If the outbound was delayed by weather (and it still counts as extraordinary even though it snows and is windy in Europe in February) then the inbound can still be covered by the EU261 exemption but a third flight involving that plane would so e.g the plane was due to fly
AMS - IAD
IAD - AMS
AMS - HKG
If the first flight was delayed by weather then the second would be excluded from EU261 but not the next flight to AMS to HKG
If the OP could supply the details of the original and replacement flight times then they would get more tailored advice.