I'm not going off of any weather maps, charts, or archives - rather...I'll go off of my experience living in Seattle pretty much all my 27 years.
Off the top, Westcoaster is right...It isnt quite as bad as people think Seattle is a non-stop rain fest. Local Seattlites love the fact the whole country thinks it rains here a lot. Especially love when Californians stay away from wanting to move here and clog up our freeways and dirty up our downtown.
As for the months
April - About 65-70% of the month it rains. The weather is pretty mild in the low to mid 60's. Occassionally, it will hit around 70 later in the month moving into May.
July - Here's where the secret is let out. Like Westcoaster said...I think it rained one day in July with the average temperature at a comfortable 72-75 degrees. The last two weeks in July this past summer averaged around mid to upper 80's...with a couple days we hit 90's. Remember, this is high temperatures with NO humidity or dryness. The air is fresh, clean, and there it is completely barable.
Oct - Being in the first part of October now..the past 3 days have been sunny in the high 60's and low 70's. Just watching the news a couple of hours ago states the average temp in October around the mid 60's. I'd say it rains about 30% in October, mostly in the later part of the month.
Like Westcoast said...Jan, Feb, and March are all rain filled months where the weather is like 80-90% rain. And its grey and cold, often in the low to mid 50's.
I dunno if Seattle coined the term 'drizzle', but, it drizzles in Seattle a lot in the first three months of the year. Drizzle meaning not quite raining, but, imagine a grey cloudy, cold day, where it is damp and misty enough where you're not quite sure to turn your windshield wipers on...even the intermittent wipers seem a bit too much. Its the kind of dizzly, rain, grey and cloudy day for days and days and days that make you wanna move to Hawaii...haha...but when the Summer months come, you realize why you love Seattle. But when it rains...it POURS!
A lot of people ask me where I'd live if I would leave Seattle...I often say Hawaii for many obvious reasons. But, even if i lived in paradise, I'd still miss the 4 true seasons we get here in the Northwest. The leaves are turning brown now..fall here is amazing. Then the Snow comes during the winter..along with the spring rains...and just when you think you can't take anymore drizzle...the sun hits..and stays from June all the way to October.