Currently, EMS flights may begin--without passengers--under Federal Aviation Regulations (FAR) part 91 rules, which allow flight even if weather conditions are not good enough for passenger-carrying commercial operations (governed by FAR part 135 rules). (17) Once a patient is collected, the mission becomes a part 135 flight, so if the weather has not improved at the evacuation scene, pilots are forced to choose between flying back without the patient or breaking FAA regulations and completing the mission.
A case for the medical team to be classed as passengers perhaps, as here in the UK. Then the stricter weather limits would apply for the whole flight.