Mountain weather in Washington can be a critical decision criteria for where to go and when to go hiking. Perhaps 20% of the time can you set out on a sunny day and feel confident that it will be like that everywhere and stay like that for an entire day. If one tries to restrict outdoor activities to days like that one will not go hiking very often. So the first thing to understand is on any given day, the weather can be horrible in one part of the state and much better in another. This page will attempt to provide a simplistic view of Washington weather patterns and resources for determining the best place to go when you have a free day for hiking.
At its most simple, weather comes from the west in Washington. The jet stream drives moisture from the Pacific Ocean right over the top of us. Nowhere is this more apparent than the Hoh river valley on the western slopes of the Olympics. A constant supply of wet atmosphere driving straight up the valley makes this one of the five wettest places on earth. The prevailing winds can vary from southwest to northwest. As a very rough rule of the thumb, the more southerly winds carry more moisture and the closer to the north, the drier it gets.
When this moist air coming from the Pacific encounters mountains, the upward flow of the air causes it to cool and release moisture. That is, it rains on the western side of the mountains more than it does on the eastern side. On the opposite side of the Olympic mountains from Hoh is Sequim and Port Townsend. These are some of the driest locations in Western Washington. In a straight line, Forks and Sequim Washington are only 60 miles apart! So if it is wet where you are, try going east to get on the dry side of the mountains.
To get a spot forecast, go to the NOAA website and search for a location. Start with this page: NOAA and enter/paste (Decimal Degrees) GPS coordinates into the Search box ("Search for places or addresses"). If you do not know what (Decimal Degrees) GPS coordinates means, I have a very short explanation here. If you do know what all this means, just paste in 47.3603, -121.4749 for a spot forecast at Annette lake.