Narrative:

Taf visibility was 3 SM between an hour before and hour after arrival. Forecast to drop to 1 SM roughly six hours after our arrival. We picked up the ATIS on arrival (1/4 mile; ceilings indefinite 002). Promptly contacted dispatch to work out a viable alternate. Discussed alternatives (ZZZ would have resulted in min fuel with ~2.3 fuel remaining); ZZZ1 is not in our ops specs; ultimately decided on ZZZ2 as our alternate. The RVR was fluctuating between 1600 and 2000. RVR dropped to 1600 (needed RVR 1800 for 19C) just prior to reaching FAF; we went missed. Dispatch wanted us to try again; which was not going to happen with ZZZ2 deteriorating as well. Proceeded direct to ZZZ2 (ATIS: vis 2 1/2; 003 overcast) and landed. Though not required; I should have requested an alternate with taf reading 3 SM; light winds; and a tight spread. [Suggest] dispatch provide an alternate whenever borderline necessary. [And] training for dispatchers on how to look out for 'unexpected' deteriorating weather; classic case-in-point above. [Lastly] monitor destination weather and communicate deteriorating weather with pilot's enroute to help the crew start planning prior to arrival when workload increases. [Suggest] company stress/communicate conservative fuel planning to dispatchers when conditions anything less than ideal. [And] CAT ii capability would have given us extra margin for safety; and it would have allowed us avoid the diversion.

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Original NASA ASRS Text

Title: Flight crew reported they were legally dispatched to IAD with no alternate; but were forced to divert to an alternate when weather deteriorated enroute.

Narrative: TAF visibility was 3 SM between an hour before and hour after arrival. Forecast to drop to 1 SM roughly six hours after our arrival. We picked up the ATIS on arrival (1/4 mile; ceilings indefinite 002). Promptly contacted dispatch to work out a viable alternate. Discussed alternatives (ZZZ would have resulted in Min Fuel with ~2.3 fuel remaining); ZZZ1 is not in our Ops Specs; ultimately decided on ZZZ2 as our alternate. The RVR was fluctuating between 1600 and 2000. RVR dropped to 1600 (needed RVR 1800 for 19C) just prior to reaching FAF; we went missed. Dispatch wanted us to try again; which was not going to happen with ZZZ2 deteriorating as well. Proceeded direct to ZZZ2 (ATIS: vis 2 1/2; 003 overcast) and landed. Though not required; I should have requested an alternate with TAF reading 3 SM; light winds; and a tight spread. [Suggest] Dispatch Provide an alternate whenever borderline necessary. [And] Training for dispatchers on how to look out for 'unexpected' deteriorating weather; classic case-in-point above. [Lastly] Monitor destination weather and communicate deteriorating weather with pilot's enroute to help the crew start planning prior to arrival when workload increases. [Suggest] Company Stress/communicate conservative fuel planning to dispatchers when conditions anything less than ideal. [and] CAT II capability would have given us extra margin for safety; and it would have allowed us avoid the diversion.

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.