Date: 201906
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: Windshear; Thunderstorm
Light: Night
Aircraft: Regional Jet 900 (CRJ900)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Landing
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Physiological - Other; Workload; Situational Awareness
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Contributing factors: Human Factors; Manuals; Procedure; Weather
Primary problem: Human Factors

Narrative: This flight was influenced by the day before. We had an 11:34 hour duty day and min rest due to weather at our destination. The next day we showed to our flight and had a maintenance issue. By the time we got to the flight we were on leg 4 and 9.5 hours of duty with minimum rest. It looked like there were thunderstorms West of ZZZ1 moving east. These storms were going to get to ZZZ1 before the forecast was calling for them. We loaded the plane and taxied out. While taxiing we were told all west bound departures were shut down. After about 40 minutes on the ramp we were cleared to take off. About 100 miles away we called ZZZ1 Tower to see how the weather was doing. He told us it was moving slowly and if we keep our speed up; we could get in before the storm. When we were 15 miles from ZZZ1; a wind shift occurred. We switched our plans for landing to a different runway. On final approach we got a Wind Shear Caution around 500 feet. We executed the missed approach. To avoid the thunderstorms to the east we made a right turn and cleaned up the missed approach. After a few vectors we entered a hold pattern. While we waited in the hold Tower was saying that the storm would pass in about 45 minutes. Both the First Officer and I were getting more fatigued as the flight continued. We made the call to go back to ZZZ. While on the approach into ZZZ I called for the Before Landing checklist. Both the First Officer and I thought we saw the gear down. At about 1;000 feet we heard the gear warning horn. We then got the gear down. We are not sure on altitude. After it was down a little later we heard the 500 foot call. We landed in ZZZ with a 13:06 duty day. We were not sure if we should have done a go around.


Second narrative: We were on a visual approach to ZZZ; we completed the before landing checklist. At some point after that a warning tone started. Captain initially thought it we had in error selected a DA. As I looked for a DA I discovered that the gear was up. I communicated that to him and he promptly requested gear down. I selected gear down. We continued the approach to an uneventful landing. The warning was after the before landing checklist but I am not sure that it was before the '1;000 feet above the field stabilized' call. This was day 3; all four legs the previous day were extremely difficult and complicated due to weather in ZZZ. We arrived at the hotel early in the morning of day 3 and were on minimum rest when we started day 3 that afternoon. Despite that I felt fit for duty throughout day 3 at the start of each flight. On the subject flight; we were on final approach and received a windshield alert. We responded to that alert and it quickly subsided only to have a second alert on the climb-out. We departed the area and entered a hold. That high workload event drained my energy reserve significantly. We ended up leaving the hold to return to ZZZ and that is where the deviation occurred. I'm not absolutely sure what sequence of events lead to gear being in the retracted position. I am also not sure how both of us missed it in the before landing checklist. Since we were not sure of if this happened before the 1;000 feet call we should have executed a go-around.


Synopsis: CRJ-900 flight crew reported after they initiated a go-around due to a wind shear alert and holding for weather they diverted to another airport and forgot to lower the landing gear until receiving the gear warning horn.