Who Cares,
If you don't have the fuel on board to cater for the weather requirement, whatever it is, (INTER/TEMPO/ALTERNATE):
1. Nominate a suitable alternate (If FOB on board allows)
2. Divert to an en-route airport to pick up extra fuel. (Insufficient FOB)
3. If the departure or any en-route airport can not be reached with required fuel reserves, declare a PAN and continue.
It's really very simple.