Hard to say! Yes we did have a heat wave but that is over. Things are back to normal which is to say day time temperaturs on the plain of 24-27C. Last night it was 12C here at 471m. Obviously the higher you go the cooler it gets.
September is a classic "changeover" month and temperatures fluctuate. So expect, on the plain, day time to be around 20C, nights around 10-12C. maybe less, depending on the weather. As soon as you go up, the temp. drops around 1° for every 500m or so. So 1500m today will be around 20C. In September there can be frost at night in the mountains. If you ar lucky (ie if it isn't raining) you will have lovely blue skies and see the new snow coming on the peaks. (In fact some fell above 2200m this week!)
But it also rains - at any time of the year. So layered clothing is important. Good solid waterproof walking shoes and woolen socks. A woolen hat and gloves. A waterproof anorak. Tee shirts, sweaters etc that enable you to take things on or off as the weather changes. AND most important a really good pair of sunglasses and total sunblock. The air is thin in the mountains and you can get a nasty sunburn.