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.
Some cars were partially buried in hail according to reports. And snow plows were dispatched. Check out this video of the flash flooding rain and banks of hail developing.
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.