Originally Posted by
rvr1800
Correct me if I'm wrong (I probably am) but I don't think there is ever a requirement to add an alternate once you are enroute. Now I'm not saying it's a bad idea. It's a very smart idea. But there is no legal requirement to.
Plenty of aircraft flying around without ACARS so they can't update the destination weather until in range of the broadcast weather.
121 requires it (also requires contact with company dispatch, which is how you learned that the destination wx was headed south before we had all the comms technology).
Back in the day it was primarily a DX function, since they could get the latest DEST WX while you were in enroute.
Nowdays with ACARS it seems that I often catch it by pulling METARS before DX tells me. Then I tell DX how much fuel I have and he does math and provides an alternate. If there's no suitable alternate then it's diversion time.