Archive | February, 2012

Colder Next Week – Why & How Long For?…

29 Feb

It just goes to show how unpredictable and varied the weather can be, particularly at medium and long ranges.  At the end of last week it looked as though there was little chance of any return to colder weather, even briefly, with my thoughts highlighting what the models where signalling as we moved into early March, which was predominantly mild.  However, subtle changes to the broader scale weather patterns are now signalling the development of colder conditions, at least for a time, during the latter stages of the weekend and into the early part of next week…

Just to re-cap the last 7 to 10 days and that has been dominated by high pressure to the south of the UK and lower pressure to the north and north-west, which has brought a persistent feed of very mild and moist south-westerly winds into the UK, well that is about to change as one would expect.  The initial changes come about due to a marked change in wind direction and air mass over the weekend;

Using the latest FAX chart for Saturday, you’ll notice that a cold front is forecast to move across the UK and it is this feature that will signal the change to a colder west or north-westerly air mass.  Now, what is of significance here is the air mass to the west of the UK.  Note on the above chart that the 1000-500mb thicknesses are less than 528dam just to the West of Ireland, but more importantly are as low as 510dam further upstream within the mid-Atlantic.  This air mass will be advected into the UK on the westerly air mass during the second half of the weekend…

I’ve discussed theta-e (equivalent potential temperature) before in previous blogs, but just as a recap, theta-e charts are used to highlight different air masses, whether these be warm or cold.  In this instance and as a you can see on the above image, which is for approximately 1200Z on Saturday 3rd of March, the black line is signalling the approximate location of the surface cold front and importantly notice how the deep blue and even pink colours are evident to the west of the UK which signals a cold, if not very cold air mass.  This can also be seen on the 850mb temperature chart;

Notice how 850mb temperatures are below -5C to the east of  the UK and further upstream are below -10C and again this cold air mass is then forecast to be advected into the UK over the latter half of the weekend and into the early part of next week.  This drop in temperature can also be seen very well on the latest ensemble graph for Manchester for example;

For at least 2 or 3 days the 850mb temperatures are at, if not below -5C.  Of interest as well is the signal for some precipitation as well in conjunction with these rather cold 850mb temperatures.  The details of the weather for the early part of next week is uncertain, but at the moment a cold and showery west or north-westerly air mass if forecast to affect the UK during Sunday, Monday and perhaps into Tuesday before stabilising from the west.

So the end result is a marked drop in temperature compared with what we have seen of late.  Maximum temperatures are likely to be no higher than 4C to 6C across northern and western areas of the UK and between 6C and 8C further south and east.  There is likely to be a return to overnight frosts as well from Saturday night and through into next week.  Wintry precipitation is possible, but given the time of year it is likely to be restricted to higher ground of northern and western areas of the UK.

So a change is on the horizon to generally colder, but also sunnier conditions as well, despite some wintry showers at times, a welcomed change for those who want to get rid of the persistent overcast conditions of late…At the moment by the middle of next week and beyond temperatures will be on the rise again, so it is expected to be nothing more than a short cold spell and there is little risk of intense or prolonged cold as predicted by some of the forecast models of late.

Regards to all,

M.

 

 

Medium to Longer Term Thoughts & Analysis – 24th Feb 12

24 Feb

Well it goes without question that February 2012 will go down as a month of extremes and producing a large variety of conditions.  The first half of the month, or so, was clearly dominated by cold conditions and temperatures dropping to between -10C and -15C quite frequently.  However, yesterday’s maximum temperature of 18.7C shows that the UK can produce some significant extremes of conditions within a matter of weeks…so what does the remainder of February and March have in store…

Well I want to start with the following image, which I produced a day or so ago which is relevant to next week;

The primary reason the  UK has been bathed in very mild air from the south-west in recent days is due to the above synoptic situation within the mid and upper levels of the atmosphere and of which, is forecast to repeat itself next week.  In the above image I have highlighted an area of high pressure not only at surface levels but also within the upper atmosphere at the 300mb level, the height of the jet stream.  Now this area of high pressure situated just to the south of the UK is the primary reason for allowing such mild and moist air to advect into the UK from the south-west.  The strong jet stream forecast on the above image for next week helps to reinforce the high pressure and aid in the advection of some very mild air from the south-west.  Last nights 120hr FAX chart ties in nicely with the above image;

