I have traveled extensively in Europe during those months and the weather for those cities except Rome will be similar - unpredictable and perhaps wetter than usual but not real cold. Layers are great for a place like London where the temps seem to change 10 degrees F whenever the sun pops out and then declines when winds blow and clouds set in - the feel of when walking i'm saying not the actual temps. Expect highs in mid 40s to 50s - in October and prepare for rain. Rome will be significantly warmer but still can rain a heck of a lot. Well that's based on my travels, not weather data. I've never let inclement weather hurt my enjoyment of these cities even though bright and sunny is better.

Steph, Just like in the USA each region has its own climate. You will be going from Boston to Atlanta on this trip--very different climates. I would research each for Oct. weather.

Europe Climate - Historic Averages for Rainfall and Temperature
Europe Climate - Historic Averages. These city charts can help the ... Another city best enjoyed in the "shoulder" season: May-June and September-October. ...
goeurope.about.com/.../Europe_Climate_Historic_Averages.htm

I have ben to all the country's you have mentioned at that time of year .I would suggest a unberella.I buy those rain coats you can get at the dollar store or a pharmacy for about $2.00 i take a couple they don't take up much room they never fold back up and go back in the bag..Just take a sweater that you can wear with all your clothes.Usually cool in morning and evening but can be warm during the day. Ck weather day before you leave for a 10 day forcast. Weather can be kind of tricky that time of year.I was in Paris once in June and needed a sweather .But i was also in Northern France one Feb. and it was like Spring.I will be in Paris Sept 27 threw the 30th . Have a safe and wonderful trip!

Nowday the weather is so unpredictable that is hard to predict what will be like anymore.

Once I was in Paris in July and after a week of swealtering hot weather, woke up in the morning was cold, damp and rainy..Fortunately I was able to find a sweater in sales at the local Monoprix.

Bring a light jacket,ligh summer clothes,sandals, walking shoes and a foldable small umbrella.

I am going to have the same dilemma..Going to La Camarque for 8 days at a small fishing village, will spend 5 days in CT, afterward will stop in Pescara, then Rome for 5 days and back again in Paris the first week of October..for few days before departing for the US.

Cant bring too much either because I will be taking a smaller plane from Rome to Paris and cant go over the weight limit.

No - Rome will be warmer and Amsterdam and Berlin will probably be chillier - esp if they're at the end of the trip.

Pack layers so you're prepared for anything from 80 down to high 40's at night and expect rain - bring a sturdy folding umbrella and 2 pair of waterproofed walking shoes.