For daily (and longer) forecasts, try:
Institute of Meteorology, Portugal – English language version giving a general forecast for the country and 10-day forecasts (select district and individual town from the drop-down menu near the top of the page).
Spanish forecaster El Tiempo also produces forecasts for Portugal (and are probably more accurate).
BBC Weather – link to the five-day forecast for Lisbon (go to Find A Forecast for other Portuguese towns.
Climate Charts.com has weather statistics for a number of key locations around Portugal.
Weather & Climate.com has a general overview of the Portuguese climate plus links to summaries and detailed statistics for hundreds of locations around the country.
Portugal’s Meteorological Institute is also an excellent site for weather stats. Some of the information is only in Portuguese (or bad English) but the charts are reasonably easy to understand.
For historic information, see climate normals then – from the links on the right – select the historic period, temperature or precipitation, and the location.