Archive | August, 2012

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.

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.