Over the last few years September has turned out to be a fine settled month. September 2017 looks like the first half of the month will be unsettled with rain and winds.
This can be partly explained to be down to a strong jet stream across the UK allowing a deep trough to develop. This will allow some low pressure systems to develop become trapped and deepen.
The jet stream becomes faster where there is a greater contrast in upper air temperatures. This starts to the South of Greenland and moves down towards the UK.
The hurricane season is underway too and it has been several years since the hurricane season was quite active. So some of the of the tail end of theses storms will move into the Atlantic. But we may see some of these storms deepening again bringing some rain and windy conditions and at times some cooler temperatures into the UK when they get trapped in the trough
I see this pattern continuing for the next 10 to 14 days.