Dash 8 Series Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 145; Flight Crew Total 22310; Flight Crew Type 7700
Fatigue
Ground Excursion Taxiway; Inflight Event / Encounter Weather / Turbulence
Human Factors; Weather
The airport was covered by approximately 2-4 inches of fresh snow. The runway was plowed. Braking action was good. Landed Runway XY; exited onto H. Tower cleared us to taxi A; F; across ZA to park. I noticed that the taxiways were not plowed. The centerline and edges obscured. I referenced the taxiway reflectors (raised) to steer. A difficult task as the reflectors are set a good distance from the taxiway edges; and the taxiways are narrow. Nosewheel steering seemed effective. Low light also reduced visual detail. It was overcast at 600 ft. ½ miles in mist. Taxi A angles 30 degrees left onto F. I began steering left; and started the after landing flow (only a few items). I might have also tried to reference the airport chart. I then found the aircraft; possibly; too far left. I applied the tiller to steer right. The left main dropped off the taxiway into the soggy grass. I idled. Maintenance suggested power. It only slid the nose gently into the grass. I shut down the engines. Ground transport took the passenger to the terminal. No injuries; orderly.Causal factors: fatigue due to short sleep the night before; therefore I 1) Didn't recall we require taxiways plowed.2) didn't fully gauge the level of fatigue3) didn't stop the aircraft to eithera.have taxiways plowed b.at least deliberately evaluate situationc.bus pax to terminal4) Multitasked when full attention should have been on taxiing.I mitigated the possibility of fatigue with caffeine earlier; but; as this situation occurred; I was feeling the effects of the previous night's short sleep.Secondary causal factors.Complacency/comfort with conditions. We'd been operating in snow for 2 months; so it did not raise my concern as much as it might have 2 months earlier.Solutions:1) Quantify a minimum amount of sleep.2) More deliberate and continuous evaluation of alertness.3) Re-evaluate sleep conditions.
DHC-8 Captain reported an excursion from a snow covered taxiway.