Low Pressure Development – 14th May 13 Case Study

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,

M.

Required Acknowledgements and thanks to these websites;

http://www.wetterzentrale.de/topkarten/tkfaxbraar.htm

http://squall.sfsu.edu/scripts/jetstream_atl_model_fcst.html

http://mag.ncep.noaa.gov/?prevPage=&MainPage=index&model=&area=EUROPE&areaDesc=Europe&page=Model&prevModel=&prevArea=NAMER&currKey=region&prevKey=&cat=MODEL+GUIDANCE

http://meteocentre.com/models/models.php?lang=en

http://meteocentre.com/sat/get_sat.php?lang=en&area=uk&map=bw_ir&siz=_50&anim=0

Summer 2013 – Thoughts and Analysis

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:

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:

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.

Summary:

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,

M.

February 2013 – What’s In Store?…

Well there goes another month and we are now well over half way through the winter of 2012/2013 with last of the winter months just around the corner.  January, will, without question, be remembered for quite a distinct and sharp change in the weather compared with the opening week or two. The discussed Sudden Stratospheric Warming event that I made numerous references to in blogs at the back-end of 2012 did occur and without question clearly influenced the weather during the middle and latter half of January.

So what about February and the last of the winter months?

Clearly these final few days of January has seen a marked change once again back towards milder and more unsettled conditions after approximately two weeks of cold weather, but I don’t expect this to be sustained.  Throughout the opening week of February I expect the following broader synoptic pattern to be in evidence;

In essence I expect high pressure to be dominant to the south-west and at times west of the UK whilst low pressure remains dominant to the north and north-east.  This is likely to maintain a west and at times north-westerly air flow over the UK overall.  This broader synoptic pattern is clearly evident on the latest GFS and ECM ensemble mean charts for the 5th;

It is beyond the opening week of February that a change in synoptic pattern may well become more dominant to introduce a renewed risk of colder conditions developing across the UK.  The expected synoptic pattern and broader synoptic evolution looks to be very different to what brought the cold/very cold conditions through January.  This cold weather was produced due to a blocking area of high pressure to the north and north-east of the UK whilst low pressure dominated to the west and south-west.  As February progresses I expected high pressure to be influential to the west and north-west of the UK whilst low pressure is dominant to the east or north-east, as highlighted on the below graphic;

This pattern is known as a meridional pattern where the jet stream often tracks far north before then tracking south.  It is on the eastern flank of the expected high pressure within the Atlantic that produces north or north-westerly winds and I do expect that this general, broader, meridional pattern to become established beyond the opening week of February and towards the middle of February.  This pattern is likely to lead to colder than average conditions becoming established with perhaps some wintry precipitation at times.  However, what is extremely difficult to comment on is just how much ridging will take place to the west and north-west of the UK.  If that area of high pressure isn’t substantial enough then low pressure systems can easily move over the top of the high pressure and down in to the UK and bringing generally milder conditions and this scenario cannot be ruled out either.

The ECMWF ensemble mean at 240hrs does highlight this broader meridional pattern.  Note how high pressure is attempting to ridge north-wards to the west of the UK whilst low pressure is digging southwards to the east of the UK and thus producing a rather cold north or north-westerly air mass over the UK.

A week or more ago there was the potential for a significant rise in pressure over Greenland through February which would have potentially brought a more sustained period of cold weather.  However, latest data has backed away from that idea and personally I don’t believe we will see a solid, sustained Greenland blocking high pressure developing through February now.  It can’t be ruled out, just like any pattern, but when looking at the medium and longer term, trends are important and without question there has been a trend away from this scenario.

The final third of February has a lot of uncertainty associated with it.  The stark variations that have been evident through this winter so far have made us realise, once again, how volatile the UK’s weather can be and how quickly it can change, as a result I have little or no confidence over the final week or 10 days or so of February at this stage, but a progression back towards more unsettled and milder conditions may well be preferred with time.

