According to Weather Underground, the weather forecast for tomorrow evening (5 PM) in downtown Seattle is 61 degrees Fahrenheit, dropping to 54 by 8 PM. Humidity will be 50% at 5, increasing to 70% by 8. 20% chance of precipitation, and 68% cloud cover.
For those intending to attend the afternoon event in Olympia the temp at 11 AM will be ("is forecast to be") 52 degrees Fahrenheit, increasing to 58 by 2 PM. Humidity in the low to mid 60% range. 20% chance of rain, and 66% cloud cover.
This being western Washington, we should prepare for anything and everything short of an ice age, or a solar flare. And I wouldn't rule out the solar flare.