From the KOMO weather blog:

North winds are generally an indicator of nice weather, while south winds are typically associated with cloudy and rainy weather. So when you see the planes coming over Downtown to land to the south, it usually means it’s cloudy or rainy (if, for some reason, you haven’t noticed the sky behind the jets.)

Actually went there in search of the elevation of Sea-Tac airport which is a) surprisingly high [433 feet] and b) variable [90′ difference between the ends of the runway].