So in summary after an unsettled and relatively mild opening week or so of February the cold may well return, but this time most probably from the north  or north-west rather than the east or north-east as high pressure becomes an influential feature to the west or north-west of the UK and as low pressure dominates to the east. A mild, wet and windy month is not preferred at all and whilst there may well be a reduced risk of any Greenland block, I still think overall that February will combine with January in particular to help bring about an overall average if not slightly below average CET value by the end of the winter, which would mean that the winter of 2012/2013 would not fall into the ‘mild’ category and essentially be another cold winter overall.

Kind regards to all,

M.

A New Month, A New Year – January 2013

Well, there goes 2012 and it sure has gone quick, especially when you look back at the variety of quite significant and extreme weather events that were ‘handed out’ to us this past year. Another year is about to start and seeing we are entering into the ‘heart’ of winter, there are some interesting developments which could make for a particularly interesting January.

The winter so far has generally been quite mixed and overall ‘average’.  Despite a rather cold first half to December with some fairly frequent frosts and even some snow in places, the second half of the month has clearly become far more unsettled.  This has helped to lift temperatures back to nearer average values and not only that cement this year as one of the wettest on records with clearly further flooding to end December and end 2012.

The month ahead, January, I believe could deliver quite a stark variation in conditions with a potentially abrupt change in the weather approximately mid-month, this is primarily due to the expected development of a Sudden Stratospheric Warming event and also other variables including the likes of the QBO (Quasi-Biennial Oscillation).

Taking a look at the first half of the month and the below is what I believe the broader synoptic patterns will look like as an overall average/mean;

Overall there is quite good model consistency between the GFS and ECMWF ensembles for high pressure to build from the south and south-west of the UK during the opening week of January and become a dominant feature just to the south of the country.  At times there is a clear signal for the high pressure to build up into the UK and thus produce predominantly settled conditions with a marked ‘ridge’ in the jet stream across more west and north-western areas of Europe.  As a result, whilst perhaps not lasting the whole 15 days, the first half of January looks set to be dominated by higher pressure bringing more in the way of settled conditions and with, thankfully, a marked reduction in the risk of further heavy and disruptive rainfall.  It should be noted that at the moment the high pressure isn’t likely to be one that brings cold and frosty conditions, at least not a first, but instead a lot of cloud is forecast within the short term with patchy rain and drizzle possible.

Now, onto the interesting bit and this relates to the stratosphere.  I have discussed the stratosphere in a blog a few months ago which highlights it has two primary states, that being a well established ‘cold’ vortex and the other being a more disrupted and ‘warmer’ vortex.  This blog and information can be read here

Latest forecast models continue to show a warming event taking place within the stratosphere into early January which is forecast to either split the vortex, displace it or perhaps even do both.  Some charts of interest are below which highlights this current thinking;

Without going into the details too much, both of the above forecast images show a significant warming event is set to take place through the opening week of January as higher pressure becomes particularly dominant within the stratosphere and with a marked increase in temperatures. You’ll notice on both images that the vortex, or what’s left of it, is well displaced from its usual position across the pole and is also split with one main region over North-east Canada and the other over or near Scandinavia. These charts signal a SSW event.

The other factor of a SSW is the reversal of what is known as the zonal winds within the stratosphere.  This is also evident by the negative blue colours on the above image from approximately 60°N and of which are extending from the top of the stratosphere (1hPa) down to near the bottom of the stratosphere and the top of the troposphere (100hPa). This is, yet again, another clear signal for a SWW event.  Finally the below image clearly shows the predicted temperatures across certain areas are forecast to rapidly rise above average into early January in association with this warming event;

Now, what I should add here, which I have also mentioned in the blog from a few months back, is that a Sudden Stratospheric Warming event doesn’t automatically mean cold weather for the UK.  This is an important variable and just because one is forecast doesn’t mean the UK will be plunged into deep cold for weeks.  However, what it does mean is that the affects on the troposphere is likely to be significant and previous SSW events and analysis clearly show that when a SSW event occurs it promotes a highly disorganised polar vortex and allows/aids in the development of blocking (higher pressure) at higher latitudes.  If high pressure can be achieved at higher latitudes then this brings the risk of colder weather affecting the UK and also many other parts of the northern hemisphere.  As an example of high pressure becomes dominant to the north-west of the UK over and around Greenland, then this ‘blocks’ the Atlantic and often brings cold/very cold north or north-easterly winds for several days.

