Archive | May, 2013

Low Pressure Development – 14th May 13 Case Study

14 May

As we all know low pressure systems often bring cloud, wind, rain and showers to the UK. However, some low pressure systems are clearly deeper and more active than other low pressure systems and of which these, more often than not, bring some potentially severe weather. Using the low pressure system today (14th May 13) as an example, find below a discussion as to how and why low pressure systems can deepen and develop. I hope this blog will then provide some insight as to the usefulness of weather charts and also aid to bring some further understanding to the development of low pressure systems and what to look for in the future.

Starting with the initial synoptic pattern on Monday 13th and also into Tuesday 14th of May;

bracka20130513 bracka20130514

A large and dominant area of low pressure was evident to the north of the UK during the 13th as both a warm front and a cold front moved south-eastwards across the UK.  Now the important development and feature of interest is if you follow the cold front back into the Atlantic on the first chart. You will note that the cold front eventually becomes a warm front and this initial development is known as a frontal wave. Frontal waves are, at least at first, notoriously difficult to forecast because the ingredients required for development are very specific and don’t always happen. As a result sometimes the wave never really develops much at all and just comes and goes without much notice. However, as with today (14th May) the ingredients evident for development are significant and noteworthy and as a result this frontal wave is developing into a fully-fledged low pressure system. You can see on the second chart that the initial cold front is now moving into France whilst at the same time the frontal wave is in a state of development just to the south-west of Ireland with a small warm front and cold front evident.

One primary importance of low pressure systems is the the interaction between the surface low pressure and the jet stream. The jet stream position can influence the development of low pressure and also high pressure in a big way, but in this instance I will be primarily looking at low pressure. The following chart is of the jet stream early on the 14th of May;

jet stream diffluent trough

Now of primary importance here is the position and also speed of the jet stream that is evident across and also just to the west of the UK.  You will note that the wind speeds are in excess of 130KT (~150mph) to the west of the UK and are digging into the rear of the surface low pressure which is essentially just to the south-west of Ireland. This particular jet stream pattern where the wind speeds are strongest on the let hand side of the trough is known as a diffluent trough and is one of the primary features to look for within the upper atmosphere with regards to low pressure development. Another example of this is evident on the below forecast chart and again note how the wind speeds are strongest to the left of the main trough, with this particular chart showing wind speeds ranging between 110KT and 130KT just to the west of Ireland.


So the obvious question is? What is the importance of a diffluent trough?

The importance of a diffluent trough is that it provides what is known as divergence within the upper atmosphere.  As many will know to get clouds and rain, the clouds within the atmosphere need to be of a sufficient depth to be able to produce rainfall.  A diffluent trough interacting with a low pressure system at the surface produces divergence within the upper atmosphere which is also then directly related to vorticity.  Think of vorticity of sort of like a hoover, metaphorically speaking of course,  in which when you switch a hoover on the mechanical parts allow air to rise at great speed beneath it. The diffluent trough is sort of like the ‘hoover’ and allows air to rise and then diverge at high altitudes, the opposite happens in association with high pressure, convergence takes places at high altitudes. So lets take a look at this more closely;

chart modifications

In this diagram, which is still representative of the low pressure today, you will see that the jet stream (diffluent trough) is producing wind speeds of approx 140mph to the west of the UK and between 90 and 100mph further south and east. The low pressure is in a prime location for vorticity which is directly associated with the rising motions of air which is represented by the diagram within the top right of the image. Meteorologists use vorticity charts to gauge the activity of frontal zones and low pressure systems for example.  Vorticity can be a great tool to discover whether a frontal system for example will be particularly active and bring thick clouds and heavy rain, like in today’s example. A forecast chart, for today, showing vorticity is shown below;


Of importance on this image is the orange and red colours located overand  just to the south-west of Cornwall.  What this image/forecast chart is showing meteorologists is where air is rising. Clearly in this instance the most important variable is rising air and the bright orange and red colours associated with the developing low pressure clearly show that there is a significant amount of vorticity associated with this developing low pressure, for the reasons discussed above, and hence the reason why this low pressure is set to bring some heavy or very heavy rain, strong winds and extensive frontal cloud to parts of the UK. The low pressure, because it is in a rapid stage of development has all the hallmarks of a rapidly developing low pressure on satellite imagery as well;

ir imagery

So, in summary. Some low pressure systems never get the change to develop because of unfavorable conditions within the upper atmosphere and how the jet stream is interacting with the surface low pressure. This example, however gives a great representation of how important conditions within the upper atmosphere are in terms of the development of surface low pressure systems.  Frontal waves, as I highlighted earlier in this blog, are notoriously difficult to predict at first, but these types of features can often bring some extreme weather to the UK if the upper level ‘ingredients’ allow for development.  There are many examples over the years were frontal waves have rapidly developed and moved into the UK bringing heavy rain, floods, gales and even severe gales. So I hope the above information has given some insight into the development of low pressure systems and when future charts are discussed some understanding of their importance will be known.

