Snow was expected to fall on higher ground in Wairarapa, south of Masterton, late yesterday and overnight.
MetService forecaster David Miller said heavy snowfalls yesterday afternoon were expected down to 400m, which would knock out the Rimutaka Hill Rd.
These would impact between 6pm yesterday and midday today, with snow accumulations of between 10cm and 20cm.
"Lesser amounts could accumulate down to 300m," Mr Miller said.
The probable weather pattern for the valley floor could include chilly showers and sleet, with temperatures plummeting to zero at ground level.
