Flash Flood in Grayson County, Virginia

Drone footage showed Grassy Creek was well out of its banks and blocking Grassy Creek Road. One culvert was washed out on Grassy Creek Road, making the road impassible, while another culvert was washed out by Grassy Creek on an adjacent driveway, blocking the road.

Multiple intense thunderstorms developed with the passage of an upper level disturbance during an afternoon of strong daytime heating. Abundant sunshine and above normal warmth resulted in surface-base CAPE (Convective Available Potential Energy) values ranging from 2,000 to 2,500 J/Kg for areas west of the Blue Ridge in western Virginia. Precipitable water values were estimated to be in the 1.2 to 1.3 inch range as southerly wind flow tapped deeper moisture pooling ahead of an approaching cold front to the west.

Flash flooding occurred in two separate areas of Grayson County as intense storms began to train over the same area. One storm produced between 3 and 4.5 inches to the east of Troutdale, while the other produced 4 to 5 inches near the community of Rugby, each during a 2 to 2.5 hour period. MRMS FLASH indicated rapid responses to the heavy rain, with CREST Unit Streamflows ranging from 500 to locally over 1,000 cfs/mi^2. With both storms, 1-hour, 3-hour and 6-hour ARIs (Annual Recurrence Intervals) maxed out at 200-year events, and estimated rainfall was more than double flash flood guidance for the area at the 1-hour, 3-hour and 6-hour intervals.