Regards to all,


Required Acknowledgements and thanks to these websites;


Summer 2013 – Thoughts and Analysis

14 May

Well, here we are again…

We are progressing through May quite rapidly and after one of the coldest spring’s in a long time and particularly March, it certainly feels as though we have progressed a long way into the year already without anything of worth in terms of warmth and more prolonged settled conditions. The middle of May, in my opinion, is often the best time of year to produce some thoughts on the up coming summer period covering June, July and August. It should be noted at this point that summery conditions can and often do continue well into September, but September is classed as the first autumn month. I should also place emphasis at this point that long range forecasts and information are far from an exact science and this particular blog is here to provide some possible insight as to the expected weather this summer. Sometimes the weather can change days ahead, let alone weeks and months, so please use this information as a possible guide and nothing more.

The summer months are often a difficult period to look at. The reasons for this is that during the autumn and winter period there are often a large number of variables that can be looked at to gauge how the winter may pan out, so summer can be difficult as there are generally less variables to analyse.  It doesn’t come with much surprise that over the last number of years the “summers” across the UK have been particularly bad and this was emphasised spectacularly last summer in terms of the persistence of unsettled conditions and also cool conditions as well. There are clearly discussions and thoughts that perhaps “something” has change with the weather to now bring the UK poorer summers and colder winters. Clearly time will tell on that, but the natural variation and changes that often occur through the summer across the UK and the North Atlantic have indeed change in recent summers. The usual trend is for the Azores high pressure to become increasingly influential through the summer months and ridge north and eastwards up into the UK and north-west Europe in general. This often combines with the jet stream disappearing well to the north of the UK and thus bringing the UK some summer weather. Clearly this trend and usual progression has essentially been none-existent for the last 4 or 5 years at least.

So what about this summer?…

As is often the case certain variables can be looked at to get an idea of potential broader term patterns within the long term, not only that there are also a variety of forecast models as well which can be looked at including the likes of the recently improved CFSv2 model for example. My interpretation of the situation as it stands now is for the summer to generally be in keeping with the last few years and that is for higher pressure to be more influential to the north of the UK rather than the south and south-west and lower than average pressure could well be influential in and around the UK once again. Last summer was clearly spectacularly bad due to the persistence of the unsettled conditions.  Whilst a repeat cannot be ruled out, I don’t expect this summer to be as bad as last summer with at least some temporary spells of better weather.

Expected Pressure Anom:

chart modifications

High pressure could well be more influential to the north of the UK in general signaling an overall -NAO summer pattern as lower than average pressure potentially affects the UK and some other central and southern areas of Europe. There is evidence from some of the seasonal forecast models for both June and July to have an overall -NAO pattern, which again would likely lead to higher than average pressure, in some shape or form, to the north of the UK.Summer

Expected Temp Anom:


The temperature forecast is particularly problematic and sometimes you can find significant variations across the UK, but as an idea I expect temperatures across the UK, in general, over the three summer months to be around or slightly below average by say 0C to 2C. The expected broader synoptic patterns don’t support a warmer than average summer and I don’t expect a sudden change from recent years for this summer to be dramatically warmer than average, so a near or slightly below average summer temperature wise is preferred.

Expected Precip Anom:

summer precip

With the signal for a possible -NAO dominated summer, or at least the majority of it, the end result could well be for the UK to experience more in the way of lower pressure than higher pressure once again. As a result this general trend and thought does then lead to the summer potentially being wetter than average, to what extent I am unsure. Clearly last summer was exceptionally wet not just for weeks, but for months, but I don’t expect a repeat performance at the present time.


So in essence I am not expecting the summer of 2013 across the UK to be a spectacular improvement on recent summers. As I have mentioned clearly last summer was particularly poor to say the least and the odds of that being repeated this year are slim, so in theory some improvement is to be expected compared with last summer. However, there is just little evidence, in my opinion, that supports a markedly improved summer with high pressure dominant for a number of weeks and for some very warm or hot weather to be a frequent occurrence. So, as ever time will tell, but a distinct average summer is expected in my opinion. I look forward to reviewing this prediction come early September and again please read and use this blog as a potential idea as to the possible weather this summer and nothing more.

Regards to all,