Archive | March, 2012

High Pressure, Cloud or No Cloud? – That Is The Question…

27 Mar

The current settled and quite exceptional Spring conditions across the UK is associated with a large area of persistent high pressure.  However, if you remember a week or two ago high pressure was dominating the weather but it was often cloudy with it, so why is this high pressure different?…

The continued unbroken sunshine that is affecting many areas of the UK is down to the fact that the air mass across the UK is very dry, unlike the other week.  Meteorologists use atmospheric soundings to show conditions within the upper atmosphere and I have attached below a sounding for Leeds during the course of this afternoon (Tuesday 27th March).  This sounding is particularly impressive because it indicates a large amount of dry or very dry air across the region which is prohibiting cloud development;

The above information should help to explain why at the moment the UK is basking in unbroken sunshine and why even during the warm afternoons not even a few scattered cumulus clouds can develop.  This very dry environment is not only evident throughout a large portion of the middle and upper atmosphere, but it is also evident at the surface.  Those with a weather station may have noticed some exceptionally low humidity and dew point readings in recent days, well this once again helps to highlight just how dry the air is at the moment and why clouds just simply can’t develop.

Now just for comparison’s sake, the weather is set to change later in the week to cooler and generally cloudier conditions as our high pressure declines to the west of the UK and allows for a cooler north-westerly air mass.  The below atmospheric sounding for Leeds on Friday afternoon does look very different compared to the above image;

So, in summary; When high pressure becomes located over the UK it is often extremely important to use and look at atmospheric soundings to gauge what the atmosphere will be like above surface levels.  Atmospheric soundings are an extremely important “tool of the trade” when it comes forecasting.  So if you ever see an atmospheric sounding present like in the first image, think plenty of sunshine with very little or no cloud.  However, if high pressure is evident but the atmospheric soundings look more similar to that of the second image, then whilst it may well be mainly dry, cloud is likely to be an issue with perhaps cumulus clouds developing during the day or perhaps stratocumulus clouds persisting in large sheets, as was evident the other week.

If anyone is interested in viewing more of these specific forecast and actual atmospheric soundings, then they can be found here;

http://rasp.inn.leedsmet.ac.uk/RASPtable.html – Just use the scroll down menu on the right hand side.

Regards to all,

M.

 

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Weekend Prospects (24th/25th) & The Rest of March…

21 Mar

In my last blog, I took at look at the medium to longer term scenario which essentially covered the rest of March and at that particular point all the signs and signals were pointing towards high pressure and anticyclonic conditions.  That trend and signal hasn’t changed and I’ll have another quick look and re-cap at the remainder of March after taking a look at the coming weekend…

Firstly we are now into spring, with the spring equinox taking place earlier in the day yesterday (20th) and the coming weekend looks set to produce a very spring-like spell of weather, as you would expect, due to high pressure…

Taking a look at this mornings 84hr chart from the UKMO along with the 120hr image from overnight, you can clearly see that high pressure develops into a significant feature over the weekend.  At this time frame there is still uncertainty over cloud amounts and for example that weak east or north-easterly flow evident on the 120hr chart may well produce more extensive cloud and cooler/chilly temperatures along the east coast of England.  In contrast, come inland, to parts of the Midlands, Wales and Northern England and Sunday looks very good indeed with pleasantly warm temperatures of around 15C or 16C and with plenty of sunshine. So what’s the reason for the high pressure?…

As is usually the case, the expected surface conditions can be traced back to the mid and upper levels of the atmosphere and the above information should give some background information as to why high pressure is forecast to be a dominant feature of the weather this weekend.  The other important feature to point out is that warm air advection (WAA) on the western flank of an area of high pressure often leads to the intensification and development of the surface high pressures.  You can see that on the first two images.  Note how on Saturday whilst evident the high pressure isn’t as developed as it is by Sunday and this is down to WAA taking place just to the west of the high pressure, and a few other ‘ingredients’ for good measure.

So in summary a decent weekend for many, but given the time of year there will be some exceptions and where there is any onshore breeze or airflow then those areas are likely to come off worse, but many inland areas of the UK look very pleasant indeed this weekend for late March.

The remainder of March is generally unchanged from when I looked at the situation a number of days ago and the below GFS and EC ENS mean at 240hr support the continued high pressure scenario;

So the remainder of March does indeed look anticyclonic, with further predominantly dry conditions leading to below or well below average rainfall totals and with temperatures generally above average overall.  There continues to be consistency however for a pattern change into early April and despite the time frames involved, I can say with some reasonable confidence that as the opening week of April progresses cooler, if not colder, and more unsettled conditions are likely to develop…

Regards to all,

M.

