Archive | May, 2012

From High Pressure to Low Pressure…

30 May

Well it goes without question that the final week or so of May has produced some of the best weather of the year so far.  Many areas have enjoyed the persistent effects of high pressure across the country, bringing some prolonged spells of very warm, if not hot, sunshine.  Clearly a strong ‘taste’ of summer for many, however, it is all about to change and unfortunately timing with the onset of the first week of the first summer month.

So before looking forward, it is of worth to highlight and review why we have had such settled conditions.  This will help give a comparison to the up-coming changes.  The below is an analysis of the jet stream from this past weekend with information and comments as to why the weather has been settled and very warm;

If you compare the above image and analysis to the below image, which is the UKMO surface analysis chart on the 26th of May, you can see that a well established and large area of high pressure (1035mb) is centered just to the north and north-east of the UK and clearly maintaining very warm and settled conditions;

http://www.wetterzentrale.de/archive/2012/brack/bracka20120526.gif

So, where do we go from here and why is the weather changing?…

Clearly to get a change in the weather we have to see the above synoptic patterns change and that is what is forecast to take place over the course of the opening week of June.  High pressure is forecast to be replaced by lower pressure and more unsettled conditions and, as usual, this can often be seen and explained within the upper atmosphere;

Now transferring the upper analysis to lower levels and you can see this very well indeed.  Using the GFS ENS (ensemble) mean as the example, note how by Tuesday the 5th of June low pressure is evident at surface levels to the east and north-east of the UK and the upper level low on the above image is directly related to that surface low pressure in the middle of the North Atlantic;

The natural progression here is for the surface low pressure in the central North Atlantic to head toward the UK from the west or south-west, as the minor ridge of high pressure in-between both systems is eroded.  This is what is currently forecast to happen and I have high confidence in this event taking place given good model agreement and consistency within the forecast models for this to happen next week.  So jumping ahead a few days and you’ll note that by the middle and latter half of next week the low pressure to the west and south-west of the UK has moved up into the UK to bring a cyclonic pattern across the UK;

So, there we have it.  From high pressure to lower pressure in approximately a week or so. Just to reinforce this prediction, find below 3 main forecast models GEM (Canadian model), GFS (USA, NOAA mode ) and the ECMWF (Reading, European model) . As you can clearly see despite some variations over the synoptic patterns, all 3 models by the middle and latter half of next week have low pressure affecting the UK;

As is often the case with the weather across the UK, it never stays the same for very long.  I think many will admit and acknowledge that the weather of late really has been stunning and many have enjoyed it, particularly seeing that for a change the excellent weather persisted into this past weekend as well.  However, given the above analysis it is safe to say the first official summer month is not going to get off to a good start and given the longer term analysis for the remainder of June I wouldn’t expecting a ‘flaming June’ either at this stage across the UK.

It’s a case of don’t shoot the messenger, but get ready for a significant change in the weather over the next 7 to 10 days…

Regards to all,

M.

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What Lies Ahead for June 2012?…

25 May

Before looking forward I must comment on what has been and that is in direct relation to the last blog I produced and the weather of late.  The middle of the month update I issued, taking a look at the rest of May, has most certainly headed in the wrong direction.  What this clearly highlights is that even with some of the best forecast model data to hand, medium and long-term forecasting is, without question, open to changes and can often be very difficult to do.  I believe, in this instance, I may have been caught “ball watching” with regards to looking at a certain set of information but not seeing the full picture.  I suggest this because at the beginning of May the outlook for the second half of the month was towards higher pressure and I quote this below, which now looking back was a pretty darn good prediction at the time!;

“So to summarize.  The first half of the month, at least, looks set to be a predominantly unsettled one with further rain, showers and rather cool conditions.  Thereafter, the second half of the month and then perhaps more particularly the final quarter of the month may see a trend towards drier and warmer conditions.”

https://matthugo.wordpress.com/2012/05/01/may-2012-thoughts-analysis/

So, really I should have stuck with my original prediction, but as ever updated model data has to be acknowledged, but clearly this time round the finely balanced scenario which developed in the last week or more has clearly signaled a significant improvement in conditions.

So moving on and what does June, or at least the first half of June have to offer?…

At the moment I continue to support a theme towards lower pressure from the west after an initially settled opening week to June.  The below pressure patterns are what I believe are likely to be the case during the opening two weeks of the month, the first image essentially covering the 1st to the 7th and the second image the 8th to th 14th;

The second image may well look rather bleak from a UK perspective, but don’t take the image too literally.  What I am attempting to highlight is a simple trend towards lower pressure and more of a trough over the UK rather than significant high pressure.  Clearly during the summer months areas of low pressure are often far less ‘intense’ than during the autumn and winter months for example, so a progression towards a slack and perhaps convective pattern is what I believe will materialise after a settled and warm opening first week.

The GFS ENS also highlights this at the moment and the below two images tally well with the above graphics;

As can be seen the first image is showing a general ridge over the UK with surface high pressure out to the 5th of the month, which again essentially covers the opening week with dry, settled and often warm conditions overall.  However, the second image is showing that progression towards a more cyclonic pattern with more a trough over the UK.  Note how the orange colours on the first image in association with the upper level ridge and surface high pressure are replaced by lighter oranges and more of an upper level trough.

