With the destination weather forecasted to be below minima for the ETA plus/minus one hour, you can stil plan and dispatch the flight to this destination aerodrome, however in this case two (2) destination alternate aerodromes are required acording to JAR OPS 1.
Fuelwise you have to plan with the destiantion alternate aerodrome requiring the higher amount of fuel. This fuel is then to be used as alternate fuel for calculating the minimum fuel required for dispatch.
On the ATC flightplan you have to enter the destination as well as both destination alternate aerodromes. On the manual ICAO flight plan form you will find therefor the field labeled "2nd ALTERNATE AERODROME".
However during flight the pilot in comand may decide to which aerodrome he diverts, this can be any aerodrome and is not limited to the alternate aerodromes mentioned in an ATC flightplan.