I started and DNF'ed in 2014 at the end of Day 3. For weather, you have to be prepared for anything. Day 1 it rained continuously from the start (5am) until well into the evening (probably about 10pm), except for the parts when it was sleeting, or freezing rain*. I live in Wales, but I wasn't prepared for that amount of rain in June. Evenings and night sections can be very cold, even if it is hot during the day, so you would need to carry decent gloves and a warm layer with you. Eastern Washington is technically a desert, and its weather can be very hot in the day time, but very cold at night.
Day 2 was quite warm, but really windy. Day 3 was mostly windy, with hot sections and cold sections. There were also a few very very heavy showers. This caused a landslide on the backside of Loup Loup Pass which delayed a few riders who walked over it. Fortunately I passed it when it was still raining so heavily that I didn't actually see it until I was in it, so just held on for grim life. At the end of Day 3 it snowed. In June. It was still snowing in the morning, so about 6 or 7 of us quit there and then. Day 4 is supposed to be a relatively easy day, at only 220km, with most of that being downhill/flat (once you've got over Rainy Pass). 220km in snow/sleet/hail wasn't desperately appealing. However, the previous event (2 years prior), the whole event was in 30C heat, with riders needing ice to cool down on the hills, so you can't really guess.
It is very hilly. 12000-15000m over the 1200km. They are mostly long drags. They aren't like European zig zags though - it's more solid steady climb. A few of the climbs have 'redundant grade' - a massive downhill in the middle of the climb - Loup Loup pass is just like this. The instructions can be a lot more sparse than you might expect from a UK Brevet. Day 2 was: R out of control, continue 45 miles to control (up a 35 mile drag).
The scenery is bloody amazing though. Seriously. It's a fantastic ride. I'm not a massive fan of pointless hills, but I can bear them for fantastic views, and scenery, which this ride has in spades.
If you do ride, invest in decent tyres and a load of tubes. I had 4 punctures on Day 1, on tyres that would have been fine for a UK ride, but the surfacing there was no so good.
Also, although the route is announced in advance, the exact route is subject to change right until the same week because they need to check the required roads are all open.
*My brother is a meteorologist and gave me a lecture on how interesting freezing rain is. It's a lot less interesting when you are soaking wet and can't feel your hands.