| Summary: | The transformation of 45 tropical cyclones into extratropical cyclones over the western Noah Atlantic Ocean between 1963 and 1996 is studied. Cases are selected from the National Hurricane Center's "best track" archive. National Centers, for Environmental Prediction (NCEP) reanalyses of geopotential height data are used to construct a synoptic-dynamic climatology of extratropical transition, or "ET". The Forecast Systems Laboratory (FSL) upper-air archive of six near-track stations is used to produce sounding composites. Primary results of the study follow. (1) A statistically-significant 1000--500-hPa warm anomaly (with respect to the 1963--96 climatology) persists for the one-week period prior to the passage of the tropical systems into the Canadian Maritime provinces. (2) A northwestward extension of the surface subtropical anticyclone exists over the Canadian Atlantic Provinces during the two-day period prior to the arrival of the cyclone. (3) The tropical cyclone's warm core and conditionally-unstable tropical airmass are maintained after transition. (4) The presence of quasi-geostrophic forcing for ascent, typically seen in extratropical cyclones, is observed during periods in which the systems are still classified as tropical cyclones. This forcing for ascent continues during the extratropical transformation, and typically occurs ahead and to the left of the storm track.
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