MJO – Influences In Forecasting

29 Jun

When it comes to weather forecasting there are a variety of weather models that are used to give insight as to the possible weather. Both higher resolution short term models help weather forecasts pin-point those more specific areas at risk of say heavy rainfall and thunderstorms, to then the ensemble forecast information from the likes of the GFS and ECMWF which out to 2 weeks or more help to gauge the possible weather trends within the medium and longer term. When it comes to longer term forecasts there are some other factors which can be taken into consideration and one of these is the MJO – Madden Julian Oscillation

This tropical feature which is characterised by an eastward movement of rainfall across the Indian Ocean and into the Western Pacific whilst it may not seem to be of that much importance to the UK at first glance, does have significant and widespread affects on weather patterns across the globe and including the UK. There are many studies on the MJO that prove this to be true and one particular analysis and seminar by Christophe Cassou in 2008 provides significant and detailed information that the MJO can and does affect the weather across the UK and the North Atlantic area in general.

What I will say at this stage as a priority however, is that the MJO is not the “be all and end all” when it comes to medium and longer term forecasting. Just like any part of weather forecasting a variety of pieces of information need to be looked at and acknowledged when forecasting the weather and the MJO can be overridden by other large scale, global weather events and features.

However, I want to use this blog as an experiment, a forecasting experiment to see whether, on this one particular occasion, the MJO can indeed be a guide as to the longer term weather trends across the UK and the North Atlantic in general. Reanalysis of past weather data has lead to a series of pieces of information that correlate to the expected phase of the MJO, time of year and how approximately 10 to 12 days ahead (Cassou 2008) it may then influence the weather across the North Atlantic. Just like weather charts that show weather forecasters how areas of low pressure and high pressure move, there are forecast charts that show how the MJO may progress as well – http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/CLIVAR/clivar_wh.shtml

So, using the last link and the forecast charts on that page you can see that the MJO Index is currently in Phase 8. For a June month, this correlates to the follow possible synoptic pattern within approximately 10 to 12 days;

JunePhase8500mb

What this is showing us is that the green areas are signalling areas where pressure is likely to be higher than average and the blue colours denoting lower than average pressure. It is interesting to note that on this chart there is a clear signal that for the UK and NW Europe pressure is likely to be high and the current expectations are that towards and beyond the opening week of July the UK will experience a progression towards high pressure leading to settled, summery and potentially very warm conditions. So this is one interesting correlation and example of how the MJO may well be highlighting/influencing this signal of better weather given the current phase of the MJO.

So lets look further ahead for a real challenge;

ALL_emean_phase_full

The latest forecast guidance regarding the progression of the MJO is that there is quite high confidence that by the time we move into early July (1st to the 3rd) the MJO will have progressed into Phase 1. Note on the above image how the forecast models (coloured lines) signal the progression of the MJO into the ’1′ region. Now, using the reanalysis charts again as a possible guide to longer term trends, this is the following possible pattern for a July month given a Phase 1 of the MJO;

JulyPhase1500mb

Now, in this instance we have a totally different possible synoptic pattern across the UK. Remember that blue colours denote generally lower than average pressure and clearly there is a region of blue/lower than average pressure located across the UK.

So, using the above information and the latest forecast models regarding the track of the MJO, I’ll make a prediction that approximately 10 to 12 days after the opening few days of July the weather across the UK will become more unsettled as the high pressure and potentially very warm conditions from the 6th and 7th of July are replaced by cooler and more unsettled conditions as we progress towards mid-July (12th to the 15th approx).

I’ll come back to this blog and prediction around the middle of July to see whether it was accurate or totally wide of the mark, but it will be an interesting experiment to see, if on this one occasion, the MJO did have some influence on the weather longer term. If this prediction is also likely to be correct, it would seem that the possible settled and very warm conditions towards the 6th and 7th of July won’t be prolonged and lasting for less than a week.

Time will tell as ever!

Regard to all,

M.

Atmospheric Soundings – Approaching Frontal System

21 Jun

In a recent blog I discussed the usefulness of atmospheric soundings in terms of aiding to predict thunderstorm and convective activity. Well, forecast soundings have many uses and one other primary one is using them to view an approaching frontal system or to see the atmosphere through an active frontal system. A great example of an approaching frontal system is in evidence today (Fri 21st June) across Ireland. So, lets take a look at some of the synoptics;

12Z synoptics18Z synoptics

The low pressure and associated frontal systems can be seen clearly to the west of Ireland on the two synoptic weather charts. At midday you’ll notice that across Ireland there is a minor ridge of high pressure in evidence, this is represented by the isobars ‘ridging’ northwards. At midday the atmosphere across Ireland is forecast to be relatively stable beneath this ridge of high pressure, but you can clearly see by 1800Z the frontal systems associated with the surface low pressure are across Ireland, the ridge has been removed and the weather by the end of the day is essentially wet and windy in association with the passage of an active set of frontal systems.

The approach and passage of a frontal system (warm front) has certain characteristics and events that take place within the atmosphere.  These include anything from increasing wind speeds at high altitudes, the backing of surface wind speeds at surface level to generally a south-westerly direction and also the introduction of cloud and moisture, initially at high altitudes and then progressively at lower altitudes. The approach of a frontal system is represented well on the following graphic;

warmfront

Now, taking a look at some forecast soundings through the course of the afternoon across Ireland;

sounding13.curr.1200lst.d2sounding13.curr.1300lst.d2sounding13.curr.1400lst.d2sounding13.curr.1500lst.d2sounding13.curr.1600lst.d2sounding13.curr.1700lst.d2sounding13.curr.1800lst.d2sounding13.curr.1900lst.d2

1200BST and 1300BST: On these two forecast soundings you’ll notice that the red line (temperature) and the blue line (dew point) are particularly separated, especially between 700mb and 500mb. On a forecast sounding this represents a particularly dry atmosphere and hence little cloud. Also notice that the wind direction is still westerly or west-southwesterly and the wind speeds haven’t really increased very much through the atmosphere. This profile is representative of being beneath a ridge of high pressure and this tallies in well with the weather chart earlier in the blog which highlights a ridge of high pressure across Ireland.

