March- Coat or Jacket?

No coat needed. A light jacket water resistant big enough to just layer underneath with a cardigan sweater or longsleeve tshirt as needed usually after sun down.

I agree with Jazz + Travels. We were in Siena in December 2015 and it was comfortable with just a lightweight jacket and a thin sweater. Evenings in the Lake Como region might be a little more chilly, but nothing requiring a coat.

Happy travels!

Here's what I do, regardless of season of travel. I take a shirt, fleece, and a rain coat (mine is a DryJoy Nike golf rain jacket, very light). Then I layer and I am ready for anything. I have done it from March through October and it works well. Sometimes I use a wool sweater instead of fleece. Then I have a nice sweater for going out to dinner and don't have to pack the fleece.

Layers are the way to go! This gives you maximum flexibility.

You may need more than a light jacket in March. Since you are traveling only with a light carryon, then you can wear the extra layers you may need in the Cabin. Remember that Cabins can get pretty cold and wearing something for warmth helps you feel comfy for the long flight and avoids over packing your carryon.

Last year in early May we did a day trip to Lake Como. I had the outer shell of my ski jacket. Last minute I threw in my down vest. On the day to Lake Como I wore both along with a long sleeved shirt. It was windy and very cold. Layering with fleece is a good idea. When wind blows over snow-capped mountains it can be bone chilling.

No way do you not need a substantial coat for this trip...

You are in northern to central Italy, from the Alps region down to just south of Florence. We've been from Florence down to south of Naples twice in early March and needed a sleeveless fleece, a fleece, a lightweight down jacket (that folds up into its own pocket the size of a softball), a stocking hat & gloves, and a cheap rain poncho. That about covered everything for us from a layers standpoint, and all of it didn't take up that much room in our suitcase. And full disclosure, we are 'pack light, pack right' proponents!

You'll have temps in the 30's/40's in the morning to maybe low 60's as a high, mostly 50's. Warmest for us was Rome, coldest was Sorrento. You just never know that time of year.

It’s hard to tell as the weather is crazy nowadays. A light weight waterproof jacket may or may not be enough and you can’t know but just days before when forecasts are more reliable.
You might need a heavy sweater under that waterproof jacket, and maybe a light long sleeve garment under that. As a general rule, in March you might get rather cold weather at night, especially on lake Como.

We were in Florence in December last year and it was freezing! I had a full length down coat and insulated, water proof knee high boots. That was necessary at that time of year.

That being said, I always look at the live webcams where I am going and see what the locals are wearing. That’s how I knew I needed a heavy coat last year in Florence. Here is a webcam of Siena. Looks pretty cold there too right now. https://www.google.com/amp/s/m.skylinewebcams.com/en/webcam/italia/toscana/siena/piazza-del-campo

I would wait until early March and check the weather then. With carry-on only, I would do my best NOT to take a coat! If you are really too cold, you can buy a sweater or sweatshirt while you are there.

I travel with a lightweight trench coat from scottevest.com that has lots of hidden zipped pockets inside. I have sprayed it with waterproofing and it is great to then layer under. March can be winter and need a coat. I have been in Florence in January when all I needed was a sweater. So layers. And a raincoat or long rain jacket (rain is VERY common this time of year) that can be layered with a cashmere sweater, polartek, long sleeved ts etc is the way to go. Be sure that you have at least one pair of waterproof shoes. I have lightweight hiking shoes from REI that are great for long days walking in cities and water proof. If you are spending a lot of time outdoors, a pair of silk long john pants for wear underneath jeans or slacks can make a big difference on a cold day.

mid-March at Lake Como? I'd take a coat. That has a great chance of being really chilly.

We were at Lake Como in late April 2016, and it was quite chilly. I dressed in layers with a raincoat, a lightweight cashmere sweater and gloves and stayed comfortable. Dressing in layers usually works well for me.

Locations on water tend to be windy; added to cool temperatures and dampness, it can require more layers than you might expect. I've been distinctly cool in the evening, waiting on a ferry to depart from Athens in early September.