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;
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;
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;
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;
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;