B737-800 flight crew experiences a rapid change in visibility to below cat I minimums during taxi ou...

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1204796
Time
Date	201409
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Taxi
Flight Plan	IFR
Person 1
Function	Captain Pilot Flying
Experience	Flight Crew Last 90 Days 219
Person 2
Function	First Officer Pilot Not Flying
Experience	Flight Crew Last 90 Days 174
Events
Anomaly	Deviation - Procedural FAR Inflight Event / Encounter Weather / Turbulence

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.