Looking at a few maps, the first chart below shows the average (1981-2010) 700 mb height field in June and July, which can be thought of as the pressure pattern at about 3 km or 10,000 feet above sea level; there is a mean trough in the Bering Sea and extending into the Gulf of Alaska, and a ridge extending up from British Columbia to eastern Alaska.
Taking the difference between this year's average and the long-term mean shows where the height has been most unusual, see below.
Having said all this, in my view the upper-air flow pattern is ultimately unsatisfying as an explanation for the warm weather, because the question immediately turns to "what caused the ridge"? Why did the weather pattern evolve this way in the summer of 2013? This is an enormously difficult question, and I only have one small piece of possible insight to offer: this summer's pattern may (perhaps) be (partially) related to the phase of the Quasi-Biennial Oscillation (QBO). The QBO is a fairly regular cycle in east-west winds in the tropical stratosphere, i.e. about 15-50 km above sea level at the equator. Over a period of about 2-3 years, the wind direction reverses from east to west and back again, and it is known that the phase of the oscillation affects weather and climate variations around the globe. This summer there happens to be a strongly positive phase of the QBO, so I looked up previous years with similar phase and found the following pattern of 700 mb heights near Alaska:
Rick and Brian may have more to add on causes of the warm summer - and any other insight or suggestions from readers would be most welcome.