I would like to clarify why some many apps and websites provide the wrong info on weather in and around the Lagos, Algarve Area.
According to my research, there are only two official internationally recognised Weather Stations in the Algarve at Sagres and at Faro Airport. Most non-Portuguese web sites providing "weather forecasts" for Lagos, Algarve, actually appear to take their readings from the Sagres weather station. Unfortunately, many websites do not take into account that Sagres, lying on a peninsula jutting out into the Atlantic has its own "micro-climate" which can result in weather conditions being totally different than those in Lagos +- 30km away. This is often the case, for example, in the case of BBC weather forecasts for Lagos.
The only reasonably accurate weather forecast for Lagos and other centres in the Algarve is from the Portuguese Met. Office where they extrapolate the data from the main stations and using local knowledge and history make reasonable accurate predictions. (though not all the time)
Portuguese Met Office: http://www.ipma.pt/en/index.html
To find local forecasts, select Faro from the "District" drop-down menu and then the nearest Algarve town to one's location from the City/Town drop down menu.