Not all planes/operators are CAT III equipped or certified. That said, ceilings/vis have been causing a lot of delays this last week in BOS, but they haven't been THAT low. What I'd bet is A) lightening reported on or near the field, B) windsheer/svr turb reported on approach or departure from the field (with the rain and wind in those METARs, that'd be my bet), or C) one of the many many airfield maintenance guys out working messed something up
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We reported +-35 kts with continuous moderate to severe turbulence from the surface to 3,000 on climbout from BOS a month or so ago. Come to find out, three planes (including one from our company who heard our report/told me this story) on the approach were spun and held for a half hour, everyone else on vectors was tucked into a corner somewhere, and all departures were halted for a half an hour.
I think it'd be pretty hard for the ceiling/vis to be low enough for them to close the airport; they just use low visibility taxi procedures, and anyone who can't start the approach is held. Lots of people might go missed, but I'd be surprised if they close the airport (ground stop, sure; closure, probably not).