Historical Perspective on Rainfall from Hurricane Florence

Hurricane Florence generated torrential rainfall in the eastern Carolinas during September 14-17, 2018 and triggered catastrophic flooding, exceeding all-time records in some locations. Damages in North Carolina are estimated at $13 billion. An historical analysis of Florence rainfall was performed based on long-term records from the National Weather Service’s Cooperative Observer Network. The purpose of this analysis was to compare the rainfall in Florence with other events from throughout the coterminous United States. Official reports of rainfall exceeded 30 inches at a few locations but rainfall across the region was highly variable. The particular focus of this analysis was on evaluating average rainfall over large areas over 4-day periods. The period of analysis was the 70-year period from 1949 to the present. We examined precipitation averaged over various areas from about 5,000 square miles to 80,000 square miles.

The results for two area sizes, 14,000 and 20,000 square miles, illustrate the historical significance. In both cases, the largest 4-day event was Hurricane Harvey and by a large amount. At 20,000 square miles, Florence rainfall averaged about 13 inches (compared to 24 inches for Harvey) and ranks as the 7^th largest of the entire 70-yr period of analysis. The 2^nd ranked event at 20,000 square miles is Hurricane Georges in 1998 (September 27-30) which produced very heavy rain in the central Gulf Coast. The 3^rd through 6^th ranked events, none of which were hurricanes, are

#3: March 8-11, 2016, northern Louisiana

#4: August 13-16, 2016, southern Louisiana

#5: October 11-14, 1962, northern California

#6: October 16-19, 1994, southeast Texas

The precipitation in events ranked #2 through #7 are similar in magnitude, in the range of 13-16 inches averaged over 20,000 square miles.

At 14,000 square miles, Florence rainfall averaged over 17 inches and is ranked #2 behind Harvey (which averaged 26 inches). At larger areas, Florence’s rank is considerably lower. For example, at 30,000 square miles, it ranks 19^th and at 80,000 sq. miles, it ranks 49^th.

The key finding is that Florence’s rainfall was unprecedented for eastern North Carolina and one of the largest on record for the entire United States for areas up to 20,000 square miles.