A B757 First Officer pointed out during the preflight weather briefing that the alternate weather was questionable but a suitable diversion airport was available. The Captain read the weather incorrectly during planning. Prior to landing Dispatch changed their destination to the diversion airport.

Narrative

While at the hotel I took a quick look at the flight plan; weather; and NOTAMS a short time before show time. I mentioned to the Captain that I also looked at the weather and NOTAMS for another airport near our alternate because I was familiar with weather patterns of this area and our alternate looked marginal. Weather at the second/diversion alternate was good VFR and there were no significant NOTAMS affecting our flight should we need to divert there. The Captain briefed that he would recheck the weather upon arrival at the aircraft. The Captain returned from break about two hours from our arrival. I pointed out that our destination alternate airport forecast did not meet our Operations Specifications. After reviewing the weather again; the Captain realized that he had mistaken the terminal forecast for another airport or misinterpreted the terminal forecast contained in the weather packet. During our discussion of other options; we received two calls from Dispatch; one to alert us about the marginal destination weather and another to change the destination alternate to the VFR diversion airport as the crew had previously discussed. We had already reviewed the diversion weather and NOTAMS and it was VFR and the release was amended for the new alternate. During the preflight at departure we were all very busy with our normal duties. Because of this we overlooked the need to confirm that the Captain had rechecked the alternate terminal forecast.