A September trip to Yellowstone! Expect big temperature swings. September averages are roughly upper 60s by day and upper 30s at night, with daytime temperatures reaching the 70s, especially earlier in the month. Freezing mornings, rain, and even snow are possible, so layers are the key to this capsule. The outfit formulas make getting dressed simple. Whether you’re hiking, exploring geyser basins and boardwalks, wildlife watching, or heading to a casual lodge dinner, you’ll have what you need. Here’s your packing list: 6 tops — 3 technical tops for active days, plus a short-sleeve sweater, plaid shirt, and sweater for casual exploring and dinners. 3 bottoms — 2 lightweight hiking pants that are polished enough to rewear for casual dinners, plus jeans. 4 layers — a hoodie, lightweight down jacket, waterproof shell, and denim jacket cover everything from cold mornings to rain and casual evenings. 3 shoes — trail shoes for hiking and park days, a casual flat to dress up the hiking pants, and ankle boots with jeans. Cold-weather extras — a beanie, lightweight gloves, and scarf for chilly mornings and evenings. Day pack + baseball cap — for layers, water, snacks, and sun protection. Accessories — a crossbody bag and bandanna scarf add a little fun to all those practical pieces. The real trick with Yellowstone in September? Don’t pack one heavy coat. Pack layers you can add and shed as the temperature changes throughout the day. Not headed to Yellowstone? Use the same outfit formulas for Grand Teton, Glacier, or Rocky Mountain National Park with adjustments for the forecast. For warmer parks like Yosemite, Zion, or the Grand Canyon, lighten up the layers and add more warm-weather pieces. Same formulas. Different layers. Always check your park’s forecast right before you pack. LTKOver40 LTKTravel