Often, precipitation forecasts associated with landfalling tropical cyclones are based on a simple algorithm where the maximum 24-h precipitation (in inches) is forecast by 100/v, where v (in m.p.h.) 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 (PV) 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, 2) Precipitation distribution is heaviest to the right of the track of the storm when downstream intensification of the ridge is important, and 3) Precipitation distribution is heaviest to the left of the storm track in a transitioning storm. Without large scale forcing for vertical motion associated with a midlatitude trough in situation 1, most of the greater vertical velocities remain near the storm core in the region of greatest diabatic heating and maximum wind speeds. In situation 2, 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). In a transitioning cyclone (situation 3) , a midlatitude trough approaching the tropical cyclone from the northwest often results in a strong baroclinic between the two systems. Once the tropical cyclone interacts with the baroclinic zone, a large region of precipitation develops in the left front quadrant of the storm in a region of strong warm air advection. Furthermore, diabatic heating from the resulting precipitation can re-distribute PV in the midlatitude trough creating a more dynamically active system with a negative tilt.