Often, precipitation forecasts associated with land-falling tropical cyclones are based on a simple algorithm where the maximum 24-h precipitation (in inches) is forecast by 100/v, where v (in mph) is the translational speed of the cyclone. This algorithm, however, provides little insight as to the precipitation distribution and intensity that can be expected in a land-falling tropical cyclone. Furthermore, several recent cases (Danny 1997, Dennis, Floyd, and Irene 1999) show that precipitation distribution and intensity can be drastically altered by interactions with mid-latitude troughs and jet streaks which often result in extratropical transitions. Occasionally, these interactions produce catastrophic rainfalls as illustrated by hurricane Floyd in September 1999. This talk will present diagnostics of results from case studies and composites of several storms from a Quasi-Geostrophic potential vorticity perspective, designed to elucidate the important dynamics responsible for the modulation of the precipitation distribution and intensity.

Preliminary results suggest that precipitation distribution in land-falling tropical systems may be characterized in the following ways. 1) Precipitation is heaviest along/very near the track of a storm when there is no significant interaction with a midlatitude trough. Without large scale forcing for vertical motion associated with a midlatitude trough, most of the greater vertical velocities remain near the storm core in the region of greatest diabatic heating and maximum wind speeds. 2) Precipitation distribution is heaviest to the right of the track of the storm when downstream intensification of the ridge is important. The intensification of the downstream ridge ahead of a weak midlatitude trough can accentuate the PV gradient between the tropical system and the downstream ridge enhancing the cyclonic PV advection (implied ascent) between the tropical system and the downstream ridge. 3) Precipitation distribution is heaviest to the left of the storm track in a transitioning storm. As a strong midlatitude trough approaches the tropical system from the northwest, it often attains a positive tilt as the progress of the trough is more effectively impeded by the tropical cyclone outflow at lower latitudes resulting in an enhanced region of baroclinicity north/northwest of the storm. Strong isentropic ascent often results as warm tropical air overruns the cool dome in the northwest quadrant of the storm.