deep.. If seeds need to be 3 in. deep to get to enough moisture, it is likely that, due to uneven soil moisture, some parts of the field will not come up until it rains due to uneven soil moisture and (probably) seed placement.
Algarve: Annual Weather Averages. July is the hottest month in Algarve with an average temperature of 24 Â°C (74 Â°F) and the coldest is January at 12 Â°C (54 Â°F) with the most daily sunshine hours at 12 in August. The wettest month is December with an average of 117.2mm of rain. The best month to swim in the sea is in August when the average sea temperature is 21 Â°C (70 Â°F).
It depends the exact time you go, as usually September is hot in the beginning of the month and tends to gets a little bit colder at the end of the month. I would say that in the south (Algarve) the weather should be around 25Â°C, and still probably beach weather. In Lisbon it would probably be colder, probably around 20Â°C, so probably not the right weather to go to the beach. More info here: September Weather Averages for Algarve, Portugal, and September Weather Averages for Lisbon, Portugal.