Tropical weather analysis - August 2, 2010

Tropical Depression Four

Tropical Depression Four was able to develop this morning from former Invest 91L. As of 5:00 PM EDT, the National Hurricane Center had it centered at 13.0°N 42.5°W, moving WNW at 16 mph, with a pressure of 1006 mb. It should be noted that this system appears to be moving more westerly as of the last couple of hours, and it will be interesting to see how the NHC will respond to this in the upcoming advisory. The newly-developed tropical cyclone is moving rather quickly, and I think that it might have even increased in forward speed from the earlier estimated 16 mph. Indeed, since the time of the previous advisory, it has jumped from 42W to 44W. The last of the visible satellite animations suggest that the cyclone may be experiencing some slight southeasterly to easterly shear, with most of the deep convection confined to the western portion of the circulation.

This shear is being induced by a developing upper-level anticyclone to the east of the depression. Another possible explanation for this (since the shear associated with the anticyclone isn't prohibitively strong, only 10 kt) is that the brisk forward speed of the tropical cyclone is making it difficult for sustenance of low-level convergence. Indeed, there have been many tropical cyclones that have dissipated for this reason, despite favorable atmospheric conditions. This remains a possible scenario with Tropical Depression Four. Indeed, a recent SSMIS microwave overpass indicates that the surface circulation is elongated and not well-defined. However, the circulation is probably still closed off, based on animation of RGB satellite loops. However, if the forward speed increases anymore, the cyclone may not be able to maintain a closed surface circulation, and also may not significantly intensify, due to lack of low-level convergence.

There is also a noticeable vertical tilt associated with the cyclone, based on CIMSS vorticity data. This further suggests that the cyclone is having trouble, and the low-level center may be outrunning the mid-level center. Despite the cyclone's recent W movement, steering data suggests that a turn to the WNW should soon begin once again, with perhaps an additional increase in the forward speed over the next 12 hours. This general motion should continue until around 72 hours, when the cyclone encounters a large break in the Atlantic subtropical ridge forecast by most of the model guidance. At that point, a gradual deceleration is expected as the aforementioned ridge weakens with the approach of a mid- to upper-level trough currently stretching from eastern New Mexico to central Kansas. This will be the key player in whether or not the tropical cyclone harmlessly recurves out to sea, or makes a strike somewhere along the eastern United States mainland.

The models all agree on bringing the trough across the western Atlantic in about three days, but differ greatly on its overall strength and amplitude. The CMC, GFS, and NOGAPS models foresee a less amplified trough, which keeps enough ridging in place to prevent a complete recurvature. The GFDL, HWRF, and ECMWF models foresee a more amplified upper trough, with less ridging and more troughing, which would tend to favor a sharp poleward turn during the day four to five timeframe, which would lead to eventual recurvature.

My current thinking is that the pattern should remain less amplified and more zonal than the latter three models are predicting. This is based primarily upon climatology, which strongly argues against such a large break occurring in the Atlantic subtropical ridge. However, one must keep in mind the unseasonably strong longwave troughs that recurved both 2004's Charley and 2009's Bill. There is greater than normal uncertainty in the track forecast for this system, and I would put the odds of an east coast strike and an eventual recurvature at 50/50. But again, I tend to favor the former scenario, due to the time of the year. Nevertheless, if consistency is shown with regards to a more amplified upper air pattern, I will have to shift my forecast track significantly to the right as the days press on.

Due to the uncertainty in the forecast track, residents all along the east coast from Florida to Maine should carefully monitor the progress of the depression over the next several days. Bermuda should as well, since even if the cyclone completely misses the United States east coast, it could still cause problems for them. Indeed, the GFDL, HWRF, and ECMWF all foresee the cyclone coming very near that island late on day four and into day five.

In the meantime, the cyclone should move north of the Leeward Islands and Puerto Rico in about two and a half days. Residents in those areas should also closely monitor the progress of this system, as tropical storm watches or warnings could be required over the next day or so, depending on how far west the cyclone moves in relation to the current NHC forecast track. All in all, my own forecast track is in strong agreement with the NHC's 5:00 PM one.

The intensity forecast is problematic. If the cyclone fails to maintain a closed surface circulation, it will probably never regenerate, due to strong upper-level shear forecast by the GFS to persist north of the Greater Antilles and Leeward Islands for the next several days. On the other hand, if it remains a tropical cyclone and the anticyclone follows it, it could withstand the shear even as it approaches that shear by 60 hours or so. None of the models, save the GFDL and CMC, are significantly intensifying the tropical cyclone. However, I will err on the side of caution and forecast gradual intensification over the next three days or so.

Thereafter, the cyclone is forecast to lose its upper-level anticyclone, and vertical shear is still expected to be prohibitive across the western Atlantic, albeit not quite as strong as what's currently forecast north of the Leeward Islands. I will call for a leveling off of the intensity at that point, though a gradual weakening could certainly commence.

There is only a small chance, about 15%, of TD4 becoming a hurricane.

Elsewhere

Elsewhere in the tropics, a tropical wave is moving across the extreme eastern Caribbean Sea. Upper-level winds support development of this disturbance as it moves WNW at 15 to 20 mph over the next couple of days. By day three, the system will be approaching a weakness at the western end of the Atlantic subtropical ridge, and will probably enter the Gulf of Mexico in about five days. The biggest inhibiting factor is proximity to South America, which has been known to destroy rather healthy tropical waves, and even tropical cyclones, in this area.

The models nevertheless eventually develop the system, with the 18z GFS calling for a rather powerful hurricane impacting southeastern Louisiana in a week, while the 12z CMC suggests that the system will eventually run into Central America. This scenario appears unrealistic for now, given the synoptic scale pattern across North America and the western Atlantic Ocean. The 12z NOGAPS hints at development as the system approaches the western Caribbean in six days. I feel the NOGAPS solution, while plausible in terms of development, is much too slow with the forward speed of the system. The 18z GFDL hints at development of this feature as well. Finally, the 12z ECMWF does not develop this system.