I will shy away from creating a graphic of how I think the synoptic patterns will evolve beyond mid-January simply because it is anyone’s guess for now.  However, the science being the SSW is sound and I do believe that given the above information and also other information, that as January progresses a far more blocked pattern will likely develop and as a result will bring an increased risk of colder conditions affecting the UK towards and beyond mid-January.  Clearly the details as to how cold, how long any cold will last and its origins are unknown and will be for a number of weeks.  But I urge caution at writing off this winter, there is a long way to go and I do believe eventually the ‘cold’ will arrive.

A Happy New Year to all followers and readers.

M.

 

Possible Stratospheric Demolition & Final Winter Thoughts…

With just a few days of autumn left and an autumn which has produce quite a few extremes, the attention now turns towards winter.  I’ll add at this junction that (time willing) I’ll produce a blog looking specifically at December in the next few days. In this blog I wanted to directly focus my attention and information on the state of the stratosphere as we now enter into winter.

Many may well have heard of the ‘stratosphere’ in recent months and also within the last year or so as this particular ‘variable’ has been given more and more scientific attention.  Some people believe that the stratospheric conditions aren’t the be all and end all when it comes to affecting the weather across the UK during the winter and in a way they are correct.  There are a large number of variables which come in to play, but equally I can disagree and argue that comment with substantial evidence, of which I have acknowledged myself over the last few years.  Without question, the conditions of the stratosphere do affect the weather across the UK and the northern hemisphere during the winter and early spring period.

In one of my previous blogs, which explains the two primary different states of the stratosphere (http://matthugo.wordpress.com/2012/10/01/stratospheric-conditions-winter-weather-analysis-information/) I give a perfect example of how the stratosphere can directly affect the weather within the troposphere and a perfect example of that was last December.  Now without going over those details, I wanted to take a look at the state of the stratosphere up to present day and what is forecast to happen in the coming days;

Initially, through this autumn, the stratosphere has cooled (as it should) at quite a significant rate to say the least and generally this progression would often lead to a pattern as evident last December.  However, for a number of factors, including the likes of the QBO (Quasi-biennial Oscillation) these particularly potent stratospheric conditions haven’t been able to filter down into the troposphere this autumn and are now highly unlikely to do so.  The rapid drop in temperatures across the polar stratospheric region can be seen on the below 2 images;

As can be clearly seen on the first image in particular the temperatures have now dropped below -80C which is pretty much as low as they can go. Within the second image you can see that at this time last year the temperatures were very similar, but perhaps slightly warmer.  However, there are clear differences, of which I highlighted earlier, this year the stratospheric conditions have been unable to penetrate down into the troposphere and aid the development of a significant polar vortex (http://en.wikipedia.org/wiki/Polar_vortex). To give an example of what the stratospheric vortex looks like at the moment, the below image shows this;

What this image is essentially showing is the temperatures within the stratosphere and also the overall ‘shape’ of the stratospheric vortex.  At the moment, whilst rather elongated, it remains a noteworthy feature and as can be seen within the bottom left of the image the lowest temperature on that chart is -83c (82.5N 2.5W) essentially near the center of the vortex over the polar region.

So what is expected in the coming 7 to 14 days to change this setup?…

Well, without going into the details too much, of which even I am still attempting to get a grasp of, the stratospheric vortex through the winter period can often be at risk of under-going ‘attacks’ from various sources.  These ‘attacks’ help to distort and weaken the vortex and the so-called “holy grail” can be a complete split and relocation of the vortex as well, you may have heard of an SSW (Sudden Stratospheric Warming) event.  At this point don’t forget that a highly disorganised stratospheric vortex has been found (time after time) to aid in the development of northern latitude blocking and a heightened risk of colder weather across the UK and other areas of the northern hemisphere during the winter period than if the vortex is well organised.

So a few more charts of interest;
1)

2)

3)

1) The first image clearly shows a very different vortex within the next 7 or 8 days compared with the actual vortex of present day.  The vortex, as can be seen, has split and of significance is the major warming event taking place over Eastern Russian in association with a major region of higher pressure across Alaska and far eastern areas of Russia.  This is represented by the lighter blue colours and also the patch of pale yellow colour of which represents temperatures of -50C or above.  This may still seem cold but clearly compare that to temperatures of below -80C and that is a marked area of warming.

