to the fact that this system should organize quicker than the aforementioned three models are saying.

Alarmingly, the GFS, as per the 18z run, still insists on creating a very favorable environment across the entire Caribbean from now throughout much of the forecast period. In fact, much of this environment could be anticyclonic in nature, and the GFS insists that this anticyclone will begin to develop by just 24 hours. This has consistently been the case on this model for the last two and a half to three days at this point. Given that I find the GFS performs the best with regards to vertical shear, I will be keeping a very close eye on this.

It already appears that there is an anticyclone attempting to establish itself across the eastern Caribbean, so this certainly lends some credence to the GFS's forecast. The CMC and NOGAPS are also showing this, though the NOGAPS almost always has a low-shear bias from my observation, so it's probably irrelevant with regards to that particular model.

All in all, the NHC's forecast of a 50% chance of tropical cyclogenesis within the next 48 hours appears to be a reasonable one. The ultimate track of this system is uncertain, and residents from the Yucatan Peninsula throughout the entire Gulf of Mexico should carefully monitor the progress of this system over the next week