The Marques de Pombol, Stylish Fellow He Is, Surveys His Beloved City
Weather
Lisbon is great in the Spring and the Fall, hot in the summer, and cold, windy, and wet in the Winter.
Don't be fooled by ideas of sun-drenched outdoor cafes if you are coming in November through March. Think rather puffy jackets, and always, always have an umbrella handy.
If you'll stay at a short-term rental, in the winter make sure it has a dehumidifier and either central heat or heat in all the rooms. Most buildings in Portugal are made of cement and brick, often without insulation, and they're cold and damp. Mold is often a problem. In summer make sure it has air conditioning in all the rooms... not just a split in the living room or bedroom. Hotels above 1 star should be fine for both heat and a/c.
Lisbon has some cool nearby beaches, but the sea water is going to be brisk: 21°C / 69°F in the summer and 15°C / 60°F in the winter.
More at Wikipedia's Climate of Lisbon page.
Clothes
More on what to wear in Lisbon from TouraMaybe not as stylish as Milan, but nice.
Think practical, comfortable, layered, understated, and smart casual.
Unless you really want to look like a tourist, consider avoiding gym clothes or any kind of activewear; sports team or college logo-wear; leggings; or that quick-dry lime green microfiber t-shirt that seemed so perfect for travel when you saw it at REI. In fact avoid anything from REI, particularly zip-off pants, hiking boots, fishing shirts, a tan multi-pocket safari vest, and walking sticks. This is a fairly sophisticated urban capital, not the Camino de Santiago.
Most important: the shoes! Not just for comfort, as you'll walk a lot, but for grip. The sidewalks in Portugal are typically made of calçadas, which are irregularly shaped 2" x 2" pieces of rock that are hand-laid, like a mosaic. They are notoriously uneven, unbelievably slippery when wet, and sometimes missing in chunks. Ladies, leave the heels at home!
In the winter you'll want a warm coat and maybe a lightweight raincoat.
The old advice has always been "bring half as much stuff and twice as much money" and that certainly applies. Think twice about having more than one rollie suitcase, and keep the one you bring to international carry-on size. We've seen lots of people struggle with monster bags as they're trying to squeeze through the narrow doors of a train, or schlepping the heavy things up and down multiple flights of metro stations. Jaw-dropping pro-tip: there are stores here. Cool ones. You can buy more clothes here!