Winter weather in Portugal is generally mild but rather unpredictable. There is no lack of rains during that time. But your holiday can also be bright and sunny if you get lucky. On the coast, the temperature rarely drops below 10 °C during the day. It also rarely falls below zero at night. Generally, coastal resorts are your best chance to avoid winter weather and stay warm.
The mountains are of course the coldest during this season, as Serra da Estrela experiences both snow and frosts all winter long. Porto and Lisbon enjoy quite long periods of good dry weather during the winter thanks to Azores Anticyclone. But there are also cold fronts bringing rain and wind. Northern regions, east of Coimbra and Oporto, are colder than the rest of the country and may occasionally see some snow. Although generally the winter is mild there too, and frosts are rare.