Thanks to Sheila Kramer for tipping me to this unusual natural occurrence, as explained on the website of the National Weather Service's office in Spokane, Washington:
On the evening of March 31st, 2009, Tim Tevebaugh was driving home from work east of Craigmont in the southern Idaho Panhandle. Across the rolling hay fields, Tim saw a very unusual phenomenon. The snow rollers that he took pictures of are extremely rare because of the unique combination of snow, wind, temperature and moisture needed to create them. They form with light but sticky snow and strong (but not too strong) winds. These snow rollers formed during the day as they weren't present in the morning on Tim's drive to work.According to the NWS, those snow rollers are 18" to 2' high. See more of Tim's photos here.