Note how pressure is higher to the south and south-west of the UK, whilst low pressure is dominant to the north-west.  Also note how the surface low pressures and frontal zones align nicely to where the jet stream is forecast to be early next week.  A significant and extensive warm sector is forecast across the UK early next week in particular with once again the air mass originating from the Azores.  So in summary next week looks to be another mild or very mild week with temperatures well above the seasonal average.  With high pressure dominant just to the south of the UK in some shape or form, than many southern areas of the UK are likely to have a predominantly dry week whilst Scotland and Ireland are at greatest risk of more unsettled conditions at times.

So what does the end of February and more particularly the first half of March have in store.  Well there has, and continues to be, good model agreement and consistency for high pressure to remain a persistent feature of the weather to the south or perhaps south-east of the UK, whilst low pressure dominates to the north-west.  This can be seen on the below image as well which highlights the 500mb means in terms of where pressure is likely to be higher (orange/reds) and where pressure is likely to be lower (blues).

 

Clearly the important features here is the ridge just to the east of the UK and the associated orange colours and also the deep trough to the east of Canada and the blue colours.  What this pattern is suggesting is that many southern and eastern areas of the UK are likely to remain predominantly dry throughout the next 7 to 10 days at least and I believe this pattern will continue out to 14 days as well.  In contrast more northern and western areas of the UK are at risk of more unsettled conditions at times, but what does seem highly likely is that temperatures across the UK will remain generally above average throughout the next 7 to 14 days at least.

The ECM ENS and GFS ENS mean at 240hr highlight the expected pattern well also;

The GFS ENS mean does have pressure higher across a large portion of the UK, but the broader pattern is similar with high pressure to the south or south-east of the UK and lower pressure to the north-west and the ECM ENS chart (top image) in particular is a good representation of what I expect the weather will be generally through the rest of February and into early March.

The latest longer term data, including the latest EC 32 day is signalling a similar pressure pattern as we progress through March with higher pressure dominant to the south of the UK and low pressure dominant to the north or northwest.  As a result there is the potential for further mild or very mild conditions through the first half of March and perhaps with further predominantly dry conditions with below average rainfall across England and Wales.  Things may well change beyond mid-March to a more unsettled and cooler regime but this is a long way off as yet.

So, expect further mild or very mild conditions in the weeks ahead and also predominantly dry conditions away from northern and western areas of the UK.  The end of winter is certainly going out on a whimper and with some confidence I can say that there is little risk of any cold and wintry weather returning to the UK within the next 10 to 14 days at least.

Have a good weekend all!

Regards,

M.

Spring-like Temps On the Way!…

20 Feb

Well the phrase “In like a lion, out like a lamb” could well sum up February by the looks.  Clearly the first half of the month or the opening week or two experience a far different spell of weather than experience so far through the winter given a continental feed of cold or very cold air across the UK.  The difference between earlier in the month and the coming week will be significant!…

The latest FAX chart for Thursday helps to highlight the reason why;

The synoptic evolution through this week will be characterised by high pressure to the south or south-west of the UK and low pressure systems developing and passing to the north and north-west through the week.  The combination of these two synoptic features later in the week will be for the development of a very mild and moist south-westerly air mass from the Azores.  I highlighted the usefulness of theta-e (equivalent potential temperature) in forecasting in a previous blog, but just to re-cap theta-e is a highly useful tool to determine different air masses with greens and yellows highlighting mild/very mild/moist air masses, whilst blues and pinks signal cold/very cold/dry air masses and as can be seen on the below image, Thursday’s very mild and moist south-westerly air mass is clearly evident!…

850mb temperatures are also set to rise as well to at least 5C to 7C, if not perhaps between 8C and 10C across parts of England and Wales later in the week;

So a very mild week ahead, unseasonably so for parts of England and Wales later in the week and the very cold conditions of early February are nothing more than a distant memory.  Whilst focusing on temperatures here, it is also of worth to highlight that some particularly wet and windy conditions will effect parts of the north and west of the UK as well through the week as a series of frontal systems and low pressures affect these areas.  So it’s not all plain-sailing and despite a likely spring-like feel in any brightness across central and southern areas of England by Thursday Scotland, Ireland and Northern England in particular look like having a very wet and windy mid-week period.

