Plan clothing layers for the temperature.
Dressing for a whole day means dressing for its coldest point and being able to shed layers by its warmest. The base garment count comes from the lower of the two forecast temperatures, a swing of six degrees or more earns an extra removable layer, and twelve or more earns two. Precipitation chance decides the rain layer separately, since a waterproof shell is a different decision from insulation.
Day-long clothing plan
Base garments from the colder forecast temperature band; add 1 layer for a 6 C swing or 2 for 12 C, plus a shell when precipitation chance is 40% or more
Dress for the coldest point in removable layers rather than for the average. A ten-degree swing is normal in spring and autumn and needs at least one layer you can take off and carry.
Around 40% is a sensible threshold for a packable shell. Above 70%, waterproof trousers become worth carrying too if you will be out for hours.
No — this plans clothing for a forecast day range. For a specific activity in wind, work from the effective temperature including wind chill.