These Nexrad images are from the hours following the Severe Weather Outbreak of April 21, 1997. Watch a swirl "develop" east of Charlotte, NC at 3:30 AM then rotate counter-clockwise until it dissipates near the RDU radar just before 6:00 AM.

What is it? A low-reflectivity mesocyclone? A reflection of a surface low pressure system? A convection-induced vort-max? Your guess is as good as mine.