September 2012 – Thought’s and Analysis…

29 Aug

Well, there goes “summer”, with unfortunately the air quotes being used again.  In summary the summer of 2012 will be remembered for being predominantly cyclonic and unsettled.  Rainfall totals over the 3 months are likely to be particularly high, especially when June and August are combined.  Despite some drier and sunnier weather during the early part of August, the second half of the month really has produced some particularly wet weather at times and this is in-keeping with a particularly wet summer overall with clearly some noteworthy flooding events at times. Despite temperatures still reaching 31C and 32C around the middle of August there has also been a distinct lack of any sustained or significant warmth this summer.  So overall, rather in-keeping with the run of poor summers from previous years and attention now turns to the back-end of the year as we progress into Autumn and eventually Winter…

So, taking a look at the first half of September in terms of broader synoptic patters and the below two images were required within this blog to highlight my expected thoughts;

The need for two images comes about due to the fact that quite a changeable opening first half of the month is expected.  What is now a near-certain event is for high pressure to become quite an influential feature across the UK and also many other central and western regions of Europe into the opening 7 to 10 days of the month.  At the same time the jet stream and low pressure retreats northwards to the north of the UK.  However, I also have quite high confidence to suggest that this build of pressure for the UK will be relatively short-lived and as we move beyond the opening week of September and more particularly towards the middle of the month, higher pressure will decline further south into Europe, allowing the jet stream to sink back southwards and bring a renewed spell of cool and unsettled/cyclonic conditions.

This pattern is represented well on the latest GFS and ECMWF ensemble charts for the early and middle part of the month;

This signal for higher pressure is then in comparison with a more cyclonic trend towards the middle of the month;

Clearly on the above GFS ensemble image out to the 13th of September, any high pressure has retreated towards the south or south-west of the UK and clearly the jet stream has sunk southwards to be essentially located across the UK.  Low pressure is dominant just to the north of the UK and hence a predominantly cool and unsettled pattern is evident.  There is support for this trend from the ECMWF ensembles as well.  So in summary I expect a reasonable start to September, or opening week or so, but with a clear trend that as the first half of the month then progresses more unsettled conditions will return across the UK.

So what about the second half of the month?…

As is often the case, this is the hardest part of the forecast and prediction, given the time frames involved and sometimes September can often be a month of quite significant and rapid changes, so the second half of the month has rather low confidence.  Despite this, I predicted, overall that the remainder of the month will keep a broad unsettled pattern going;

The expected pattern is similar to the pattern that I expect to develop as we progress towards the middle of the month.  The Azores high pressure may well ridge east or north-eastwards at times to bring somewhat better weather to parts of England and Wales, but at the moment I don’t expect a high pressure dominated second half of the month, with the broader pattern being an unsettled westerly-type across the UK.

So in summary, with the exception of a potentially decent start to the month, overall I expected September 12 to be a particularly ‘average’ month overall with temperatures likely to even out at near average through the month.  Precipitation totals may well be slightly above average across northern and western areas of the UK in particular given the expected unsettled pattern, but southern and eastern areas may have precipitation totals near or slightly below average overall.

As usual I’ll attempt to produce an update during the middle of September looking back on the first half of the month and how that panned out and also taking a closer look at the second half of the month.

Regards to all,

M.

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One Response to “September 2012 – Thought’s and Analysis…”

  1. commuterjohn August 29, 2012 at 6:41 pm #

    I like a forecaster who does not commit to more than 14 days in advance. Sensible.
    John.

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