Date: 201512
Local time of day: 1201-1800
Location: IAD.Airport, DC
Flight conditions: IMC
Weather elements / visibility: Fog
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B IAD
ATC / advisory: Tower IAD
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Distraction
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue; Inflight Event / Encounter Unstabilized Approach; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented
Contributing factors: Human Factors; Weather
Primary problem: Human Factors

Narrative: We were dispatched without an alternate. Enroute; the weather at IAD did not improve as forecast and was below alternate requirement mins on arrival. Despite my best efforts to save gas; we were going to land 1000lbs down from the flight plan at Dulles; assuming one approach to a landing. During the descent; the FO and I agreed that if we had to go around; the safest action would be to divert to BWI and we told Dispatch this via ACARS during cruise.Traffic in Dulles was packed and ATC gave us min spacing to the point we were worried we would have to go around being too close to plane in front of us and behind us. The FO and I discussed this and I varied my speed and configuration to prevent this since a divert would put us below 45 mins of fuel. We put the gear down and I called for the landing checklist 2 miles prior to the marker. I delayed final flaps intentionally since I was concerned about the fuel burn and was doing my best to comply with ATC's speed instructions while also trying to maintain spacing with the airplane in front of us. Inside the marker; ATC told me to increase speed by 10 knots which I did. This is the same time I'd normally call for Flaps 30. To be honest; I'm not sure whether or not I called for this; even though it's part of my normal habit patterns. We were distracted due to our spacing concerns; without a doubt. We were both also concerned increasingly concerned with how a go around would affect our fuel state. We popped out of the weather around 1000 feet AGL with the visibility at about 2 miles with fog. Shortly after that we got the GPWS 'Too Low Flaps' warning. I called for flaps 30 and the approach was stable so I continued and we landed without incident inside normal touchdown parameters.After the flight we debriefed this incident and I told the FO that I felt it was safer to land; than to divert into an unknown situation to BWI and land with less than 45 minutes of gas; assuming everything went perfectly. He agreed. However; I also pointed out that I deviated from procedures and that this is not something to be taken lightly. The flight manual calls for a go around. We also debriefed ourselves on checklist discipline and talked about not putting the list away until it's complete. I take responsibility for the checklist/configuration/speed errors as the Captain particularly since my FO is brand new. We did our best to thoroughly complete the debrief with honest self-critique; so that these errors do not happen in the future. In the end; I think what I did (deviating from the FM) was the best course of action and the safest way of recovering from our errors and landing the airplane in the safest condition possible; considering fuel and weather.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported they continued the approach to a landing even though they selected final flaps below 500 feet AGL.