Heavy Rain in Roanoke (c) County, Virginia

The Roanoke ASOS (ROA) set date rainfall records for three consecutive days: May 19, 20 and 21 with 2.84 inches, 2.76 inches and 2.72 inches, respectively. These were also the 5th, 6th and 7th wettest days in May in the period of climate data for Roanoke back to 1912. The 3-day total of 8.32 inches ending at midnight on the 21st was easily the highest on record in May. The previous 3-day record for May was 5.70 inches in 1940. For any 3-day period it was the 5th wettest on record, trailing August 14-16, 1940 with 9.31 inches. Per NOAA Atlas 14, for a 3-day period this rainfall total is in the 90 percent confidence interval (7.27 to 8.65 inches) for a 50-year recurrence event (.02 annual exceedance probability).

An upper-level low became cut-off over the southern Appalachian region for several days leading to persistent rainfall and eventually some flooding. Four-day (96-hour) rainfall amounts ending at 800 AM EDT on May 22nd ranged from 3 to over 12 inches in the most favored locations. Some of the 11-inch+, 4-day rain totals in Roanoke County were within the 200-year recurrence interval (9.78 to 11.8 inches) per NOAA Atlas 14. The flooding that ensued was almost all in the form of river flooding that took several days to develop. Nearly every major river basin saw some flooding, but the most notable occurred in the New River basin where crests ranged from the 4th to 8th highest on record at the Virginia gaging stations along the river. Stages on the upper New were higher than in October 2018 when the remains of Hurricane Michael transited the region. The incredible rainfall, runoff and numerous debris flows produced widespread damage to roads in the region with VDOT reporting over $5 million in damages.