What does seem to be evident however is that despite this likely and/or possible trend towards a more cyclonic pattern I haven’t seen anywhere that suggests temperatures will drop to or below averages throughout the first half of June.  As a result I expect the first half of the month to be relatively warm, particularly the opening week with perhaps some very warm temperatures at times. The latest GFS ENS 850mb temperature graph for London, supports this prediction;

The second half of the month is shrouded in uncertainty.  As I highlighted in a previous blog during the summer months the pressure patterns become less distinctive than during the winter for example, so it is sometimes difficult to determine whether a more positive or negative pressure anomaly is expected.  However, I will put my neck on the line and suggest that after a potentially more unsettled mid-month, the second half of June may well see high pressure return and bring predominantly settled and warm conditions. This is based on some long-range forecast data but also a broader analysis of some seasonal forecast data as well, combining with what may well happen during the first half of June.

As ever, a mid-month update will be issued in mid-June to take a more detailed look at the second half of the month. Overall whilst the month may become quite changeable, generally June may well turn out to be a reasonable summer month, but as ever, time will tell…

Regards to all,

M.

 

Mid-May Analysis – Longer Term Thoughts…

15 May

Over the last month or two I have attempted to look at the medium and longer term trends both at the end of one month and approximately during the middle of the next month.  Through April and into May there has been clear trends in terms of the expected broader synoptic pattern, but this outlook is considerably more difficult to summarise and analyse.  The primary reason being, the longer term signals are less significant or clear.  This is often a feature of the summer months when pressure patterns become very slack and ill-defined which makes for attempting to find the broader scale patterns very difficult….However, I’ll give it a go!…

Clearly, and without question, this spring is likely to go down as being one of the most unsettled and most probably the coolest, or coldest in quite some time and clearly bucks the trend of recent springs.  With the exception of some glorious weather in March, which may offset that prediction, the rest of spring has been very wet indeed, as low pressure dominated the weather.  At the moment, despite the potential for some temporary settled conditions at times, I expect that the rest of May will remain predominantly unsettled.

The above image, I believe, will generally continue to summarize the broader synoptic pattern through the rest of the month.  There are clear signs that high pressure will become influential to the east and north-east of the UK and also perhaps to the west at times as well within the Atlantic, which may bring some temporary respite from the unsettled conditions.  However, this still doesn’t detract from the fact that the broader pattern is likely to be that of a trough over the UK and likely lower pressure overall.

The GFS ENS in particular, highlights this well also within the latest forecast chart for the 30th of May.  When looking at medium and particularly longer term, the details are sort of ignored with emphasis placed on attempting to find the broader patterns, particularly if there is consistency over a specific pattern.  This is the case at the moment.  The GFS ENS and the EC ENS both are consistent in terms of having a trough over the UK, whilst ridging within the jet stream is evident to the west and also the east of the UK and you can see this within the below image;

The other noticeable feature of the weather of late is the depressed temperatures, is this likely to continue?…The overall answer is yes, but with a likely trend in temperatures to recover to nearer average as May progresses.  What there isn’t any sign of during the second half of May is a surge or significant increase in temperatures into the very warm or hot category.  The generally unsettled theme is likely to sustain temperatures around average at best, but when compared with of late, it will feel warmer.

So in summary, despite a potentially improving picture compared with the first half of May, the second half of the month looks set to remain predominantly unsettled with further rain and showers at times and with temperatures nearer average, but with little risk of any significant or sustained warmth.

Into early June and unfortunately the scenario, at the moment, looks set to continue.  It really is near impossible to summarize the expected pattern in graphical form for early June, but the latest longer term data at the moment keeps a more negative pressure anomaly across the UK rather than higher pressure.  As a result the early expectations for the opening week of June are forecast to be for a predominantly unsettled regime to continue with temperatures near average at best.

Hopefully over the next 7 to 10 days a better understanding of what may develop into early June will develop and as a result I look forward to looking at early and mid-June more closely later in May, but for the time being it’s more of the same unfortunately…

Regards to all.

M.

May Bank Holiday – Thoughts & Analysis

2 May

The first Bank Holiday of May is nearly upon us and no doubt many are looking forward to the long weekend ahead.  Unfortunately the weather won’t deliver an early taste of summer, in fact it’ll deliver what is likely to be a very (very) late taste of winter!…There are some interesting reasons why and these are discussed below.

The broader scale weather patterns are shifting towards a far more blocked pattern.  High pressure is forecast to become slow-moving to the north-west of the UK, whilst low pressure is slow-moving over Scandinavia and primarily due to a weak and disorganised jet stream, something I’ll discuss a little later.  For now though, taking a look at the last synoptic chart for Friday;

As mentioned you can see high pressure dominating to the north-west of the UK with atmospheric pressure forecast to be over 1024mb across the North Atlantic, whilst pressure is lower across the Azores.  As a side note, this synoptic pattern is indicative of a -NAO (North Atlantic Oscillation) and of which, if this were winter the UK would be looking at a potentially noteworthy and prolonged cold spell.  But returning to what is expected in the here and now and the feature of interest is the cold front lying across Ireland and into North England on Friday.  It is behind this cold front that a markedly colder air mass is forecast to filter south from an arctic source.

