September weather can change quickly, espcially at high altitudes. I recommend layers. I wear a moisture wicking t-shirt and a waterproof jacket with ventilation panels. I pack a windproof fleece sweater and windproof glove liners in my bag. If the temperatures are in the mid-high twenties you can open your ventilation panels. If the temperature drops below 15 degrees you close the panels and if it goes below 8/9 degrees you can put on your sweater and glove liners. I ride like this pretty much all winter, until the mountain passes are covered in snow and closed.
BTW... it snowed at Passo Rolle yesterday and Passo Manghen still has over three meters of snow. Usually the mountain pastures are already open and green... weather is a bit odd this year.