Tropical weather analysis - October 6, 2010

Otto

Subtropical Storm Otto developed today from what was previously Invest 97L. As of the latest NHC advisory (5:00 PM AST) from the National Hurricane Center, here is the latest information on Otto:

Wind: 60 mph, with higher gusts
Pressure: 990 mb
Movement: N at 5 mph
Location: 23.2°N 68.3°W
Category: Subtropical storm

Infrared satellite loops indicate that Otto may be beginning to acquire tropical characteristics, as an equatorial outflow channel has recently appeared and expanded on the southern side of the storm. This indicates that the system is spreading out warm air in the upper levels, signature of a warm-cored system. Deep convection is also firing closer to the center, but is still located relatively distant from it.

In addition, AMSU temperature profiles from CIMSS and cyclone phase diagrams from FSU suggest that the cyclone is gradually improving its mid- upper-level warm core.

Water vapor imagery indicates that the upper low responsible for Otto's subtropical characteristics is gradually weakening and deamplifying, and is quite elongated at present. Consequently, the surrounding airmass has moistened, indicating less dry mid- to upper-level cold air and more warm moist air. As a result, Otto should gradually acquire tropical characteristics over the next 6-12 hours.

Conditions appear favorable for continued intensification, especially now that the upper low is moving out of the picture. I expect that by morning, Otto will have fully acquired tropical characteristics. There is now a distinct possibility that Otto becomes a hurricane, as most of the models are reflecting this. I agree rather well with the NHC's current intensity forecast:

FORECAST POSITIONS AND MAX WINDS

INITIAL 06/2100Z 23.2N 68.3W 50 KT
12HR VT 07/0600Z 23.8N 68.5W 55 KT
24HR VT 07/1800Z 24.5N 67.9W 60 KT
36HR VT 08/0600Z 25.5N 66.4W 65 KT
48HR VT 08/1800Z 26.8N 63.7W 65 KT
72HR VT 09/1800Z 30.3N 55.8W 55 KT
96HR VT 10/1800Z 34.5N 44.5W 40 KT...POST-TROP/EXTRATROP
120HR VT 11/1800Z 39.0N 31.0W 35 KT...POST-TROP/EXTRATROP

$$
FORECASTER STEWART

Otto is forecast to be extratropical in about four days as it interacts with a large and powerful baroclinic system to the west.

Otto is located in a weak steering regime between a well-defined and slow-moving longwave trough to the west and the subtropical ridge to the east. Such a steering pattern favors a continued slow generally N motion over the next 18-24 hours, though there could be some wobbles to the NW during this time as well. Thereafter, the global models unanimously predict a gradual acceleration to the NE as Otto becomes well-embedded within the southwesterly steering regime associated with the aforementioned trough, which is forecast to move closer to Otto over the next day or so.

In about two and a half to three days, the models diverge, with some accelerating Otto more NNE, with others accelerating him ENE. I still feel the trough is amplified enough to bring Otto along the eastern edge of the guidance envelope. The HWRF, UKMET, and ECMWF models are on the western edge of the guidance, forecasting Otto to move well west of the Azores. I tend to favor the more eastern camp, composed of the NGFDL, GFDL, and GFS models.

I am unsure what is causing the GFS to move Otto sharply east near the end of the forecast period, and so I will discount that solution for now. My forecast track takes Otto within 100 miles west of the western end of the Azores in about five days, and interests there should closely follow the progress of this system.

Last night I had mentioned the possibility that Otto could become absorbed into a large extratropical low pressure system over the next few days, but this solution now appears unlikely. While interaction with this low will ultimately cause Otto to transition into an extratropical storm, global models suggest that Otto will remain a separate entity, distinct from the aforementioned baroclinic low throughout the forecast period.

Because Otto will be extratropical as it approaches the Azores, impacts will be felt well away from the center, particularly to the northeast. Hence, the center of the cyclone should not be focused on in this instance. Otto could also ultimately impact western Europe sometime next week.

Elsewhere

Elsewhere in the tropics, several of the global models continue to indicate the possibility of tropical cyclogenesis in the southwestern Caribbean in the next 5-7 days. However, they have become less aggressive today. If this becomes an established trend, it may mean that this system will not become as strong as previously thought. This possibility will have to be carefully assessed over the next few days.

The exact details of the track are uncertain, but right now I'd expect a track similar to Nicole.