Quick thoughts on next weekend and looks cooler and generally more settled, at least for a time, but there’s uncertainty over specific synoptic features and developments at this stage…

Have a good week all!

M.

Cold Weekend Ahead – Thoughts & Analysis (18-19th Feb 12)

14 Feb

Clearly the start of this current week has seen a change in the broader scale weather patterns than previous weeks and despite the winds being north-westerly this has resulted in milder conditions than of late as the origin of the north-westerly wind hasn’t been from a cold source.  However, there continues to be growing signs for a change to colder conditions as an air mass with more of a polar origin develops over the UK towards the coming weekend and potentially persists into the early part of next week.

Last nights FAX chart at 120hrs covers Saturday and this, at the moment, highlights the expected synoptic pattern quite well;

The development of a minor ridge of high pressure to the west of the UK is currently forecast to combine with low pressure to the north or north-east of the UK to produce an increasingly cold north-westerly air mass and this may well becoming northerly across parts of the north in particular.  Notice on the above image that if you follow the isobars back that the winds and air mass originate from near Svalbard or just to the East of Greenland, clearly a cold source!.

The cold front on Saturday will be the primary feature that signals the introduction of colder air from the north-west.  The timing of this feature is open to question at the moment, but it would seem that process will take place during Saturday itself.  One very useful variable that meteorologists use to determine different air masses and also where fronts will be is the variable theta-e  or equivalent potential temperature at the 850mb level.  In a nutshell this variable highlights regions of differing air masses by colour with blues and pinks highlight cold or very cold conditions and greens, yellows and oranges warmer.  The latest 06Z GFS Det highlights the cold air behind the southward moving cold front very well during Saturday;

Again at the moment it would seem that the cold front will move down across the UK between 06Z and 18Z on Saturday with an increasingly cold air mass following on behind.  Also of note is the predicted drop in 850mb temperatures which can be highly useful to gauge the risk of wintry precipitation, again note how 850mb temperatures do drop below -5C across most areas of the UK towards the end of Saturday;

By the latter stages of Saturday evening and overnight 850mb temperatures across Northern England, Northern Ireland and Scotland are ranging between -7C and -10C which is a clear signal that any precipitation will be wintry and down to quite low levels as well.  Now the wind direction favours Scotland, Ireland, Northern England and perhaps parts of Wales with regards to the regions at greatest risk of showers.  At this time frame it is very difficult to say how many showers or how much precipitation is likely to develop within the north-westerly flow.  However, given the current conditions which are expected then some sleet and snow showers seems likely and where these are most frequent and heaviest overnight into Sunday then some snow accumulations are possible.  At the moment I would say parts of Scotland, Northern Ireland and Northern England are at greatest risk of some accumulating snowfall, but again this is subject to change given further model runs and guidance nearer to the time in question.

Interestingly, the 06Z GFS Det does highlight the above scenario well and using the predicted ‘weather type’ variable you can see that the areas I mentioned above do match the forecast graphic at the moment;

Weather type GFS Sun 19.02.2012 12 GMT

The other issue which may well limit precipitation or at least signal the reduction in showers is if the ridge of high pressure to the west of the UK builds into parts of the UK more quickly by Sunday for example, than any showers could well be killed off completely or at least reduced in intensity or frequency.  So the longer any ridge to the west stays to the west the better.  Just finally after a likely week without any frosts or icy patches, the latest guidance is for a widespread moderate frost on Saturday night and also perhaps on Sunday as well into Monday morning so don’t put the de-ice away just yet.

