KL doesn't have a single "monsoon off switch" like east-coast islands, but October–December is the wettest window on the west coast — afternoon thunderstorms 3–5pm most days, haze possible in September from regional burning. November is workable if you front-load outdoor stuff before noon and keep mall/museum backups. Worst for open rooftops: steady all-day rain during northeast monsoon peaks (can happen 2–3 days straight in Nov). Best dry months: June–August and February. April is brutally hot (35°C+) but dry. Plan Petronas and walks for morning; save Aquaria or Pavilion for post-lunch storm hour.
My Weather app was wrong half the week — radar (MyMet) more useful than monthly averages.
November visit — afternoon downpours daily but mornings were clear. Packed a pocket umbrella, kept going.