Florence Sets Preliminary North Carolina and South Carolina Tropical Cyclone Rain Records; Third, Fourth States to Do So in 12 Months

An observer with the CoCoRaHS network near Elizabethtown, North Carolina, about 50 miles northwest of Wilmington, tallied 35.93 inches of rain from Florence through Sep. 17.

If this preliminary total is confirmed by meteorologists, it would crush the previous tropical cyclone rainfall record for North Carolina in records dating to 1950, previously held by a reporting station near Southport, measuring 24.06 inches of rain during Hurricane Floyd in September 1999, according to David Roth, a meteorologist and tropical cyclone rainfall expert at NOAA's Weather Prediction Center (WPC).

CoCoRaHS is a non-profit, nationwide volunteer network of observers trained to take precipitation measurements to supplement conventional weather observations.

According to the CoCoRaHS network and the WPC, 15 North Carolina reporting stations measured rainfall in excess of the previous state tropical cyclone record. 

In neighboring South Carolina, observers near Loris, about 25 miles north of Myrtle Beach near the border of North Carolina, reported over 23 inches of rain.

Six other reports of 18 to 22 inches of rain were received by the WPC in Chesterfield, Dillon and Marion counties.

If these preliminary totals can also be confirmed, they would all have shattered South Carolina's tropical cyclone rainfall record, previously held by Tropical Storm Beryl in 1994 (17.45 inches), according to Roth.

To put this in perspective, Dr. Robert Rohde of the University of California, Berkeley, estimated 10 trillion gallons of rain fell in the Carolinas during Florence.

Florence's slow movement and training bands of rain were responsible for these heavy totals. A tropical cyclone's rain potential is largely dependent on its forward speed, not its wind intensity. The slower it moves, the heavier the rain totals.

(MORE: Three Reasons Slow-Moving Tropical Storms and Hurricanes Are the Worst)

For comparison, Hurricane Matthew produced up to 18.95 inches of rain in the Carolinas in October 2016, leading to flooding that topped levels from Floyd along the Neuse River at Goldsboro and Kinston, North Carolina.

Hurricane Floyd in 1999 wrung out torrential rain on ground saturated from Tropical Storm Dennis a few weeks prior, leading to massive flooding in North Carolina.

Two Other Recent State Records

Tropical cyclones and their remnants are notoriously heavy rainfall producers. 

In August 2017, Harvey set a U.S. tropical cyclone rainfall record, dumping 60.58 inches of rain near Nederland, Texas, topping the previous record from 1978's Tropical Storm Amelia, which dumped 48 inches in Medina, Texas, according to NOAA's David Roth.

Almost exactly one year after Harvey, Hurricane Lane dumped 52.02 inches of rain at Mountain View, Hawaii, a gauge located on the Big Island of Hawaii at an elevation of 1,600 feet above sea level. Another private weather station reported 58.8 inches of rain during Lane.

If those Hawaii totals are confirmed and the North and South Carolina totals are also confirmed, that would make four state tropical cyclone rainfall records smashed in just over 12 months' time.

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