2) The second image shows the zonal wind speeds (m/s) up through the troposphere and more specific the stratosphere in relation to latitude.  Of significance here is the negative (blue) colours and contours above 70N across the right of the image.  This highlights a progression towards a highly disorganised and weak stratospheric vortex.

3) Finally the bottom image and more importantly the top 3 graphs show how the temperatures are forecast to rise into early December compared with at present to either near or even above average. Of significance, don’t forget that a warmer stratospheric vortex essentially equates to a weaker vortex and a greater likelihood of potential blocking (higher pressure) within the troposphere.

In conclusion;

Gathering the above information and more together the situation through this autumn has been quite unique.  The behavior of the vortex would (more often than not) have led to a likely period of zonal conditions across the UK as the autumn progressed, but clearly this hasn’t happened.  Despite the highly organised and very cold stratospheric vortex there are now a variety of variables which are forecast to “attack” the vortex within the next 7 to 14 days of which, if successful, may actually demolition the vortex almost completely.  For this to happen at this time of year would (combined with other factors) lead to a heightened risk of more meridional and blocking conditions developing.

So, and finally, combining this data and with other information I believe that the winter across the UK between December and February will end up being colder than average with a greater frequency of blocking patterns leading to cold outbreaks.  Clearly details are not possible, but particularly as we progress through the second half of December and also particularly into January if the stratosphere fails to recover from this potential “attack” then I wouldn’t be surprised to see at least one major and noteworthy cold outbreak across the UK.

It’s all to play for, but I certainly feel that compared with last winter, the coming winter will be markedly colder for the UK.

Time will tell as ever in meteorology and I look forward to looking back on this particular blog come next March to contrast and compare.

Regards to all,

M.

The Importance of High Pressure On the UK In Winter

Well autumn is nearly behind us and attention is most certainly being directed towards the start of December and the first of the winter months.  Clearly there has been a lot of talk about a cold start to the month and that seems increasingly likely.  One of the primary reasons for this is the potential development of high pressure (anticyclone) in and around the UK.  I have produced this blog to bring attention to the importance of high pressure across the UK through the winter and whilst this isn’t a forecast as such, may highlight some of my thoughts for the coming weeks.

As we all know high pressure brings more settled conditions than compared with low pressure.  Now from a UK’s perspective high pressure during the winter is important because it aids to bring the risk of colder conditions from a variety of sources.  Due to the location of the UK geographically and the way air masses move around high pressure, more often than not, higher pressure to the north of the UK in some shape or form, is the main ‘key’ to unlock the door to much colder conditions from the arctic.

Whilst this particular chart is at the far end of the GFS model’s time frame, it is still a good chart to use as an example to illustrate the above comments;

 

Of primary importance and significance here is the area of high pressure situated to the north-west of the UK.  The high pressure is extensive and extends from eastern Canada (1035mb) to Greenland (1030mb) and then all the way up towards Svalbard and into the arctic (1040mb).  Air masses around high pressure systems flow clockwise which is particularly important because if you get high pressure to the north-west of the UK for example, then in theory the air mass to the right of the high pressure would be from a north or north-easterly source, clearly of which is a cold source through winter.  The other important factor is the low pressure over Scandinavia here.  This is sort of acting to combine with the high pressure to allow a particularly cold region of air to move down across the UK from the north-east.

So this is one way that high pressure is important to the UK in winter, get the high pressure at a more northerly latitude to the north of the UK then the chances of colder weather certainly increase, but isn’t a certainty, then again what is in meteorology!?

