I believe it depends on the airlines fuel policy.
This is a quote of the fuel policy of a particular airline "
....at all time in flight onboard fuel shall not be less than :
- flight fuel
- alternate fuel (if required)
- fixed and variable reserve
- weather fuel (when required)
Therefore (for this particular airline) I believe it means that if the forecast deteriorates to below the alternate minima, you would now need alternate (or holding) fuel if you want to continue to the destination.
Thats my interpretation anyway