 

 

Medium to Longer Term Outlook – Rest of March 12

14 Mar

Well so far March has been quite a benign month with nothing really of any significance or great interest.  The primary and dominant feature of the weather overall so far has been high pressure and anticyclonic conditions and it would seem that within the medium to longer term, high pressure will remain, or at least re-develop, into a dominant feature…

The high pressure conditions of this week are set to decline for a brief period of time over the coming weekend as more unsettled and also colder conditions develop.  This change can be seen within the forecast jet stream patterns and the first image highlights what we have been having at present and compare that with this coming weekends jet stream pattern;

The information within these two images should help explain why we progress towards a more unsettled and cyclonic pattern and last nights 96hr FAX chart from the UKMO for example, clearly highlights a far more cyclonic pattern with low pressure, troughs and frontal systems evident over the weekend;

Moving beyond the coming weekend and into the final third or so of March and the primary question is; will we continue to see cyclonic and more unsettled conditions prevail across the UK? – The simple answer is; no we won’t…

The primary reason why, whilst referring back to the first image in this blog, is that as next week progresses the jet stream once again begins to ridge northwards across the UK and as a result surface pressure rises.  The below two images are the 240hr GFS and EC ENS charts, as you can see both signal a marked region of high pressure affecting the UK;

As you can see the 240hr time period is currently covering Saturday 24th of March.  So the medium to longer term outlook is one of high pressure and settled conditions, particularly away from the far north and west of the UK in particular, with unfortunately very little rainfall expected for England and Wales.  Now one of the talking points of recent days is the lack of sunshine, could this be the case with this next high pressure? – The answer is simply, yes it could…

Unfortunately it is impossible to be able to say with any degree of accuracy what amount of cloud will accompany this next anticyclonic spell.  Given the current broader pattern it does seem as though quite a lot of mild and moist air from the south 0r south-west will be associated with this high, which isn’t a good early sign, so at the moment it could well be another mild, but often cloudy spell of weather.

The generally settled conditions do look set to continue overall until the end of March, so in my opinion, particularly for England and Wales, this March looks like it will end up being a particularly dry month and also potentially quite a warm one as well, depending on sunshine amounts in subsequent weeks in particular.  The sunshine amounts are important at this time of year because without the sunshine temperatures can still be depressed beneath cloud as many have been experiencing in recent days.

So for those wanting rain, and lots of it, unfortunately it seems unlikely you’ll get any…at the far reaches of forecasting, there are early signs that the opening week of April may see something more unsettled and cooler, but this is a long way off as yet.

Regards to all,

M.

 

Medium to Longer Term Outlook – Spring Sunshine!

5 Mar

Spring can often be a time for extremes across the UK.  The progression away from winter often leads to a greater risk of some milder conditions developing at times and clearly a progression away from long, dark nights.  However, some late winter weather can often return at times as well, but in this instance that isn’t the case.  The outlook is looking extremely promising indeed for those in need of some spring sunshine…

The early signs with regards to this up coming spell of high pressure dominated conditions was evident around 3 or 4 days ago, but at that time there was little model consistency at all over the specific synoptic evolution.  However, recent model runs, particularly in relation to the GFS and ECMWF ensembles have continued to show signs of high pressure become influential across many areas of the UK as we progress towards the middle of March.  Some of the global models are now also picking up on this signal which is likely to develop over the coming weekend….Find below a few charts of interest;

The above images are the UKMO and the ECMWF deterministic models at the 144hr time frame, which at the present time covers Sunday 11th of March.  As can be seen there is a marked area of high pressure becoming well established across the UK at this particular time, with surface pressures of at least 1035mb.  The other noticeable feature as well is for the potential development of some mild or very mild air from the south-west, this can be seen on the below chart in terms of the 850mb temperatures reaching at least +5C from the south-west;

As I highlighted there is model consistency and agreement for this synoptic pattern to develop now over the coming weekend and continue well into next week.  Whilst it is very difficult to say how long these anticyclonic conditions will carry on for, the longer range ensembles, at the moment, signal the settled conditions continuing until at least the 17th and 18th of March.  This pattern can be seen well within the GFS ensembles at the moment and take the following two images;

Just using London as an example, but note on the first image how there is a distinct lack of precipitation ‘spikes’ evident across the bottom of the graph throughout a large portion of the forecast period which takes us up to the 21st of March. Also of significance as well is the second image and the predicted pressure.  At the moment the ensemble mean (white line) has atmospheric pressures at or above 1020mb across London for the next 2 weeks!…

So what does all this mean?…Well in summary it would seem that the UK is going to progress towards a sustained period of anticyclonic conditions which will lead to sustained dry weather with some likely decent sunny spells at times and also some pleasantly warm days with temperatures potentially ranging between 12C and 15C into the early part of next week for example.  As ever, there is always a negative to a positive and from a drought point of view it does seem as though March could now well turn into a very dry month indeed for parts of England and Wales in particular…

If you don’t think spring has sprung just yet, you will do by this time next week!…

Regards to all,

M.