Working with American clients is brilliant, I'll go through how I usually work when collaborating with an American client. I love working with web design clients from the US for the following reasons. Clients always follow my client process detailed on the process page of my portfolio, but it can change depending on the timeframes and client needs.
With clients from the states, I usually make a special exception and work past 7 to have an initial meeting and talk on the phone or over skype/google hangouts; this allows the client and myself to put a voice and sometimes a face to an anonymous email address. The meetings are never out the way for the customer (e.g. 4 am US time) I always change my schedule to meet them, because that's the right way to do it.
The time frame becomes an advantage at this point! I'll start on the designs for a project and get them to the client by the end of my business day which means their morning. When the customer (you) wakes up, you have a nice email greeting you with designs awaiting your feedback. The time difference gives you the opportunity to either discuss with team members or even give the designs to friends and family - what I've noticed the most is the feedback is much more thought out and in-depth. Then once the feedback arrives in my inbox, I'll just be starting my business day bright and early with full, thought out and planned feedback - perfection!
Whether you're looking at San Franciso design agencies or ones based in New York, you'll always find a cheaper alternative when you cross over the pond. I can always guarantee my rates will be cheaper than my San Francisco counterpart; this is all down to living costs and less bloated tech salaries - but the same great talent.
So why wouldn't you hire a web designer based in London for your next project? It's got a great feedback loop, cheaper rates and of course the accent!