Tropical weather analysis - October 9, 2010

Otto Tropical Storm Otto continues to move rapidly northeastward over the central Atlantic Ocean. As of the latest (11:00 PM AST) advisory from the National Hurricane Center, here is the latest information on Otto: Wind: 70 mph, with higher gusts Movement: ENE at 38 mph Pressure: 988 mb Location: 33.9N 48.7W Category: Tropical storm Infrared satellite loops indicate that Otto continues to look less and less like a tropical cyclone, as it begins to undergo extratropical transition. Indeed, deep convection is elongated about a sharp SW to NE axis due to strong southwesterly shear associated with a mid- to upper-level trough located to the west. In addition to the shear, conventional satellite imagery implies that there is a substantial area of cool, stable air just to the west of the cyclone. The aforementioned vertical shear is acting to pump said dry air into Otto's circulation, hampering thunderstorm activity near the center and promoting extratropical transition. The official NHC forecast calls for Otto to complete extratropical transition within 24 hours, and I tend to concur with this forecast. Otto is currently being steered by an upper trough to the west and a ridge to the east. By 18-24 hours, the global models indicate that Otto will begin moving more to the ENE as it feels less influence from the trough and more influence from the ridge. By mid day two or early day three, most of the models, with the exception of the GFDL and NOGAPS, slow the system down and swing it south to southeastward. I will compromise between the two extremes and forecast a slow but steady south-southeasterly motion during that time. This track also happens to agree well with the current NHC one. Otto could stick around in the vicinity of the eastern Azores for some time, and interests in and around those islands should closely monitor the progress of this system over the next several days. Interests in the Madeira Islands should follow Otto closely as well. Invest 98L A small but steadily organizing area of low pressure located about 160 miles southeast of the Honduras/Nicaragua border, labeled "Invest 98L" by the National Hurricane Center, is a threat to develop into a tropical depression. Analysis of shortwave infrared satellite imagery indicates that the center is located along 13N 81W. In addition, infrared satellite loops show that deep convection is firing near the estimated center position, and this convection shows some organization. An upper-level outflow channel has become established to the west and north of the center, with some evidence along the eastern semicircle as well. Outflow remains restricted in the southern and eastern quadrants, however, especially the former. Surface observations and satellite data indicate that the low-level circulation associated with this system is becoming better-defined. Upper-level winds are gradually easing over Invest 98L, and the GFS suggests that the upper wind environment will only further improve, particularly by 24 hours. The environment might not be anticyclonic, but it will at least be of light shear, enough to allow steady intensification of the system. The shear is forecast to remain generally light throughout forecast period. At later periods, the primary inhibiting factor arguing against significant strengthening of 98L is the presence of very dry, stable air across the western Caribbean and Gulf of Mexico, noted on water vapor imagery. Regardless, dry air will not be an issue for at least the next few days, and I concur with the NHC's assessment of a 60% chance of tropical cyclogenesis in the next 48 hours. Regardless of development, 98L will spread heavy rainfall across portions of Central America, Jamaica, and the Cayman Islands over the next couple of days. These rains could cause life-threatening flash floods and mudslides across these areas, and interests in the aforementioned areas should closely follow the progress of Invest 98L over the next couple of days. 98L is currently moving very slowly to the NW, underneath the influence of a weak low- to mid-level ridge. I expect this motion to more or less continue for at least the next two to three days. Thereafter, the models diverge, with some taking 98L more equatorial toward the Belize/Yucatan Peninsula area, with others taking it more northward, closer to western Cuba. It is impossible at this point to tell which of these solutions is correct, especially given the location of the system and the time of year. Caribbean systems are notorious for meandering during October as steering currents remain generally weak. The GFDL brings 98L up to a Category 2 hurricane in four days as it nears western Cuba, but this solution is hard to accept at this time. However, if 98L remains in this portion of the Caribbean long enough due to weak steering, it certainly has the potential to become a hurricane.