Archive | September, 2012

October 2012 – What’s In Store?…

28 Sep

Well there goes one of the autumn months already and September has certainly produced some extremes, as is often the case with September’s.  Many areas of the UK experienced some late summer warmth just after the opening week of the month with temperatures widely above 20C.  This was then in comparison to the middle and latter half of the month which has clearly felt far more autumnal with temperatures generally average at best.  Clearly the stand-out feature of the month was the potent area of low pressure around the 24th and the 25th which clearly brought yet further large and extreme rainfall totals which is in-keeping with the months prior to September.

So what about October?…October can often really bring the ‘true’ signs of autumn with a greater frequency of frosts, especially across northern areas, an increased risk of autumn gales and also perhaps a more notable risk of some wintry precipitation across northern areas as well.  At the moment I expect the following pattern to dominate throughout the majority, if not all of the month;

There is only one particular image to summarise October at the present time, simply because I couldn’t find a distinct difference between the first half of the month and the second half.  Clearly, and just as a quick reminder, September was most definitely a month of two halves, which was highlighted, but October, I believe, looks set to be an unsettled one throughout.  What this forecasts doesn’t highlight is clearly more short-term variations where perhaps higher pressure temporarily builds into the UK before low pressure returns, so that is something to bear in mind, as unsettled conditions from the 1st to the 31st of October is unlikely.  That being said, there is a clear signal for October this year to be predominantly unsettled and cyclonic.

The GFS and ECMWF ensemble charts highlight this trend throughout the first half of the month in particular;

Both these models signal lower pressure to be dominant to the west or north-west of the UK as high pressure is influential further south, which tallies with my original image.  As a result of this broader pattern, the first half of the month in particular is likely to be most unsettled across northern and western areas of the UK with precipitation totals at their greatest here.  This is contrast with more southern and south-eastern areas which may generally experience more in the way of drier and brighter intervals interspersed with some rain and showers at times.

Looking at the longer range GFS ensemble, which covers up to mid-month and the broader unsettled pattern persists;

With the temperature gradient increasing across the North Atlantic as is always the case through October, as temperatures drop quite rapidly across higher latitudes, this increases the risk of more noteworthy and deep areas of low pressure. So given the broader unsettled pattern it does seem likely or at least possible that somewhere down the line the UK will be at risk of some particularly potent low pressure systems, clearly details on this is unknown at this stage. This signal for low pressure throughout the first half of the month can be seen quite clearly on the latest GFS ensemble graph for central areas of the UK;

Despite some spread in the ensemble members which does lead to some uncertainty, at the moment the general trend is towards lower pressure with little signal for higher pressure to become influential. The second half of October, as is always the case, has a lower confidence given the time frames involved and the synoptic evolutions can change and as usual I’ll review the first half of October and look more closely at the second half of the month around the 15th.

So, in summary.  I expect October 2012 to be quite a typical October month across the UK with lower pressure dominating rather than higher pressure and thus bringing quite a cyclonic month.  Temperatures may well vary from north to south, with still the risk of some warmer days across the south as areas of low pressure to the west or north-west bring up some temporary warmer air from the south-west, but equally cooler, if not colder, north-westerly winds may develop at times as well.

Regards to all.

M.

 

Winter 2012/2013 – Thoughts & Analysis

18 Sep

Of late there has been many discussions and pieces of information released with regards to the possible weather during the up coming winter.  This time of year is often the start as to when certain ‘variables’ can be used as to give an idea as to the potential broader synoptic patterns through the winter.

What I will add at this point is that this is NOT a winter forecast. I am using this particular blog to highlight features of interest that can give signs and indications as to the expected up-coming winter and discuss them so others know what to look for in the coming weeks and months.  As a scientist I put faith in forecast models and the analysis of these models and like many areas of science there are limitations and saying what the weather will be during the first or second half of January at this point, in my opinion is simply not possible.  Meteorology is not an exact science and the atmosphere can always bring about some unexpected surprises.

I was recently made aware of this publication and the information within this publication really is worth a read as it gives some clear thoughts and information regarding the longer term and how specific variables can be used;

http://iopscience.iop.org/1748-9326/7/3/034031/pdf/1748-9326_7_3_034031.pdf

One of the more recent documented features is the state of the sea-ice or lack of it across the polar regions at the end of this summer/start of the autumn.  There have been recent studies conducted, in conjunction with the severe winter of 2009/2010 that low Arctic sea ice can have a direct effect on the AO (Arctic Oscillation) through the winter in that low-level heating can aid to bring about a -ve AO pattern.  For those unsure about teleconnections, in a nut-shell a -ve AO pattern is directly associated with higher pressure across polar regions which thus aids to bring a more blocked pattern and hence a greater risk of colder weather affecting the UK, than compared with a more zonal (west to east) flow.

