Well not many sectors are limited to WAT
conditions. Most short haul aircraft (except perhaps transcontinental in the USA) are limited by max ZFW. Most aircraft can take full pax and baggage (though we sometimes have problem with A320-100) and some freight. The planners know about how much freight to accept, and the dispatcher can offload it on the day if Max ZFW is exceeded. Only sectors over around 3 hours can be a problem with Max TOW., and then usually because the airline has a limited Max ZFW for economy. (When you buy an aircraft you can specify the max TOW. More TOW costs money. Airlines specify lower TOW to save on landing fees.).
On long haul the scenario changes because the runway can be the limiting factor. But this is taken into account months before when the planners work out the payload and how many pax or how much freight to accept.
This can be hit on the head on the day with an icy/slippery runway, which can reduce TOW by 30 tons.
I depart B777 on a 12 hour sector every day and with a dry runway there is always weight to spare. Max pax and full freight holds and still 30 tons under the limiting weight. (Max ZFW or Max TOW or Max LDW. whichever is the lowest).
Then about 2 hours before departure a Computor flight plan is generated based on the expected ZFW and weather. The pilots look at this about 45mins before departure and choose a fuel figure. Occasionally if there is a limiting weight they may wait until the Final ZFW is produced before filling the tanks to Max TOW, but this is not so common nowadays.