could be a number of things,air lock in system,heater radiator could be knackered, thermostat could be closed,get it checked out before the snow comes.
Hi,I recently had a similar problem on my 95 mondeo,I cured it by fitting a new thermostat,...the old one was jammed in the open position,allowing the heater water to be 'cooled' before it reaches its correct tempreture(92 degrees centigrade),..no problems since.( it is a common problem on Ford zetec engines..hope this helps !