Tropical weather analysis - August 20, 2010

Invest 95L

A broad area of low pressure, associated with a large area of disturbed weather within the ITCZ, is centered a couple hundred miles south-southwest of the Cape Verde Islands. This area was designated "Invest 95L" by the National Hurricane Center early this morning. Satellite animations suggest that the system is gradually becoming better organized, but shower activity is not persistent yet, and in fact, has waned markedly in recent hours. Additionally, vorticity data from CIMSS indicates that, although the low- and mid-level circulations are gradually becoming better organized, this system still has quite a bit of work to do, namely detaching itself from the active ITCZ in which it is currently embedded. The aforementioned vorticity data reveals that 95L is embedded within a large scale cyclonic circulation associated with an active ITCZ.

95L is currently moving W under the influence of a well-defined low- to mid-tropospheric ridge. This general motion should continue until a more WNW component becomes established in about two days. By about four days, the models are in unanimous agreement on a highly amplified mid- to upper-level pattern, as an unseasonably powerful trough erodes the subtropical ridge. At this point, 95L (by then a powerful hurricane) is forecast to approach this weakness and begin turning more poleward with a decrease in forward speed.

However, this is where the forecast gets tricky. Not all of the models agree on there being enough troughing to completely recurve the system, at least insofar as Bermuda is concerned. The CMC depicts the strongest trough, allowing 95L to completely recurve well east of Bermuda, and the GFDL and GFS follow suit, though depict a somewhat weaker trough. The HWRF, NOGAPS, and ECMWF depict a less amplified trough, and keep a more zonal flow in place, maintaining the Atlantic subtropical ridge enough to prevent complete recurvature prior to reaching the longitude of Bermuda. In fact, the 12z ECMWF is indicating that 95L will approach the southeast United States coast in approximately ten days, though this may be related more to the fact that it keeps the system quite weak until this time, and therefore it follows the low-level flow.

Regardless, there are still a number of possible scenarios for this system, though eventual recurvature seems probable, it's just the degree and timing of such that's uncertain at this juncture.

95L is currently experiencing about 15 kt of easterly shear. The 18z GFS keeps the shear quite weak throughout the forecast period, and even starts to develop an 200 mb anticyclone over the system in as little as 24 hours. The anticyclone becomes particularly well-established by 48 hours, and continues to strengthen throughout much of the forecast period. It should also move in tandem with 95L, particularly beyond 48 hours. These conditions should allow 95L to ultimately become the season's second hurricane, and quite possibly, the first major hurricane, as well.

In the short-term though, 95L has work to do. I feel the models are developing this a little too quickly, with some doing it as early as tomorrow. I will forecast continued slow development, and I expect this system to become a tropical depression on Monday afternoon. It should be noted that the weaker the system stays, the more westward it goes.

Residents along the Leeward Islands and Puerto Rico, as well as Bermuda and the east coast from North Carolina to Atlantic Canada should monitor the progress of this system very carefully over the coming days, given the inherent uncertainties ever present in long-range forecasts.

Elsewhere

There really isn't much going on in the tropics at the moment, other than Invest 95L. However, most of the models agree on a piece of 850 mb vorticity detaching from the aforementioned trough in about four days, the one forecast to recurve 95L. This trough split is forecast to occur along the warm waters of the Gulf Stream offshore the east coast of the United States, so this bears watching.

There are also a couple of decent looking entities over Africa, at around 8 to 11N, which may be identified as tropical waves later on as they emerge into the eastern Atlantic. These features have some rather decent cyclonic circulations, so they certainly bear watching as they move into the Atlantic over the next several days.