1400BST and 1500BST: As the afternoon progresses the primary importance and change here is that the space/distance between the temperature and dew point line is decreasing within the upper levels of the atmosphere in particular. This is indicative of cloud increasing within the upper levels of the atmosphere as the frontal zone approaches. If you refer back to the earlier image showing the profile of an approaching frontal system you will see that cirrus and cirrostratus clouds are first to be expected and that is what these forecast soundings are indicating, an increase in upper level frontal cloud. Also note how the wind direction at lower levels has backed significantly and is now south-westerly

1600BST onwards: From around 1600BST onwards you’ll notice that the temperature and dew point lines get progressively closer and closer and by 1900BST are practically the same around the 700mb level. This is a clear example of how the frontal system has continued to approach the region introducing thicker cloud at increasingly lower altitudes and by the end of the afternoon a large portion of the atmosphere is moist and cloud laden in association with the frontal systems. You’ll also note that the wind speeds have increased within the mid and upper levels of the atmosphere in association with the jet stream and low pressure system.

The difference between the first image at 1200BST compared with the image at 1900BST is significant. The atmosphere goes from being stable and particularly dry indeed around the middle of the atmosphere (500mb) to being almost completely saturated by 1900BST in association with extensive frontal cloud which will clearly be producing frontal rain at surface levels.

Regards to all,

M.

Acknowledgements:

http://www.southamptonweather.co.uk/fronts.php

http://www.avbrief.com

http://rasp.inn.leedsmet.ac.uk

‘Loaded Gun’ Atmospheric Sounding

7 Jun

The term ‘loaded gun’ may well seem quite a strange one, but is quite an appropriate one for this particular kind of sounding. Clearly a loaded gun can “go off”, just like the atmosphere can under the right conditions and of which the below illustrates;

sounding 1

Atmospheric soundings, or tephigrams as they are also known, have many uses, but one of the important uses during the summer is to gauge potential instability within the atmosphere and to discover whether there is a risk of some heavy and thundery conditions possible from large cumulonimbus clouds.  In this instance, I use a forecast sounding for Exeter and I’ve highlighted two sections of the sounding which are of primary importance;

A = This area of the sounding is indicating that this portion of the atmosphere from approximately 6,000ft up to nearly 30,000ft is unstable.  When I say unstable I simply mean this area of the atmosphere is cable of allow large clouds to develop and air to rise and given the height at which clouds could well rise to (30,000ft) then these would be very large cumulonimbus (thunder clouds). However, for the clouds to properly develop the whole of the sounding needs to become unstable and there is an important part of this particular sounding that is stopping convection and the large clouds from developing…

B = Whilst this region of the sounding looks quite insignificant, it is in fact very important. You’ll notice on the image that near 4,00ft to 5,000ft the temperature (red line) increases slightly after decreasing from the surface. This is known as an inversion and whilst it looks insignificant this is prohibiting convection from occurring. Think of being in a room with the central heating on and all doors and windows are closed.  The heat from the radiators rises and will continue to rise until it reaches the ceiling and then can’t go any further, the heat is essentially ‘capped’ at the level of the ceiling, this is the same principle in this case. The warmth and humidity at surface levels is attempting to rise but is unable to rise past that level near 4,000ft to 5,000ft due to the inversion.

So, how do we get past this problem and allow the atmosphere to become completely unstable and for those big thunder clouds to develop?

Well there are a number of ways, but one of the primary ways is from the surface and that is with further warming through the afternoon. Additional warming of the surface will increase the temperatures and will then essentially override that inversion and once that is the case, the entire atmosphere then becomes unstable, as is evident on this forecast sounding for an hour or so later:

sounding 2

Note how the dashed line is now visible from approximately 3,000ft all the way up to beyond 30,000ft.  This is a particularly impressive atmospheric sounding in terms of the instability. Temperatures at the surface and humidity have increased enough to allow complete instability and that ‘capping’ near 5,000ft has been removed from the equation and all the heat and humidity within the lower layers of the atmosphere, that has built up, is now free to rise rapidly and significantly, in essence the “loaded gun” has “gone off” and more often than not, it would usually go with a ” bang” in the form of heavy and likely thundery showers.  I’ve also highlighted, at the top of the image, one variable known as CAPE (J). CAPE stands for Convective Available Potential Energy and is a measure of the potential instability within the atmosphere.  In essence the larger the value the more unstable the atmosphere is and in this instance a value of 992 is evident.  Anything over 1000 from a UK’s perspective is pretty impressive and does signal some possible significant convective activity. Of interest CAPE is derived from the area between the dashed line and the solid red (temperature) line on a forecasting sounding.

So if you ever see soundings like the above images posted on twitter or from other sources then think convection, heavy showers, cumulonimbus clouds and possible thunderstorms. The next 3 months are the prime time of year to experience significant convection across the UK that more often than not leads to some very lively weather.

Regards to all,

M.

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.

gfs_europe_009_300_wnd_ht

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;

QQ_GZ_UU_VV_018_0500

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,

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

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:

temps

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.

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?…

29 Jan

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;

Jet stream short term

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;

Rz500m7

Reem1681

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;

jet stream long term

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.

Reem2401

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

30 Dec

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;

First half of January

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;

ecmwf30f240

ecmwf10f240

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.

ecmwfzm_u_f240

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;

temps

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.

 

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