The other important feature of high pressure and the UK is if the high pressure isn’t able to get a northerly location but becomes situated across the UK.  This type of pattern in winter brings little risk of wintry precipitation simply because the anticyclone is dominant across the UK and brings settled conditions, as would be expected.  However, what is important is something which is known as an inversion, which develops beneath areas of high pressure and this can still lead to particularly cold conditions at the surface, especially over a period of days.  I have explained this below in a graphic;

Air within a region of high pressure is sinking and warms as it does so until a point is often reached in the lower atmosphere where an inversion develops, where the temperature then suddenly cools (or warms if you’re rising up from the surface). What is important here is that beneath this inversion cold air can be come trapped, particularly after a period of days because as each overnight period comes and goes, as long as the skies are clear, then cooling takes place and over a period of days that results in a gradual drop in temperature. As an example, when the high pressure builds into the UK the initial maximum temperature may be 8C with temperatures falling to near or slightly below freezing at night (as long as skies are clear).  However, the net affect is for the temperature to steadily drop seeing the nights are longer than the days and if the high pressure persists for several days then the maximum temperatures within 4 or 5 days may be nothing higher than 3C or 4C and minimum temperatures fall more significantly below freezing.

So, there we have it.  High pressure is particularly important during the winter period and it would seem that given latest forecast data the end of the month and into early December will see high pressure become an increasingly important factor.

Regards to all,

M.

Rest of November 12 – Thoughts & Analysis

Well there goes the first half of November and with only 2 weeks left of autumn, until December starts, this autumn has been quite an unusual one.  The distinct lack of unsettled and zonal conditions has been noteworthy, despite quite potent stratospheric developments which can aid the development of this trend towards more zonal conditions through autumn.  We are yet to see any significantly cold conditions, despite some frosts at times.  The first half November has followed suite really with a mix of conditions being evident.

The second half of November has clearly been a talking point for some time, particularly in relation to the potential development of colder weather and a broader change in synoptic pattern.  In this blog I’ll attempt to summarise my thoughts and information to give an idea and suggestion of what I believe will develop during the second half of the month.  As an early disclaimer, of sorts, it should be noted that the second half of November is a forecasting challenge for many reasons.

For the time being I expect rather unsettled conditions to continue across the UK and the reason for this is the following broader synoptic pattern across Europe;

High pressure remains influential across southern and eastern areas of Europe as low pressure remains slow-moving in the vicinity of the UK.  As a result for the UK the next week or so looks unsettled.  Without going into the details, areas of low pressure look set to bring showers, longer spells of rain and some windy conditions as well.  This is highlighted well by the latest GFS ENS mean chart for the middle of next week;

So now that is out-of-the-way, what about the all important final quarter or so of November and perhaps into early December?…Is it likely to turn colder I hear you ask?…

My answer at the moment is; yes.  I also expect it to become somewhat drier than average as well.  The below image was a tricky one to try to put together but I believe it summarises the broader synoptic patterns by late November and into early December.  Clearly I expect higher pressure to become dominant to the north or north-west of the UK whilst pressure becomes lower across more southern areas of Europe.  There are definite uncertainties over the final location of any higher pressure and this will be crucial to any potential cold spell.

 

 

The longer range forecast models, of late, have been persistently showing a trend towards higher pressure to the north of the UK in some shape or form.  This type of synoptic develop at this time of year often leads to what is known as a negative North Atlantic Oscillation (-NAO) and essentially ‘kills off’ any low pressure systems and zonal conditions that the Atlantic can bring to the UK.

I have to admit the final location of any higher pressure is unknown, but to give one example and that is with higher pressure more directly over the UK is the 00Z ECMWF model;

This is one of a number of possible solutions towards the end of November.  If this synoptic pattern were to materialise then generally the UK would potentially enter into a period of colder conditions with a risk of frosts and fog by night, depending on cloud amounts.  At this time of year high pressure across the UK more often than not doesn’t equal mild conditions, particularly if the high pressure becomes relatively slow moving across the UK over a number of days.  What you should also note on that ECMWF chart is how there are similarities to my chart.  Note how pressure becomes lower across southern Europe and Mediterranean, whilst clearly pressure is rising in and around the UK.

So in summary, for the time being I expect predominantly unsettled conditions to carry on with a particularly unsettled week next week for many areas of the UK, but especially across the north and west.  I will then ‘stick my neck out’ and make a bold suggestion of a significant pattern change by late November and into early December which will take the UK into a colder and drier spell of weather.  Clearly, and as mentioned earlier, the details are unknown, but my outcome is for late November to now turn colder, particularly compared with of late.

As ever it’ll be interesting to see how things develop and materialise, but the winter woolies may well be needed by late November and into early December.

Regards to all,

M.