Wunderground will give you live weather information for places along the route. Here is Bilbao for example. But the end of August is a long way off and who knows what will be happening then? Another useful site is weatherspark.com which has lots of past weather data which helps you know what range of weather conditions you can expect.
In Bilbao, the summers are comfortable; the winters are long, cold, wet, and windy; and it is partly cloudy year round. Over the course of the year, the temperature typically varies from 41°F to 76°F and is rarely below 34°F or above 84°F.
There are lots of weather websites with this information but you still will never know what you can expect especially these days. As an example I walked in 2012 and it was 30-35C minimum each day, without a cloud in site. I walked. In 2015 I walked again. When I got to the Meseta it was exactly 3 years later to the date. A coincidence huh? Every day the weather was between about 4-8C with rain every day.At points it was absolutely pouring with winds in our faces of about 35-45K an hour according to the weather reports. It may be hot, it may be a little cool, it may rain or it may not. But at that time of the year, if you have a poncho that covers your pack and a layer or two you should be able to handle anything.
To paraphrase a past member of the forum, it will be the same as last year, just on different days. More, what its like now will have little relevance in six to eight weeks time.
There are good climate sites around that can tell you what to expect. I prefer the Spanish State Meteorology Agency site, www.aemet.es as a source. From the link, find the menu item for 'Climate Services' then 'Climatological Services' and explore from there.
It’s “green Spain” for a reason. Your going at a very pleasant time, but I remember glorious downpours. If you have a good waterproof covering for yourself and your pack, walking in the rain can be a sweet pleasure. Enjoy!