What with the horribly hot, dry summer, is anyone willing to hazard a guess as to what the weather will be like in the Île-de-France, Brittany, and Normandy this October? We're going for a month and want to pack extremely light, so we hope to avoid taking lots of clothing options. Ou rêve-t-on ?
Impossible to guess... At the moment, rain gear is essential! The dry spell is over for good. You'll need layers.
Pack for rain. It's currently wet, wet, wet and the Atlantic storm season is starting to kick in. Whilst October can be warm and sunny you're visiting a maritime climate so be prepared for very changeable and unpredictable weather. You won't experience snow or very cold weather though.
It’s difficult to pack light for October as you need clothes for warmer days plus cool nights and probably wet conditions. You need 2 pairs of waterproof shoes - that part of France is green for a reason - it rains! I certainly wouldn’t take just hand luggage for a month away.
Before moving to France I spent most of a September and October in various parts of country, including the regions you mention. I had packed only a carry on bag with a personal bag (small backpack) in addition.
It wasn't difficult so you should be able to do it as well.
As long as you have a rain shell (preferably with hood) that you can layer warmer stuff under, you should be fine. Just wear it onto the plane and stuff it in the overhead compartment while flying. You'll probably want to pick up an inexpensive (~15€) collapsable umbrella at a Monoprix or similar store to have on hand -- takes up little space in a backpack, but can be invaluable if you're caught in a downpour.
If it helps, in Normandie it's currently intermittent showers with sun breaks and periods of heavy rain (had a 30 minute gully washer yesterday), with highs in the low 20s and lows in the mid teens. Météo-France's 15-day prediction is similar, perhaps expecting a slight cooling. For October, probably count on around 5 degrees cooler (all temperatures in celsius), with similar precipitation.
I don’t find it difficult to pack light for colder weather. Just substitute long sleeved shirts for short sleeved, then take a warm fleece and hooded waterproof jacket, which you can wear on the plane. You might also consider these extras: warm hat, gloves and long underwear, none of which take up much space. With a hood, you don’t need an umbrella.