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This really is a rather impossible question to answer. It is like me saying what is the weather going to be like in Garmouth on Sept 15th this year - the answer is no one knows. Expect Autumnal weather, especially as you are going further north. Pack layers then you are covered for all eventualities.
You should certainly expect some really warm weather if the seasons are normal but be prepared for a cooler time as well. Remember that the leaves change colour because of the shortening of the days and the lowering of the temperature and as this is why so many people do this trip at this time of the year. Like A of N says take layers and at the moment expect the unexpected as far as the weather is concerned. Nei.