An important forecast problem for the east coast of North America is that of predicting whether a tropical cyclone subsequently reintensifies in a baroclinic environment. The intensification of such an extratropical transition (ET) is the focus of this research.

We conduct a series of forecast sensitivity experiments using the Canadian Mesoscale Compressible Community model (MC2). The control run, using the full physics of the model with 35-km horizontal resolution and initialized at 0000 UTC 5 September 1998, succeeds in forecasting ex-Hurricane Earl's observed explosive reintensification of 44 hPa in 36 h. The downshear environment (in the central Atlantic Ocean) of ex-Hurricane Earl consists of the remnants of ex-Hurricane Danielle. Using a piecewise potential vorticity (PV) inversion technique, we remove the downstream PV anomalies associated with Danielle in the initial state. The consequence of this altered initial state is a forecast in which the 24-h deepening has been reduced from 25 to 17 hPa with a track that has changed to a location 250 km north of the control low location.

The sensitivity of Earl's intensity and track forecast is comparable to the sensitivity shown for initial condition removals of upstream positive PV anomalies. Contrary to the conventional wisdom that a well-observed upstream environment insures credible forecasts of east coast cyclogenesis, our results suggest that it is also necessary to provide credible three-dimensional analyses of the downstream marine environment.

The generality of our results is illustrated by composite analyses of 25 deepening versus 25 non-deepening ETs in the same region. The deepening ETs are preferentially characterized by a downstream cyclonic predecessor. Physical mechanisms for this preferred structure of cyclogenesis will be discussed.