Ask HN: webtools for new website design/flow

I am trying to help a friend fully design a new webapp prior to getting bids on programming. By "fully" design, I expect programmer(s) to be able to use this design to identify 90% of the visual assets and easily infer the functional specs based on the UI layout and flow.

I expect to employ a good web designer to do this first phase but do not expect the web designer to be in the same part of the world as the customer. There must be some great web-based solution for the customer to iterate over the design and flow of this new app with the designer. I can't seem to find such tool(s).

The app is mostly data-backed forms and lists with common page structure throughout, no streaming media, flash, etc...just standards based HTML/CSS/maybe JS. Documenting the design, flow, and correct elements on each form is the highly iterative part we want to do with a designer prior to inviting coders to the mix.

Any advice? If there are some good designers that are comfortable with such an undertaking, feel free to contact me.