"Reason this won't work" Document, document, document!  

Document, document, document! WX onservations and forecasts, your experience in assessing the risk and the alternatives you have/had. This gives management ground to assess and instruct as appropriate. Of course, you should also go over these things with the crew.

I don't remember the quote source, but it was to the point that it's better to wrong and safe on the ground than wrong in flight.

Whatever happens, don't create the impression that you are 'controlling' their decisions. You build trust when everybody takes part in the decision. And the decision process is very different aviation/med crew. A point I think some pilots forget is that you won't have the responsibility for the patient care when things go South on the transport leg...