If you start in Lausanne in January, the winter is quite mild with temperatures around 5°C in the day and 0°C at night. The weather changes a lot: sunny, cloudy, with chances of heavy or light rain. When you reach Vevey, Aigle, and Lake Geneva, it gets a bit colder and there can be frozen rain.
As you go through Saint-Maurice, Martigny, Orsières, and Bourg-Saint-Pierre, it gets even colder with a lot of snow, especially at night. The Great St. Bernard pass is very cold and rainy.
By the time you get to Italy, it starts to get warmer. You'll see higher temperatures in Aosta and Piedmont, lots of sun, some clouds, and possibly some frozen rain. Lombardy and Emilia-Romagna are usually foggy with sunny spells and temperatures between 5°C and 7°C in the day, dropping to near freezing at night.
In Tuscany, temperatures rise to around 9-10°C in the day and 2°C at night, with a mix of sun and rain. It may even rain more when you reach Lucca and San Gimignano.
When you reach Rome, you can expect warmer and sunnier weather, with temperatures up to 14°C in the daytime and 5°C at night. There may be some showers and clouds, but there are usually more sunny days as you reach the end of your trip.
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