A forecast for five days out is a guess with a confidence percentage attached. The traveler who packs for the forecast packs for the argument the forecast lost. The traveler who layers packs for every version of the trip at once.
The layering system has four pieces: a thin base that breathes, a mid layer that warms, a shell that stops wind and rain, and one warm hat. Any two of the four handle most days; all four handle everything else. Buy each piece cheap and well — you are packing insurance, not a fashion spread.
Check the forecast for the day you arrive, then pack for a version of it that is slightly worse. If you never need the shell, it cost you a few hundred grams in the bag. If you do need it, it saves the whole trip. That is how packing works: small premiums against large disappointments.