Well, right now the Seahawks and 49ers are delayed due to lightning. Obviously rain isn't the concern, its the explicit lightning.
Another answer:
The complete cancellation of an NFL game is extremely rare, but the league will suspend play when there is dangerous lightning in the vicinity of the stadium. On 9-15-13, two games (Saints/Bucs in Tampa and 49ers/Seahawks in Seattle) were both delayed by lightning for over an hour each.
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