Flash Flood — Roanoke (c), Virginia

2021-08-19 · near Roanoke, Roanoke (c), Virginia

Event narrative

Portions of Salem Avenue and Campbell Avenue were flooded with up to 4 feet of standing water as heavy rain overwhelmed the storm sewer system. Five water rescues were performed as cars drove into the flood waters, causing the vehicles to stall. No injuries were reported. Broadcast media reported that multiple cars parked along Salem Avenue experienced water intrusion into the vehicle passenger compartments and exhaust systems, with some of the vehicle needed to be towed.

Wider weather episode

Widely scattered thunderstorms developed in an environment of strong daytime heating on August 19th, allowing for deep instability as surface-based CAPE values were observed to be in the 1,500 to 2,500 J/Kg range during the evening for areas mostly along and east of the Blue Ridge. The thunderstorms were fueled by deep tropical moisture, with precipitable water observed to be 1.90 inches per the August 20/00Z upper air sounding conducted by the NWS in Blacksburg. One thunderstorm developed over the city of Roanoke shortly after 8:00 pm EDT, followed by another that moved across the city around 9:00 pm EDT. Doppler radar estimated that between 3 and 4 inches of rain fell across portions of the city, including the downtown area, during a 75- to 90-minute period. Two automated rain gauges, located nearby the areas of radar-estimated greatest rainfall, recorded between 2 and 2.5 inches of rain within 75 minutes, both of which are between 10- and 25-year rainfall events per NOAA Atlas 14 Point Precipitation Frequency estimates. Given the urbanized nature of where the rainfall occurred, much of the rainfall converted directly into runoff, overwhelming the city's storm drain system. Multiple streets near the downtown area became flooded with 1 to 2 feet of mainly standing water, but water was an estimated 4 feet deep in a few spots. Numerous cars parked along the flooded streets had water enter the passenger area. At least 5 water rescues were conducted by the City of Roanoke Fire & Rescue as cars attempted to drive through the flood waters and stalled. No injuries were reported. About an hour after heavy rain ended, the flooding had abated and most of the previously-flooded roads were passable again. Most of this rain fell across the Tinker Creek watershed, causing the stream gauge for Tinker Creek Above Glade Creek to register a 4.5 foot rise to crest just above Action Stage (7.0 feet), before water receded again. The National Water Model estimated the peak flow at this stream gauge to be around 2,000 cfs, which is well under a 2 year event per USGS Stream Stats.

View location on OpenStreetMap → (37.2712, -79.9490)

Source: NOAA Storm Events Database, event_id 981411. Narrative written by the NWS forecast office that issued the report.