Weather: aka how to pack

We were in the Dolomites ( the mountains, not Bolzano) and Vennice from Sept. 22 through the end of October last year. it was "short sleeve" weather the whole time; I hardly needed the warmer clothes I brought, at least not in daytime. I carried a light jacket when we went out at night. I did need my rain jacket on a few occasions ( never travel without one). But this year could be different, so you should take a long-sleeve shirt or sweater just in case. For the 6-week trip which included everything from mountain hiking to opera, we packed in carry-on size bags.

Unless you go high up on the mountains near Bolzano, all those locations should have warm day temperatures during that period, however rain is definitely possible. Nights and early mornings might require a light sweater or light jacket. I would pack a very light jacket, preferably good for rainy days. A travel umbrella is recommended, however as soon as the first rain drop hits the ground there will be scores of African street vendors that will appear from nowhere selling small umbrellas for €10 (bargain hard and you'll get one for €2).
The best strategy however is to check the 10 days forecast for all those locations the day before you depart. That should give you a better gauge rather than relying on generalizations since sometimes the weather is crazy. For example these past couple of weeks I have experienced one of the coolest weather in July in my memory. Temperatures during the day in a Florence were in the low 70's, at least 20 degrees below seasonal average.

If you are smart (we were not once) you pack for a range of weather with a series of light layers including a light wool sweater, one long sleeve, etc. No way to predict the weather. Remember to have an average temperature you have to have extremes on both sides.

I've visited N Italy in late September/October for the last 3 years. There have been times I've been cold – particularly early morning and night in Venice.

I bring a lightweight fleece zip-up and a hooded rain shell (eg, from REI). If it's particularly cold, the combination of those two keeps me warm, and they weigh almost nothing.

This is usually the time of year we travel. We could always count on rain at least part of one day in Venice (all 3 times!). Otherwise, we always seemed to be blessed with sunny warm days. Rome was downright hot (carry water bottle and fill for free at the drinkable fountains). And let me say, we were in Florence the 3rd week of Sept - I was wearing a skirt and tank top and was still hot. The nights do cool off, so it might be advisable to have a pashmina/scarf or a light cardi for when the sun goes down.

I was there about those same days last year. Lake Como, Venice, CT, Pisa, Florence, Siena, Montepulicano, Orvieto, Civita and Rome. All I bought were long sleeve dress shirts. I only had the sleeves rolled up a couple days. Most were comfortable for long sleeves and I wasn't sweating. I prefer just to travel with those then take any short sleeve shirts. I even hiked from Vernazza to Monterosso in long sleeve shirt. I did bring a sweater and ended up using it 2 nights (1 in Siena and 1 or Orvieto).

I've had plenty of warm weather later into October in Italy, and the occasional warm downpour in Rome. Light, versatile layers are best. I would be more likely to bring a cotton sweater than a wool or fleece sweater. Bolzano is where you might be most concerned, but also where you'll be able to get the most timely forecast before departure.

Last year there at the same time... 12 days and it was like late summer every day. Did not even need a lightweight wrap in the evening...however, I know it was lousy weather the week before I was there. Actually..just looked at your dates and I was there the middle of Oct.