You can see this more clearly on the latest thete-a forecast chart for Friday.  I have discussed theta-e before, but in summary it is essentially a forecasting tool and variable that allows meteorologists to determine whether an air mass is warm and moist or colder and drier and in this instance, note the deep blue and even pink colours filtering south into the northern half of the UK behind the cold front.  This is clearly indicating a change to a colder and drier air mass from the north;

Now earlier in the blog I highlighted that the broader pattern across the North Atlantic is becoming more blocked with a weak and disorganised jet stream.  Well, and as is often the case, the jet stream is directly related to the expected surface conditions.  So, find below the latest jet stream chart for the weekend and note how there is a ridge in the jet stream within the central North Atlantic in conjunction with the surface high pressure, whilst across the UK there is a trough.  More often than not when the jet stream dives down across the UK and we become located beneath a trough in the jet stream the weather often turns colder and is predominantly unsettled.

 

So what does all this mean in terms of actual weather?;

Clearly combining all the above information the Bank Holiday Weekend across the UK is forecast to be dominated by a particularly cold air mass from a northerly direction.  Some scattered showers are possible in places and given the increasingly cold air mass these may well fall as sleet and snow, particularly above 250m across Scotland and Northern England.  For most though, the weekend does look predominantly dry, particularly by Sunday and Bank Holiday Monday with some bright or sunny spells.  Clearly despite the time of year and the increasingly short nights, the air mass will be cold enough to lead to a risk of some widespread moderate and noteworthy frosts for the time of year.  Using the latest GFS Det model as an example, you can see that many inland areas and particularly rural areas of the UK look set to experience sub-zero Tmin values over the weekend, with perhaps some places experiencing -3C to -5c potentially;

 

So despite the rather cold air mass across the UK, it won’t be all that bad and in any sunshine by day the strength of the sun at this time of year should offset the chilly air mass somewhat and should be pleasant enough.  That being said, any gardeners out there beware of the frosts mind, as they are likely to be quite potent for the time of year.

Whatever you’ve got planned have a good Bank Holiday Weekend…

Regards to all,

M.

May 2012 – Thoughts & Analysis…

1 May

I’ll start with this well-known phrase:

“Ne’er cast a clout till May be out” – Despite some excellent May months in recent years, this one does indeed look very different and the well-known saying may well have some credence this month!

So, the final spring month is up and running.  What has been evident this spring, so far, is the variety and often extreme conditions across the UK.  The first spring month clearly brought us some extreme warmth and also very dry conditions in what was the 5th driest March overall.  Within a matter of a few weeks we were back to winter in early April with some snow and the rest of April clearly was dominated by low pressure which brought some serious amounts of rainfall to England and Wales and thus produced one of the wettest April months in a very long time.  So for those who like variability then this spring has and looks set to continue to deliver just that.

Taking a look at May and the first half of the month in particular generally looks set to continue where April finished and that is on an unsettled and predominantly cool, if not rather cold, note.  The below is what I believe is likely to be the broader jet stream pattern during the first half of May in particular;

It certainly seems that high pressure will ‘setup shop’ to the west and north-west of the UK for at least the first 7 to 10 days of the month, this can also be seen on both the GFS and EC ensemble mean charts below;

The broader effect of this general signal for higher pressure to the west and north-west of the UK is that the UK itself then becomes stuck in the trough located to the east of the higher pressure to the west.  Referring back to the original image in this blog, you can see that low pressure is expected to be located over the UK and also over north-west Europe in general in association with a trough in the jet stream.  This pattern, I believe, will dominate throughout the first half of May and continue to produce a predominantly unsettled spell of weather with temperatures slightly below or even below average at times, particularly during the first week of the month.  I won’t go into details over the expected weather in the short-term, but it’s safe to say that the rest of this week, into the coming weekend and early next week won’t be warm that’s for sure and some night frosts are possible with a risk of wintry precipitation in the north.

The latter half of the month has a lot of uncertainty evident, as is always the case when looking at longer term trends.  At the moment however, I expect a broader rise in pressure to take place across Europe as the upper trough across the UK and surrounding countries is potentially replaced by a rise in pressure. It can sometimes be more difficult to find trends as we progress towards the summer months because the general weather patterns become more ‘slack’ and disorganized than compared with winter which often produces more significant variation and distinct regions of higher and lower pressure.

So to summarize.  The first half of the month, at least, looks set to be a predominantly unsettled one with further rain, showers and rather cool conditions.  Thereafter, the second half of the month and then perhaps more particularly the final quarter of the month may see a trend towards drier and warmer conditions.  Overall though the final month of spring doesn’t look particularly ‘exciting’ if you are after a repeat of the weather which has been in evidence in recent May months over the last few years.

Regards to all,

M.