Date: 201902
Local time of day: 1801-2400
Location: SEA.Airport, WA
Flight conditions: Marginal
Weather elements / visibility: Snow
Light: Night
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower SEA
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Situational Awareness; Communication Breakdown; Distraction
Second reporter function: First Officer
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Diverted; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Company Policy; Procedure; Human Factors
Primary problem: Company Policy

Narrative: Due to winter weather conditions in SEA the Takeoff time in order to get to ZZZ prior to runway closure was exceeded. The crew's focus on winter weather ground operations; distracted our attention away from Dispatch destination requirements. Upon arriving to our gate of departure for ZZZ; a recent SEA ground stop due to heavy snow and reduced visibility had just been lifted. Thus; our passengers and crew were not delayed boarding. During the final minutes of the boarding process; I made a PA to let the passengers know what to expect for our taxi out plan; and although we were still waiting on a de-ice plan; we knew we would need to de-ice either at gate or remotely. That done; we ran our checklists while waiting to hear back from SEA Iceman as to our de-ice plan. We eventually got the word that we were in queue for remote operations.Upon contacting Ramp for push; it seemed to us that our clearance would be granted after the aircraft to our right was clear. Watching them attempt to push; multiple times in the snow with slick conditions under the tug; gave us a good understanding of what we might be in for as well. Also [Air Carrier] had a plane to their right-rear that was stuck and impeding the flow of traffic. All of this is to say that ground operations were slow going. When I had made the PA to the passengers; we had not assumed being in this lengthy of a delay. I thus made an additional PA re what was happening around and affecting us. We got a message from Dispatch reference ZZZ runway closure. This being prior to our physically getting pushed out from the gate area; I called Dispatch and received the message to continue operations going to ZZZ. The time frame for getting out and off with a plausible appropriate ZZZ runway time was still thought to be very doable. Also; and although; during my phone conversation with Dispatch the fact that he stated that ZZZ will not extend their hours of operation did not sink into my takeoff calculations! I basically deflected that data; assuming that the company could make something happen. Although the heavy snow and snow in general had ended; we had previously looked at a holdover time of down to six minutes. If the snow was to begin again after our de-ice; that would be a holdover time we would have to contend with. With this in the background of our minds and upon clearing de-ice; Tower commented that our flight plan dropped out of system. We were issued a new beacon code and cleared for takeoff. Dispatchers should emphasize the current status of operational limit intent in non normal situations. Pilots should insist on a defined plan of action in non normal situations. In this situation; the message that was received of 'continue ops for KZZZ ... push; de-ice; etc...will get paperwork to you soon'. Could have been stated 'will get paperwork prior to departure' or 'do not depart until paperwork received'.Pilots should ask more questions along the lines of when can I expect the new paperwork. Not time based but rather operationally based. Push; Taxi; De-Ice; Takeoff. The Tower should not clear an aircraft for Takeoff when the flight plan dropped out.


Second narrative: Due to SEA ground delays caused by a winter snow storm; we departed Seattle beyond a departure time that would allow us to take takeoff; fly to and land at our destination before the runway would be closed. We were extremely focused on the safety surrounding the ground operations as other airplanes were already stuck on icy ramps or taxiways. Wind shear was also reported so there was a multitude of distractions involved. When I arrived at the airplane I turned on the overhead speaker so I could listen into ATC and ops/deciding to get an idea what was going on. It appeared deicing was occurring at the north deice pad. I informed the captain when he arrived and we called to get in the queue. We were told a few times from Iceman that they would get back to us with a plan in 10-15 min (we had to keep calling them because it was so chaotic and they would forget about us). At one point we heard that deice had been suspended due to the heavy snow and a ground stop. Runway was closed for snow removal during this time. While at the gate we received an ACARS message from dispatch informing us [what time] we needed to be airborne to make the runway closure time at ZZZ. The captain called dispatch and spoke about the message but I'm unaware of the exact details of that conversation. We finally got a slot up at the pad and called for push but had to wait for a company Aircraft to push first and their tug driver was having difficulty getting them pushed all the way out due to the slick conditions. There was also [another] jet under tow behind and to the right of this company Aircraft who was already stuck on the taxiway. Once we finally got our push clearance we began to push when the shear pin on the tow bar sheared off. We informed Ramp control who canceled our push while the ground crew retrieved a new tow bar. We then got clearance again to push and pushed very slowly. The captain informed dispatch we were pushing. They replied that they may have to send us to an alternate but that they would get back to us. We deiced at the pad and the captain informed dispatch that we were deicing (just keeping them in the loop). Dispatch said we were likely going to go to ZZZ1; and then 'leaning towards' sending us to ZZZ2 but that they would get back to us shortly. It was under my impression that we were still going to ZZZ and that a decision to amend the release to a new destination would come while enroute if we could not land at ZZZ. After deicing tower informed us that our flight plan dropped out but they had a new squawk code for us and we were now cleared for takeoff. We took off and saw another ACARS message that we were going to ZZZ2. Captain called dispatch on satcom and we became aware of the miscommunication.  Dispatchers should emphasize the flight is no longer released and to await further plan of action. Or 'captain call dispatch prior to departure.'  Pilots should call dispatch to clarify. Texting back and forth via ACARS is not the best communication method. Assumptions get made when they shouldn't.


Synopsis: Air Carrier flight crew reported departing without knowledge the release was to an alternate destination instead of the planned destination.