Persistent snow is forecast to set in across Hull and East Yorkshire for much of the weekend.
The Met Office predicted the worst of the weather this evening and into Saturday morning - but lighter downfalls will continue into next week.
Snow may ease on Sunday, but wintry showers and sleet are forecast on Monday.
Temperatures will remain around freezing over the weekend, with a high of 3C, but could fall as low as -8C on Monday.
Many schools serving more isolated areas of the East Riding closed early on Friday and the evening rush hour passed without major problems.
A Stagecoach bus crashed on the A165 at Ganstead this afternoon but it is understood the weather was not thought to be a factor.
But with conditions deteriorating, commuters are being warned of potential transport disruption on road and rail.
EYMS said tonight its buses were suffering delays on rural routes, with the Wolds and parts of Holderness particularly hazardous, including the road between Hedon and Withernsea.
Hull City Council's gritters have been out since 6.30pm.
East Riding Council said it was prepared for up to four inches of snow over the weekend and had fitted ploughs to its gritters.
Nigel Leighton, director of environment and neighbourhood services, said he was expecting significant accumulations of snow, particularly in the Wolds.
He said: "With snow forecast and continued sub-zero temperatures, the council's focus will be on keeping the main highway network open to motorists, businesses and the travelling public."
Motorists were advised to drive with "extreme caution" on untreated minor roads.
On the railways, Northern Rail - the main local provider - said it was running a "good service" and expected trains to be running as normal on Saturday.
East Coast Trains and First Hull Trains lifted ticket restrictions to help passengers travelling between Hull and London.
Passengers were urged to check the National Rail website before travelling.
Humberside Airport was open but reporting some delays. Travellers were asked to check with their airline for further updates and allow extra time for travelling to the airport. Updated 7pm.
• East Yorkshire schools closing early as heavy snow forecast
