The weather in the Washington Cascades is very predictable; it is either raining now or will be shortly. Seriously though, mid August will generally be 70s-low 80s in Seattle, factor in an average adiabatic lapse rate of 3.5 degrees F per 1000 feet and you can expect day time temperatures on the summit to be 40 -50 F during the afternoon. If you have poor weather or high winds it can be significantly colder. Expect colder night time temperatures, perhaps as low as freezing at high camp.
This year there is less snow than previous years at this time and some glaciers are more broken, others are just fine. You didn't specify which route, I am guessing either Fisher Chimneys or Sulphide Glacier both of which I have climbed into late August and September in years past.
If doing FC, I would anticipate icy conditions. Winnie's Slide (I knew some cats in college who bowling balled down that thing. I rappelled it once.), gaining the Upper Curtis and Hell's Highway will be the steepest sections and may require belaying. Bringing 2 -3 ice screws as well as other glacier kit would be prudent, although a second tool would prolly be overkill. Sulphide should be pretty chill, I got a good look at it on 7/30/2013 and it looked to be well filled in still.
As far as cell reception, Verizon is generally considered to have the best reception in the Cascades. I have had cell reception from the summits in the Pickets, which is about as remote as the Cascades get. I suspect you may have trouble depending upon which aspect of the mountain you are climbing, but you prolly will have reception from the summit. I believe there is a cell tower at the Mt Baker ski resort which is on Shuksan arm. I recall skiing the North Face one winter and I had cell reception, I think.
Have fun. In my mind Shuksan is the most iconic peak in the North Cascades. Every route I've done is worthy and the views are pretty stellar.