B737-800
Flight Crew Last 90 Days 219
Situational Awareness
Deviation / Discrepancy - Procedural FAR; Inflight Event / Encounter Weather / Turbulence
General None Reported / Taken
Human Factors; Weather
Initial Dispatch Release and weather forecast did not require a takeoff alternate. During pushback; fog began to move in and there was a runway change; and during taxi the ATIS changed; visibility was 1/2 mile. The First Officer accomplished the takeoff data and we discussed the change. Holding short of the ILS critical area; we re-briefed the departure and takeoff data. We discussed visibility requirements and because I am a new Captain with close to 300 hours; we discussed the legality of low visibilities. We were cleared on to hold and the visibility was then reported as RVR 800. The First Officer asked if I was legal and I replied yes. During our discussion about my legalities and reviewing runway change and departure; we overlooked the fact that a takeoff alternate would now be required. We did not expect the rapid weather change. Once the weather got bad; I should have sent a message to Dispatch and coordinated with them. They would have then reminded me of the need for an alternate.
We then discussed HUD takeoff requirement and determined it was not required. Takeoff was uneventful. After departure; the Captain realized we actually needed a takeoff alternate due to the reduced visibility below CAT 1 mins.
B737-800 flight crew experiences a rapid change in visibility to below Cat I minimums during taxi out for takeoff. The requirements for the low visibility takeoff are discussed but the crew forgets to request a takeoff alternate from Dispatch.