A DHC-8 First Officer reported detecting two flat nose gear tires on taxi out. The tires appeared normal on walkaround inspection.

Narrative

Upon arrival to the aircraft I commenced my preflight walk around inspection; everything on the aircraft appeared normal. I took my seat and started getting the aircraft ready for departure. The Captain and I completed the departure review; checklist and weight and balance. We handed the paperwork and called for pushback. The pushback and engine start were uneventful. Once both engines were started we ran the after start checklist and called for taxi with ramp control. On the turn we felt a small vibration like the one you get when taxiing over snow and ice; and since snow and ice was all over the ramp we figured that's what was causing the vibration. We called Ground for taxi; obtain taxi clearance and started taxiing when the vibration started to occur yet again this time more severe. The Captain and I determine we had a flat tire. We stopped the aircraft; notified Ground Control; company and operations. I notified the flight attendant as to what was going on and made an announcement to the passengers. We secured the aircraft and deplaned the passengers. Passengers were loaded on a bus and returned to the terminal. It's a difficult discrepancy to notice when both nose gear tires are flat because both tires appear normal. Suggestion is if they do appear a little low or one looks less inflated than the other notify maintenance and let them determine the tires status.