Weather conditions in mainland Portugal are expected to reflect a typical winter pattern in the final days of the year, under the influence of an anticyclone located south of Iceland.
Between December 29 and 31, the country will experience cold temperatures, persistent inland fog and conditions favourable to frost formation. The weather pattern results from an anticyclone extending towards western Europe, channelling polar air masses into Portuguese territory.
According to the Portuguese Institute of the Sea and the Atmosphere (IPMA), this atmospheric configuration is producing a weak easterly airflow, leading to cold nights and mornings marked by mist and fog. These conditions are expected to be most persistent in the inland north and centre, particularly in the north-east of Trás-os-Montes and Beira Alta, where fog may linger until late morning or even throughout the day on December 30 and 31.
Elsewhere, skies are expected to be mostly clear or partly cloudy. However, on the afternoon of December 29, cloud cover may increase temporarily, especially inland, with a low chance of showers. If precipitation occurs, it may fall as snow at the highest points of the Serra da Estrela.
Winds will generally be light, becoming moderate at times in higher ground. From the afternoon of January 1, the approach of a North Atlantic depression is expected to bring stronger winds, particularly in elevated areas along the central coast and in the south, with occasional strong south-easterly gusts.
Minimum temperatures are set to fall across the country on December 29, dropping by between two and four degrees Celsius, with further localised decreases until January 1 in the north and centre. Overnight temperatures are forecast to range from 2°C to 4°C across much of the country, rising to between 4°C and 7°C along the coast. In inland northern and central regions, minimum temperatures may fall to between -3°C and 1°C.
In the north-east of Trás-os-Montes and Beira Alta, the combination of fog and sub-zero temperatures is likely to lead to rime formation. Frost is expected to be widespread in inland areas.
Maximum temperatures are not expected to change significantly, except in areas affected by persistent fog or low cloud. In these regions, daytime highs may remain between 3°C and 5°C.
Elsewhere, maximum temperatures should range from 11°C to 15°C, reaching 16°C to 17°C in the Algarve, particularly along the coast. On January 1, increasing cloud cover from south to north may cause a slight drop in daytime temperatures in central and southern areas.
Sea conditions will remain relatively calm. On the west coast, waves of one to 1.5 metres are expected, shifting to a south-westerly direction from the morning of January 1. Along the Algarve coast, waves will initially come from the south at around one metre, easing on December 29 before increasing again to between one and 1.5 metres from the south-east on the first day of the new year.
