It certainly is not unheard of, but there is still frost in the soil across northern North Dakota and some isolated pockets elsewhere as well. Most of the frost in around 30 inches in depth and the recent cool weather has kept that frost from melting. The warmer temperatures from the past couple of days has helped a little, but the rain that is expected in the next couple of days might be what is needed to finally get rid of the frozen soils. Frost in the soil has been known to linger into June on some of colder springs and on rare occasions even into early July. Based on the weather, that seems doubtful in 2018, but in these parts, never say never.