Heavy precipitation associated with landfalling tropical storms

Landfalling tropical storms are often associated with exceptionally heavy precipitation and flooding. Heavy precipitation can continue to occur well after storm landfall and well after hurricane-force winds have abated. In some landfalling hurricanes more people perish from fresh-water flooding well inland than die from high winds and storm-surge associated flooding along the immediate coast. On occasion heavy precipitation and flooding can also occur well prior to storm landfall (e.g., Agnes 1972 and Fran 1996). Forecasting heavy rain with landfalling tropical storms remains a challenging task. Forecasters at the National Hurricane Center (NHC) predict the expected 24 h rainfall amount on the basis of a simple rule of thumb as follows: Rainfall (inches)=100/V where V is the forward speed of the storm (in mph). Forecasters at the Hydrometeorological Prediction Center (HPC) of the National Centers for Environmental Prediction use 10" (25 cm) of rain in 24 h as an upper-bound estimate in conjunction with a landfalling tropical storm (upwards of 100 cm of rain fell with Danny in 1997 in the Mobile Bay area). These "simple" rainfall amount forecasting procedures as used operationally at NHC and HPC do not distinguish between precipitation intensity, precipitation type (stratiform versus convective), precipitation duration, and precipitation location relative to the track of the storm.

This talk will focus on the dynamical basis for understanding the expected distribution, duration, location, and intensity of precipitation relative to the track of the tropical cyclone prior to and subsequent to landfall. A perspective based on "potential vorticity thinking" will be used to demonstrate the importance of multiscale interactions with the storm on the quantitative precipitation process. Illustrative examples will be taken from historical storms which proved very troublesome to forecasters and were associated with severe flooding (e.g., Camille, 1969; Agnes 1972; Marco 1990; Fran 1996; Danny 1997).