EMB ERJ 145 ER/LR
Dispatch Dispatcher
Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR; Inflight Event / Encounter Weather / Turbulence
General Declared Emergency
Weather
Flight ORD-ZZZZ was en-route to ZZZZ with a listed alternate of ZZZ.  While flight following; a new TAF for ZZZ came out that was below alternate minimums. I sent an ACARS to the crew advising them of the situation and asked for their FOB and what they were showing landing with.  Based off of the numbers they provided me and the available airports to use to change their alternate to; there were no legal options for an alternate.  I advised crew that there was no legal alternates that we could use and advised them that we could not legally continue without a legal alternate and needed to divert to get fuel.  Captain then advised me that the TAF for ZZZZ was no longer calling for CAT II conditions and wanted to know if that changed anything.  I advised him that it did not and that we still needed to divert due to not having a legal alternate for the flight.  Captain then stated that he thought that the alternate minimums were only for dispatching and not applicable once airborne.  I advised him that the alternate needed to remain legal throughout the flight and again told him that we had no legal alternates available for use to continue the flight.  At this point he said that he was using his Captain's emergency authority and would continue onto ZZZZ.  Flight landed in ZZZZ without incident.
A Dispatcher informs a flight crew; after takeoff; that their alternate was no longer legal. Captain questions if he needs an alternate anymore as his destination was no longer reporting CAT II conditions. Captain decides to declare an emergency; and continue to destination after Dispatcher requests they divert for fuel to a different alternate to stay legal.