That was from one of our county flood sensors near Sligo Creek Pkwy & New Hampshire Ave (MD 650). I've never seen water levels rise that quickly at that location.
I actually wound up requesting the upgrade to a flash flood emergency. Our flood sensors inside the beltway just went bonkers so fast. Cabin John Branch, Rock Creek, Sligo Creek, and Northwest Branch just started raging in under half an hour.
This was the worst flash flood event I have ever worked in my entire career. Had this persisted for just another 20 to 30 minutes, we would've had real problems in Montgomery County. The flood sensors we installed, combined with the Layhill and College Park mesonet sites raised alarm bells quickly, which resulted in us getting roads closed and water rescue assets staffed quickly. It was one of the few times in my career that I actually felt helpless as things went sideways.