Late season tropical cyclone formation over the northeastern Atlantic Ocean

Tropical cyclones (TCs) which develop away from the tropical oceans and land receive little attention aside from shipping interests. This is understandable given the necessity of protecting people and property by accurately forecasting track and intensity for land-threatening systems. However, late-season systems can become Category 1 and 2 hurricanes, posing a significant threat to maritime interests. Such systems are not handled as well as TCs in the deep tropics, particularly in regard to intensity. One possible reason for these difficulties is that late-season TCs in the subtropics frequently originate as extratropical or subtropical cyclones. Over a period of hours to days they transition to TCs, though some retain subtropical characteristics even after officially becoming tropical.

The study period begins in 1975, the year of publication for the Hebert-Poteat and Dvorak techniques for satellite classification of tropical and subtropical systems, respectively. The northeastern Atlantic is defined as the portion of the Atlantic north of 20°N and east of 60°W. Purely subtropical storms are excluded from the study to focus on the conditions for tropical transformation. A total of 20 TCs fit this parameters.

Four types of incipient systems develop into TCs in this region. Non-Frontal Tropical (NFT, three systems) originate from tropical waves. While not strictly baroclinic in origin, NFTs do not appear fully tropical. Non-Frontal Baroclinic (NFB, two systems) develop from weak non-frontal surface lows into short-lived minimal tropical storms. Frontal-Weak (FW, ten systems) develop from relatively weak (maximum winds of less than 45 knots at 30 h prior to transition) frontal lows with strong mid-level cyclones. Frontal-Strong (FS, five systems) had strong mid-level and surface cyclones, and had maximum winds of 45 knots or stronger 30 h prior to transition.

The environment of each transitioning system is examined for the thirty hours prior to attainment of tropical cyclone status with minimum winds of 35 knots (the transition period). Wind fields are calculated on a 13x13 Lagrangian grid with 2.5° spacing. Wind shear is estimated as the vector difference between the wind at 850 hPa and the wind at 300 hPa and 200 hPa, respectively. Also calculated are the areal coverage of the 8- and 12 m s-1 contours and the relative locations of shear minima to the system. The central nine shear values are averaged for quantitative comparison of systems. Average sea surface temperatures are calculated for the same area and plotted with wind shear. Local static stability is estimated using the 0.995 sigma level temperature and the 300- and 200 hPa temperature. The temperatures at 300- and 200 hPa are examined for signs of warming in the storm's upper core. Lastly, the CAPE and equilibrium level are calculated for each transitioning system.