The Mountain Road will be closed from midnight tonight.
Police originally said the closure would apply from 6.30pm, but the plan was changed after a revised forecast from the Met Office.
The Island has escaped the worst of the snow which has caused major transport problems in many parts of the United Kingdom.
However, the latest forecast from Ronaldsway says sleet or snow tonight will leave a covering over the hills, and at lower levels, in some central and western parts of the Island in particular.
Gritters have been out on the main roads and police are telling motorists not to travel unless the journey is essential.