Narrative:

Flight was dispatched from airport with 300 lbs. Greater than minimum fuel. Weather in the destination airport of boise; idaho was forecast to be at greater than 6 SM thus no alternate was legally required. We flew to the destination and about 20 minutes out; after having briefed the 25R CAT I approach; we were advised by center that the visibility in boise; identification had rapidly dropped to 1600 runway visual range (RVR). Thus; myself and the captain ran the procedures checklist in preparation for a CAT ii approach. Within about 10 minutes before our scheduled arrival into boise; we were then advised that the visibility had dropped to less than 1200 RVR at the airfield which was less than our company certified RVR for CAT ii.the captain decided to divert to salt lake city and things became very very busy in the cockpit. After having climbed out from 8000 feet back to 25;000 feet while the captain was neck deep in workload; I did some rough calculations and had learned that we will land in salt lake city with about 300 lbs. Of fuel. After discussing with me that salt lake city will have company facilities that can accommodate the will be displaced passengers; I told him that I disagreed with the slc decision overall due to my fuel concern and instead suggested 2 nearer options. I was not able to pull the charts for twin falls; so I suggested pih airport with a 9000+ foot runway. Captain did not argue with me and employed excellent CRM and respectfully chose to go to pih per my suggestion. We advised ATC minimum fuel plugged and briefed and diverted to pih airport. Despite all our efforts; we had still received a red EICAS warning message on both tanks upon arriving downwind of the traffic pattern at pih and landed thirsty with 1200 lbs. Of fuel with a red fuel lo EICAS message for tanks 1 and 2. No further incidents after landing.suggestions: further and more accurate studies of the meteorological progressions of weather in boise; idaho. We were told that weather in that airport has had a history of deteriorating rapidly; and so; albeit legally done it does not make sense to dispatch aircraft to that airport without an alternate and alternate fuel until we can get more accurate prognostic reports of the weather in that region. Weather was forecast to be 6 miles visibility but it was not the case when we got there. No one did anything wrong or illegal or inappropriate; from an operational standpoint but it does not seem right for this to happen again.

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Original NASA ASRS Text

Title: Air carrier First Officer reported an emergency divert due to deteriorating weather at destination; no planned alternate; and resulting in landing with less than legal minimum fuel.

Narrative: Flight was dispatched from airport with 300 lbs. greater than minimum fuel. Weather in the destination airport of Boise; Idaho was forecast to be at greater than 6 SM thus no alternate was legally required. We flew to the destination and about 20 minutes out; after having briefed the 25R CAT I approach; we were advised by Center that the visibility in Boise; ID had rapidly dropped to 1600 Runway Visual Range (RVR). Thus; myself and the Captain ran the procedures checklist in preparation for a CAT II approach. Within about 10 minutes before our scheduled arrival into Boise; we were then advised that the visibility had dropped to less than 1200 RVR at the airfield which was less than our company certified RVR for CAT II.The Captain decided to divert to Salt Lake City and things became very very busy in the cockpit. After having climbed out from 8000 feet back to 25;000 feet while the Captain was neck deep in workload; I did some rough calculations and had learned that we will land in Salt Lake City with about 300 lbs. of fuel. After discussing with me that Salt Lake City will have Company Facilities that can accommodate the will be displaced passengers; I told him that I disagreed with the SLC decision overall due to my fuel concern and instead suggested 2 nearer options. I was not able to pull the charts for Twin Falls; so I suggested PIH airport with a 9000+ foot runway. Captain did not argue with me and employed excellent CRM and respectfully chose to go to PIH per my suggestion. We advised ATC minimum fuel plugged and briefed and diverted to PIH airport. Despite all our efforts; we had still received a red EICAS Warning message on both tanks upon arriving downwind of the traffic pattern at PIH and landed thirsty with 1200 lbs. of fuel with a red fuel lo EICAS message for tanks 1 and 2. No further incidents after landing.Suggestions: Further and more accurate studies of the meteorological progressions of Weather in Boise; Idaho. We were told that Weather in that airport has had a history of deteriorating rapidly; and so; albeit legally done it does not make sense to dispatch aircraft to that airport without an alternate and alternate fuel until we can get more accurate prognostic reports of the weather in that region. Weather was forecast to be 6 miles Visibility but it was not the case when we got there. No one did anything wrong or illegal or inappropriate; from an operational standpoint but it does not seem right for this to happen again.

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.