I'm heading to Mt. Hood in a few days and I have no clue what to bring. I was going to wear a T-Shirt and flannel skiing after looking at the forecast, but whenever I watch edits there everyone is dressed in tons of layers like it's winter.
The weather looks pretty intermittent this week. I would suggest a t-shirt and light waterproof jacket and a warm layer to put on underneath if you get chilly. A lot of days this time of year we will get nice warm pockets of sunshine, a few cold doses of sideways rain/slush/wind and some dense fog. A typical run on a day like that often consist of sweating balls at the bottom, freezing at the top of the chair and repeating so just get used to being uncomfortably hot or cold. Maximum sun protection also suggested. On sunny days I would still recommend a long sleeve because the salty snow will eat you up if you go down.
Honestly wear a Long/Short sleeve underneath a Hoodie and a Shell, It gets warm around noon from my experience but in the morning its cold especially if its windy. Oh yeah and dark dark tinted sun glasses or goggles and sunscreen even if its cloudy you will burn.
Don't wear short sleeves. Even if you plan on taking it easy, some dude at the end of the day will convince you to try a cork 7 on a tiny windlip because everyone else is hitting it, and when you lay it down on your side you will lose a lot of skin. Wear long sleeves.