Low villages lose snow weeks that high ones keep. That is the planning fact. Les 3 Vallées says 85 percent of its domain is above 1,800 m. Val d’Isère says 60 percent of the linked pistes are above 2,200 m, with two glaciers on the domain. La Plagne says more than 70 percent of its skiing is above 2,000 m. Those are the resorts’ figures, and they are why a Christmas week and an April week are not the same bet in Megève as in Val Thorens.
They are not a climate model. This page will not invent a degree of warming, a closure date, or a moral score for travelling. Read Météo-France and the resort snow report for the week you go. Snowmaking covers some pistes and not the off-piste. A glacier that is open in a brochure can be shut for wind on the morning.
The practical response is to pick altitude for the dates you have, and to keep a valley day in the plan when the top lifts close.