Late September and Early October…Thoughts and Analysis.

14 Sep

Well there goes the first half of September and in summary it has been quite a mix of conditions.  The opening week, as highlighted was a possibility in the last blog, was high pressure dominated and warm or very warm, with a late taste of summer in the air.  However, the second week of September has certainly seen a marked changed with much cooler and more unsettled conditions becoming dominant.  So is this trend likely to continue throughout the rest of the month or are we in for another burst of late summer-like conditions?…

September can often be a month of increasing extremes.  The residual warmth from summer across the near Continent can still pay us a visit, yet equally the rapidly cooling polar regions can also bring us a notable chill in the air.  It is often a time of year of varying pressure patterns as well, but I have quite high confidence over the expected pattern for the rest of September;

I expect the broader pattern to be as highlighted above and that is for low pressure to bring predominantly cool and unsettled conditions to the UK, whilst higher pressure is more influential further south.  I have high confidence over this prediction given good model continuity over the expected pattern for a marked upper trough to be dominant across the UK and northern Europe in general.  There may well be some minor fluctuations to the pattern, but generally the remainder of September looks set to be unsettled.

This pattern is evident well on some of the latest ECMWF and GFS ensemble charts, of which can be seen below;

Both the GFS and ECMWF ensemble models have been consistent in showing a marked upper trough to be dominant across the UK throughout the remainder of September and this can be seen on the above two images.  As a result the outlook for the rest of September looks set to be dominated by low pressure bringing cool and unsettled conditions with showers or longer spells of rain at times.  So unfortunately there is little risk of any late summer weather returning, in my opinion, throughout the remainder of September and in general the outlook looks distinctly autumnal. A particularly cool/chilly week looks likely next week, as an example, and the below graphic which shows 850mb temperatures highlights this well with high confidence for 850mb temperatures to drop below the climate average (solid red line);

So what about early October?…October is often the ‘true’ start of autumn, with an increased risk of overnight frosts and also the threat of some increasingly potent low pressure systems affecting the UK bringing gales and heavy rain.

At the moment I expect a very similar pressure pattern to continue into early October that is likely to be evident at the end of September. The higher pressure across central and southern Europe may well decline and allow unsettled conditions to become quite widespread across Europe, but at the moment in time there is little evidence to support a change in synoptic pattern to something more settled or high pressure dominated.

A synoptic pattern of this type during October would more than likely lead to temperatures being average at best and with precipitation totals generally at or slightly above average, so a predominantly unsettled theme is possible into early October.  It is difficult to advance on this further at this stage given a lack of forecast charts to discuss, but a cool/chilly and unsettled start to October looks distinctly possible.

So, and in summary, the outlook most certainly looks distinctly autumnal throughout the rest of September at least.  Those wanting or hoping for some (very) late summer warmth and sunshine may well be disappointed and along the way given the broader unsettled pattern the UK may well be affected by some particularly potent low pressure systems bringing heavy rain and also perhaps some stormy conditions as well.

As usual I’ll take a closer look at October at the end of September, but for now it may well be of worth to dig out some of the jumpers and sweaters that have been pushed to the back of the cupboard over the summer, they may well come in handy in the next 7 to 14 days in particular.

Regards to all,

M.

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