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Variations in time of rainfall start along the U.S. coast during tropical cyclone passage

Authors: Corene Matyas*, University of Florida, Michael Heslar, University of Idaho
Topics: Climatology and Meteorology
Keywords: rainfall, tropical cyclones, landfall
Session Type: Paper
Presentation File: No File Uploaded

Although the National Hurricane Center provides estimates for the start time of gale-force winds, time of rainfall start is not available. Thus, we examined more than 40 TCs that made landfall over the U.S. since 2002 when Stage IV gauge-corrected hourly radar-estimated rainfall totals became available. Rainfall start time at each coastal point was determined using two thresholds: a 0.5 mm/hr trigger with 6 hour duration, and 1.25 mm/hr trigger with three hour duration. We calculate the distance between the coastal point and storm center as well as time relative to landfall. On average, rain begins 10 hours prior to landfall, but this time can exceed 24 hours. Rain events lead the arrival of gale-force winds when TCs are weaker and younger, relative humidity is higher, and the westerlies are stronger. Storm centers are 260 km away on average when rain starts. To best classify the conditions associated with the range of distances, we divide locations into four spatial groups. Distance averages 300 km when rain starts at east coast locations, where vorticity, vertical wind shear and zonal winds are stronger coupled with faster storm forward motion for larger distances. This coincides with extratropical transition. West Florida cases with larger distances also indicate interaction with the middle latitude westerlies with strong divergence aloft. Higher total precipitable water and lower vertical wind shear associate with farther distances for east Florida landfalls. Distance averages 200 km for western Gulf cases. Here, relative humidity is highest and zonal winds are lowest.

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