The Timing of Hurricane Rain Events Along the Coastal United States

The National Hurricane Center issues wind-related watches and warnings along coastline segments, partitioned by breakpoints, to provide lead time so that emergency preparations and evacuations can be completed before the arrival of potentially damaging winds. Rainfall, however, may occur before damaging winds arrive and hinder preparation efforts making the practical window of time for completion less than what a warning provides. This study examines the start time of rainfall at NHC designated breakpoints for 18 landfalling hurricanes between 2002-2010. Hourly raingauge-corrected radar estimates of rainfall are available from the Stage IV dataset. The rainfall start times at breakpoints along the coastline and within 500 km of the landfall location are compared to the time of landfall of the storm and to the time when warnings were issued for those breakpoints receiving a hurricane or tropical storm warning. In all, 320 breakpoints are sampled with 177 receiving hurricane warnings, 84 receiving only tropical storm warnings, and 59 receiving no warning. We define the start time as the hour when rainfall accumulation reaches 1.21 mm (once the TC is within 500km) which is the hourly rainfall rate at 25 dBZ using the tropical Z-R equation. Results show that median (mean) rain event start times for warned breakpoints occur 14 (17) hours before official landfall. Prior to the implementation of the current warning criteria in 2010, hurricane (tropical storm) warnings were issued when hurricane (tropical storm) force winds were expected within twenty-four hours. Using this criteria, the median (mean) times to the onset of a rain event after hurricane and tropical storm warnings were 19 (18) and 14 (10) hours respectively. Median values of rain event start times after hurricane and tropical storm warnings were found to occur significantly sooner than 24 hours according to rank-sign tests. Rain events started within 18 hours after warning in 51% of all hurricane warned breakpoint areas and 60% of tropical storm warned breakpoint areas, 6 hours sooner than the expected onset of the respective winds. In future work, we will compare the onset of rain events with the observed onset of hurricane and tropical storm force winds as well as other measures of storm size such as the radius of the outer closed isobar.