Tropical weather analysis - October 5,, 2010

Invest 97L

An area of low pressure, located just north of Puerto Rico, dubbed "Invest 97L" by the National Hurricane Center, is producing a large and elongated area of showers and thunderstorms to the north and east of the estimated center. Analysis of shortwave infrared satellite imagery, buoy observations, and various other data indicate that the system does not yet have a well-defined surface circulation. However, the low-level center appears to be gradually becoming better-defined, as noted by low-cloud motions observed on satellite imagery.

The center appears to be located near 22N 66W according to the various data I've been able to analyze.

Infrared satellite imagery shows that 97L is still disorganized, but the convective bands to the north and east of the center appear to be becoming a bit more collocated with the center, a sign of increasing organization.


CIMSS analysis depicts 20 kt of westerly vertical shear blowing just to the north of the circulation center. This shear is responsible for the current subtropical appearance of the storm on satellite.

In addition, water vapor imagery shows that a large area of dry air surrounds 97L, and its counter-clockwise circulation is acting to pull in some of this dry air, which is also inhibiting any appreciable convective activity from developing and persisting near the center.

AMSU data from CIMSS along with cyclone phase diagrams from FSU indicate that the system currently lacks a well-defined warm core, with only a shallow warm core at best noted in the imagery.

Nevertheless, 97L appears to be gradually acquiring subtropical characteristics, and this system could become a subtropical cyclone at any time over the next 12-18 hours.

Based on the various steering data I've analyzed, I expect 97L to move NNW to NW over the next 36 hours or so with a gradual decrease in forward speed. Thereafter, the models unanimously forecast a stall as a well-defined longwave trough currently over the western Atlantic moves eastward and weakens the steering currents. After 48 hours, steering currents will once again become well-established as 97L/Otto becomes well-embedded within the southwesterly steering regime of said trough.

As a consequence, 97L can be expected to move NNE to ENE with a gradual acceleration after 48 hours. The models are split in their respective tracks. While it's impossible to tell at this juncture which camp of models is correct, I'll go with the easternmost camp, comprised of the NOGAPS and GFDL models, which would eventually bring the system near the western end of the Azores. This is based on the amplification of the aforementioned trough, which I feel will be quite significant.

By day four, the models forecast 97L/Otto to lose tropical characteristics (or subtropical characteristics) and become extratropical as it interacts with a large baroclinic system associated with the western Atlantic trough. 97L should ultimately become absorbed into this low, potentially impacting western Europe in about 7 days.

Vertical shear is gradually easing, and is forecast to lessen further over the next couple of days. In fact, the 18z GFS suggests that a weakly anticyclonic environment will develop over the system during the next day or so, and persist until 48 hours. Any potential transition to a tropical cyclone will occur then, when the trough will be relatively distant from the center and the anticyclone (high pressure ridge) established aloft, pumping warm air into the system at the upper-levels.

I do expect 97L to make the transition to a fully tropical cyclone sometime over the next 36 hours.

By 48 hours, vertical shear is forecast to increase sharply as the trough and associated cold front moves closer to the system. It is at this point that the system will lose its anticyclone.

It is possible that 97L becomes a hurricane, but this looks highly unlikely.

Elsewhere

Elsewhere in the tropics, the GFS, ECMWF, and NOGAPS forecast the development of a tropical depression in the western Caribbean Sea in about 5-7 days. The eventual path of such a system is uncertain.