Clearly if there is any truth to this connection, then seeing the state of the Arctic sea ice is at its lowest ever recorded, then clearly this is one variable which  may well signal a more blocked pattern and higher pressure across places like Greenland and to the North of the UK, as per winter 09/10.

For any hardcore readers or analysts out there, then this is also another well constructed publication – http://www.tellusa.net/index.php/tellusa/article/view/11595/html

Another feature, which is highlighted in the above initial publication is the SST patterns across the North Atlantic and how these correlate to either a +NAO pattern (mild, wet, windy and zonal) or whether, as per 09/10 a -ve NAO pattern (blocked, colder and wintry).  At the present time the SST pattern is favorable for the latter to take place and develop.

The QBO (Quasi-Biennial Oscillation) is, without going into the details, also another feature that can affect the weather across the UK and the northern hemisphere during the winter.  The QBO has two phases, a westerly phase and also an easterly phase.  It is believed, again through various studies and publications, that the easterly phase of the QBO tends to be directly associated with a greater likelihood of blocking patterns and generally colder, less zonal/unsettled winters. The QBO phases of late can be seen on the below image for example;

https://i2.wp.com/www.geo.fu-berlin.de/met/ag/strat/produkte/qbo/qbo_wind.jpg

The grey areas represent the westerly phase and the white areas represent the easterly phase.  Interestingly note how during last winter the general phase was westerly, but how if you extrapolate across through the rest of the year and into 2013, the pattern at the moment is for the phase to become easterly during this coming winter.  So again another variable which points towards a more blocked and potentially less zonal pattern.

Another important, but quite varied feature is ENSO.  The problem with this variable is that there has been numerous examples of colder weather affecting the UK both during El Nino and also La Nina events. At the moment there is still some uncertainty as to what state this will be in during the coming winter, but at the moment a weak or near neutral ENSO is expected, as highlighted on the below image from NOAA for example;

( http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf )

Overall a near neutral ENSO through the winter period isn’t a bad thing and again when coupled with the other features continues to provide more speculation and reason for suggesting the coming winter could well be a rather cold and blocked one.

What can’t be discussed at this stage is how the stratosphere may affect the weather through the winter period, it is simply too early.  We have seen numerous examples of how a sudden warming of the stratosphere (SSW) can have direct and significant effects on the weather at surface levels in the following weeks and I believe that the stratosphere can over-ride the above variables at times.  Clearly a warmer stratosphere is directly related to a -ve AO and a greater risk of blocking across northern regions.  As a comparison, last December was clearly very mild, wet, windy and zonal.  This is due to the fact that the Arctic was particularly cold (+ve AO), this can be seen on the below image where the red line was below the average near Nov and Dec.  However, note that by mid-late January the red line suddenly became much warmer than average in association with a stratospheric warming event;

This warming event during the middle of January this year then resulted in a much colder start to February across the UK and also some bitterly cold and extreme conditions across other parts of Europe as the very mild and zonal pattern of December was left behind.  What this shows is that changes in the conditions of the stratosphere can have a direct and extreme effect on the weather across the UK and many other areas of the northern hemisphere during the winter period and for the time being it is too early to see what this is likely to be during the first half of the winter.

So, and in summary…

At the moment in time, combining the expected ENSO conditions with the expected QBO phase, the arctic sea ice state and SST’s the end result is a good one if you are looking for a colder and a more blocked winter.  Scientific studies, like I have highlighted earlier in this blog, have been carried out on these variables and they have to be acknowledged when looking at the expected longer term and seasonal trends.  As I highlighted at the start of this blog, this is not a winter forecast, I am simply highlighting the variables of interest which can give ideas and signs of the expected weather in the months ahead but again, at the present time, the current signs are leaning towards a colder, more blocked winter.

In relation to the stratosphere and how this may affect the weather during the early part of the winter, then this can start to be analysed approximately a month or so from now, so during the middle and latter half of October onwards.  Time willing  I will attempt to create a blog on this when the time comes.

Regards to all,

M.

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.