In reply to glaramara:
It's not really a valid concept.
The higher areas on Snowdon / Crib y ddysgl, and the higher areas of Ogwen and Cwm Lloer remain snowy longer than other areas but none are in any way reliable - it's all about the prevailing weather conditions at the time.
We had winter conditions, with fairly good snow cover (some skiing) and some winter routes in condition a few weeks ago but since then the snow has all gone and it's been nearer spring weather and conditions, till the last few days, when it's turned cold but very dry.