Meteorologist Jim LaDue sets the stage in an excellent blog post on how this happened:
Slow moving supercells are common in the high plains of North America where low-level winds oppose midlevel winds to result in slow storm motion. This was no exception on 2012 April 11 when a supercell formed on a stationary front just north of Amarillo. The supercell dropped huge amounts of marginally severe sized hail (~1”) on Rt 287 south of Dumas, TX. In fact, Rt 287 had to be shut down because drifts of hail covered the road several feet thick.
Video courtesy Storm Search 7 Storm Chaser Doug Black posted to YouTube
While the hail in Texas was impressive, AccuWeather’s Jesse Ferrell documents an occasion when hail accumulated an incredible 15-20 feet high in New Mexico on his blog :
An extreme example . . . was the Union County, New Mexico hail storm of August 13, 2004, which dropped a foot of hail but piled up to 15 to 20 feet ahead of a culvert pipe . . . The hail apparently remained there for nearly a month despite summer-like temperatures.
In the picture of the towering hail on Ferrell’s blog, you could easily mistake the hail piles for glaciers. You have to see to believe.