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.
The twenty late-season tropical cyclones (LSTCs) are classified by type of incipient system. Type I (six systems) originate as pre-existing, non-frontal and non-tropical cyclones. Type II (seven systems) develop along dissipating frontal systems. Type III (three systems) develop directly from an occluded frontal cyclone, while Type IV (four systems) have tropical origins. Types I and II predominate in the study area.
The environment of each LSTC is examined for the thirty hours prior to attainment of tropical cyclone status with minimum winds of 35 knots. 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 500 hPa, 400 hPa, 300 hPa, 250 hPa, and 200 hPa, respectively. The shear values are averaged on a 3x3 grid for quantitative comparison of systems. Average sea surface temperatures are calculated for the same area and plotted with wind shear. Divergence and relative vorticity are calculated at 500 hPa, 400 hPa, 300 hPa, 250 hPa, and 200 hPa. Lastly, the local static stability is estimated using the 0.995 sigma level temperature and the 200 hPa temperature. With these data the environmental conditions prior to the formation of LSTCs are analyzed.