So in summary a change to colder weather looks like over the weekend and some areas exposed to the north-westerly air mass will likely experience some wintry showers with sleet and snow possible and perhaps with some accumulations as well in places.  This setup does favor more northern and western areas  of the UK and I expect many southern and eastern areas of England will experience nothing more than a cooling trend with a much lower risk of any significant wintry precipitation.

Regards to all,

M.

 

 

 

Medium & Longer Term Outlook (Mid-Late Feb 12)

10 Feb

It goes without question that the weather over the last 2 weeks or more across the UK have been markedly different from earlier in the winter.  Without diverging away from this main topic, the primary reason being has been changes within the stratosphere which were acknowledged at the end of December and into early January.  Gone has the prolonged +NAO pattern with low-after-low steaming into the UK and clearly we have been experiencing a far colder and more blocked pattern.  As an example the following chart highlights the pattern in the last week or more;

High pressure has been dominant to the north-east of the UK and we have been experiencing a persistent feed of cold air from the near continent which clearly has been combining with low pressure and frontal systems from the west to bring a far more wintry spell of weather.  So what does the remainder of February have in store?…

The outlook for the remainder of February is now more changeable.  The least ECMWF 32 day forecast is signalling a far more progressive and changeable pattern from week to week through the rest of February and into early March.  This model has been excellent all winter and has picked up on patterns a good number of weeks in advance.  Unfortunately what the changeable signal does bring however is uncertainty, specifically uncertainty over the details of the synoptic evolution as each week ticks by.  What can be said however is that more unsettled conditions are predicted by the ECMWF 32 day model, but without returning to a ‘proper’ zonal Atlantic.

We can see the variation in conditions during the course of next week and into next weekend on these 2 charts which are the GFS ENS mean;

Again despite uncertainties over the details the weather is about to change.  The significant block to the east and north-east of the UK is on its way out and we now turn our attention more towards to the west and north-west during the second half of February.  Next week is likely to deliver a temporary northerly air mass during Tuesday and Wednesday but as can be seen on the first chart the block to the west isn’t stable enough to support any prolonged cold weather.  This is then likely to collapse down into the UK before a potential ‘re-load’ scenario develops by next weekend.

In fact there is model agreement for next weekend (18th-19th) to be potentially quite snowy for some given an unsettled north-westerly air mass which is forecast to be quite cold, take the 850mb temperature ensemble mean from the GFS ENS as well;

Towards the latter half of next weekend (at the moment) the 850mb temperatures are at or below -5C across a large portion of the UK and as ever this is often a reasonable indicator for wintry precipitation.  So a changeable and more mobile weather pattern is forecast to develop during next week and into the following weekend with some rain, showers and also sleet and snow in places as cold north-westerly winds develop.  This is backed up by the ECM ENS mean as well;

The signal for this changeable pattern to continue then follows into the final full week of February with again the majority of the weather coming from a westerly quadrant.  Low pressure is likely to be a feature to the north of the UK in some shape or form as high pressure persists to the west or south-west.  At the moment there is little sign for any significant height rises across Greenland and I do believe and suggest that any significant northern blocking within this area is unlikely to happen during the remainder of February, despite perhaps temporary ridges of high pressure to the west and north-west.

So in summary the middle and latter half of February is set to see a change, a change away from the continental blocked pattern to a far more changeable setup.  Some milder days are forecast, particularly compared with of late, but equally there is still a clear sign for colder weather as well from potential north-westerly or northerly winds at times which may well bring further wintry precipitation, particularly seeing that it is likely to become more unsettled with it. 

Have a good weekend all.

Regards,

M.

Freezing Rain v Snow 9th Feb 12

10 Feb

Thursday 9th of February produced quite a varied amount of precipitation types from good’ol rain across Scotland and Ireland to a wintry mix of freezing rain, sleet and snow across parts of England and Wales.  One distinct variation was the difference in precipitation type across Northern England and generally over relatively short distances.

