Late Summer, Early Autumn Outlook…

14 Aug

Well apart from approximately two weeks to go, summer is pretty much done and dusted. I use the word summer very lightly mind as clearly, despite a somewhat improved second half of the summer compared with the first half of the summer, this will no doubt be one of the worst summers the UK has experienced in many years.  My last blog, at the beginning of August, highlighted a predominantly unsettled first half of the month.  Overall this was the case, but the UK did experience a last minute anticyclonic spell just after the first week of August for approximately four or five days.  Clearly a welcomed last minute development given the summer as a whole…

So, this mid-month blog will take a look at the final few weeks of summer and also a generalised look at what the opening week or so of autumn and September may well bring.  So starting with the latter half of August and the expected broader synoptic pattern;

Unfortunately the second half of August doesn’t look too clever, which may well not come as a surprise anymore.  Generally the broader jet stream pattern is that of which has dominated the weather this summer overall and that is for the jet stream to be crossing the UK and other northern areas of Europe and hence bring a predominantly unsettled/cyclonic pattern to the UK.  This is in contrast with higher pressure across more central and particularly southern areas of Europe.  This broader pattern is evident within the GFS and the ECMWF ensemble mean charts out to 240hrs which is approximately up to the 24th of August:

Both of these forecast charts clearly show an upper trough and surface low pressure affecting the UK and many other areas of northern Europe, whilst pressure is higher further south.  This broader pattern is what I believe will dominate the weather throughout the final few weeks of summer and August bring a predominantly unsettled spell of weather to the UK with further showers or longer spells of rain at times.  Precipitation totals are likely to be trending above average and whilst the start of the forecast period (mid August) may well be quite warm there is also a clear trend within the various pieces of forecast data for a cooler west or south-westerly air mass to become dominant as August progresses.  As a result I expect temperatures across the UK to gradually cool to near or even slightly below average by late August.  This trend can be seen using the 850mb temperature graph for London for example;

Whilst clearly not surface temperatures, 850mb temperatures are a highly useful tool to gauge expected temperature trends and differing air masses and clearly the above image compared with the next 2 to 5 days shows a marked cooling trend towards the end of August.

So what does the opening week or so of autumn and September bring?…Well to be honest the signal is relatively weak at the moment, despite some consistency for a negative pressure anomaly to be affecting the UK and other northern areas of Europe.  So with rather low confidence, the below is the expected broader synoptic/jet stream pattern;

At the moment in time I don’t envisage a rapid change in pattern from the rather cool and cyclonic one that is forecast and expected to develop during the latter half of August.  As a result the broader pattern of unsettled conditions affecting the UK and northern Europe in general I expect to continue with further areas of low pressure affecting the UK bringing showers or longer spells of rain.  Temperatures are likely to be average at best, if not trending slightly below average at times, with perhaps quite an autumnal feel to conditions developing into early September.

I will reiterate that early September is uncertain due to the fact that the signals for one specific pressure pattern are rather weak, but there has been consistency within the longer range models for lower pressure, rather than higher pressure to affect the UK into the opening week or two of September.  So for the time being the first half of September is unlikely to deliver a late taste of summer.

As usual at the end of the month I’ll take a more detailed look at September as we begin to move into yet another autumn period with all the usual interesting meteorological possibilities that can arrive across the UK during autumn.

Regards to all,

M.

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One Response to “Late Summer, Early Autumn Outlook…”

  1. Gavin August 14, 2012 at 11:56 am #

    Great read. Thanks Matt. 🙂

    My August forecast was for a cooler and wetter than average month. This looks a busted flush, but it’ll be interesting to see how close we are to it in the final analysis.

    Interesting that your not yet seeing signs of an especially warm and sunny start tp September. The CFS model has also been quite consistent about the weather being relatively cool through the start of September, though with no clear signal in terms of pressure distribution.

    A cooler, more unsettled September would be quite a change on many of the Septembers in the last decade or so.

    Regards

    Gav

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