North-west England in particular, including Lancashire and Cumbria for example picked up a large amount of freezing rain which in some places lasted all day.  Yet just over the Pennines and the afternoon produced quite a different setup with snow falling across many parts of Yorkshire, so why was there such a difference and why didn’t North-west England get snow?…

…the answer lies up in the atmosphere.  Before I advanced further on this, for those that have not really heard of freezing rain are unsure how it forms and develops then I recommend taking a look at this video from the BBC’s Nick Miller which explains what freezing rain is…

http://www.bbc.co.uk/weather/feeds/16956208

In association with this explanation the following is what is called an atmospheric sounding.  It allows meteorologists to view what is ‘going on’ within the upper atmosphere.  Now what is important here is how the temperature actually rises with height.  Note that at the surface level the temperature (red line) is at or even below freezing, yet if you following that line up the temperature rises (the line progresses to the right).  Precipitation was thus falling through this warmer layer, melting but then not having enough time to re-freeze before it reached surface levels which were at or below freezing and hence causing the freezing rain across North-west England where this atmospheric sounding is from yesterday afternoon.

Now across Yorkshire the atmosphere was different.  This rise in temperature wasn’t evident or not to the extent that was present across Lancashire and Cumbria for example.  As a result when yesterday’s frontal systems reached Yorkshire the precipitation through the day was primarily of snow, particularly during the afternoon simply because of the lack of this rise in temperature.  The precipitation was able to fall through the atmosphere without melting and thus at surface levels was snow.

The above atmospheric sounding is a text-book example of a freezing rain ‘environment’, something which doesn’t happen often in the UK, particularly on the scale of yesterday and lasting for a large portion of the day as well. So if you ever see freezing rain being mentioned again within the forecast think of surface temperatures at or below 0C, but as one rises up through the atmosphere the temperature actually rises.

Regards to all,

M.

Mid-month weather – What should we expect?

7 Feb

It goes without question that over the last week or two forecasting the weather has been both nail biting and also highly frustrating in association with an uncertainty over the specific synoptic evolutions.  With continued changes on a daily basis within the forecast models it made even the relatively short-term forecasts difficult, let alone attempting to get the longer term trends.

Once again we are now looking at a similar trend with regards to constant model changes and divergences within a relatively short time period over the weather at just 4 or 5 days ahead.  Over the last few days there has been a trend for the potential for a mid-Atlantic or even Greenland block of high pressure.  This would bring the risk of further colder conditions as we progress towards the mid-month period.  However, almost the opposite could be said at times with the models the next day showing a more unsettled and Atlantic dominated period of weather which would bring relatively mild conditions, particularly compared with present conditions…so what is the likely outcome?…

Today’s forecast models are interesting, take the UKMO and the ECM for example;

Both these models are attempting or actually do develop a ridge within the Central-North Atlantic and up towards Greenland.  There is also some modest support from the GFS as well, but in a more confused and uncertain state;

Could we be looking at the start of a trend?…This is a distinct possibility but as I highlighted at the start of this blog the models have been subject to constant and often daily changes particularly within the medium to long-term.  When it comes to forecasting the medium and long-term we need to look for forecast model agreement, which there is today as I have highlighted above, but we also need model consistency.  Consistency over a specific synoptic evolution between the various forecast models is highly important to come to a conclusion as to the expected weather within the medium to long-term. Clearly we don’t have any consistency as such with regards to this blocked pattern which would allow the risk of colder weather from the North or North-west.

The latest update of the ECM 32 day model does continue to signal a blocked pattern to the North-west of the UK as well, but in a generally less significant state than previous updates, but this signal has been evident within this model for a good 2 weeks at least.  There have also been signs and signals from other sources that as February progresses the general progression of the synoptic pattern would be for blocking to take place to the North-west of the UK.

The 64 million dollar question is; will this block actually develop and allow for further cold weather?…

…Well with relatively low confidence, given that lack of model consistency, but I do expect blocking to the North-west of the UK in some shape or form towards mid-February.  Whether this unleashes a very cold North or North-westerly air mass across the UK is unknown, the block could simply not be sustained enough to allow this to happen as well.  But in summary I do believe there is a realistic probability that the above charts will become reality and what we need now in the coming days is model consistency over this synoptic evolution.

As ever in meteorology…time will tell.

Regards to all,

M.