Our change in the weather and how the jet stream is driving it

13 12 2013

After a quiet spell of weather courtesy of a slow moving area of high pressure, we are now entering an unsettled period as a series of Atlantic depressions are expected to pass close to the northwest of Britain during the next week.

High pressure has now moved away and is settled over Europe and a powerful jet stream is developing over the Atlantic which will be the main driving force behind this spell of unsettled weather.

What is the jet stream?

The jet stream is a band of fast moving westerly winds high up in the atmosphere which circle around the pole in the northern hemisphere. It can feature winds of up to 200 knots (230 mph) or more, and these winds tend to guide wet and windy weather systems which come in off the Atlantic.

The jet moves around a fair bit and its position can have a big impact on weather here in the UK depending on where it is.

If the jet is over the UK or just to the south, we tend to get a lot of wet and windy conditions as it brings weather systems straight to us. If the jet is to the north of us, it guides that changeable weather away to the north to leave the UK with more settled conditions.

What’s the jet stream doing now?

Unsurprisingly given the outlook for the next week, with a succession of Atlantic depressions passing by to the northwest of Scotland, the jet is positioned to the northwest of the UK too.

As you can see from the picture below, the jet currently swoops east from Canada – swinging northeast over the Atlantic towards the UK.

Forecast position of jet stream at midday Saturday 14 December 2013

Forecast position of jet stream at midday Saturday 14 December 2013

Closer to the ground very cold air is also streaming south from Canada and meeting warm air moving north from the Caribbean. It is where these two air masses meet under the jet stream that powerful Atlantic depressions form and are blown across the ocean towards our shores.

It is these depressions that bring a significant risk of severe gales and heavy rain affecting at least the northwest of the UK at times.

What’s the weather outlook?

Currently, Met Office National Severe Weather Warnings have been issued for wind across some northwestern and northern areas for the weekend. Gusts of 60-70 mph are likely with a risk of gusts to 80 mph or more across exposed parts of northwest Scotland.

However, at this stage there remains uncertainty regarding the extent of the strongest winds and these warnings will be updated as the weather develops over the weekend.

Looking ahead, while we expect further depressions to develop it is not possible to say exactly how vigorous they may be or pinpoint where they will be in a week’s time. This means it is too early to say which areas will experience the strongest winds and heaviest rain, however there are indications that  areas further to the south of the UK may be affected at times.

You can stay up to date with what to expect with our detailed forecasts out to 5-days and our weather warnings, as well as a general view of what we expect out to 30 days and find out what to do in severe weather

You can find out more about the jet stream in our YouTube video.

 





The worst storm in years?

28 01 2013

Various articles in the news today said that the weather over the weekend was the worst storm to hit the UK in years, and that there is more to come this week. There was indeed a very deep area of low pressure in the Atlantic over the weekend. At its deepest, on Saturday 26 January, the central pressure of the depression was 932 millibars and it was sitting some 1,800 nautical miles west of the UK. It came closest to the UK during the day yesterday with a central pressure of 950 millibars but was still around 600 nautical miles to the north west of Scotland.

Satellite image from 26 January 2013

Satellite image from 26 January 2013

To put this into context, the storm that affected the UK on 3 January 2012 had a central pressure of 953 millibars but was centred right on the west coast of Scotland and brought winds in excess of 80 mph to the Central Belt and a gust of over 100 mph in Edinburgh. Property was damaged, as well as trees, and there was disruption on the road network and with ferry crossings. Power supplies were also affected significantly.

The storm in January 2012 was therefore much more disruptive and severe than any wet and windy weather we have seen so far this year.

Much of the recent severe weather has been attributed to the phrase “Weather Bomb”, which is not a perfect meteorological term but is defined as an intense low pressure system with a central pressure that falls 24 millibars in a 24-hour period. This happened to the depression over the Atlantic during the weekend but as it was miles away from the UK its impacts were minimal. A better description can be more directly linked to the meteorological phenomena known as rapid cyclogenesis. This is where dry air from the stratosphere flows into an area of low pressure. This causes air within the depression to rise very quickly and increases its rotation, which in turn deepens the pressure and creates a more vigorous storm.





Storm caused by most intense low to cross UK in September in 30 years

26 09 2012

The low pressure system that has brought heavy rain, strong winds and flooding to the UK is the most intense to cross the UK in September for more than 30 years, with the lowest air pressure of 973mb being recorded on Tuesday morning.

Pressure chart at 6am on 25 September 2012

To find a similarly intense low pressure system that affected a wide part of the UK in September you need to go back to 1981, when pressures below 970mb were reported across central parts of the UK.

Like this week, this low pressure system brought unsettled weather as it crossed the British Isles – tracking east over the Isle of Man before heading north to Cumbria, Northumberland, eastern Scotland, Orkney and Shetland.

But what do we mean by ‘the most intense’? The intensity of a low pressure system is measured as the lowest pressure recorded at the centre of the system, as this gives an indication of how active it may be. This will relate to the rainfall amounts and wind strengths associated with it.

However, pressure is only one indicator of how much wind and rain there will be, so it is possible that other systems have resulted in stronger winds or heavier rain in some places than we have seen over the last few days.

Although the storm we have seen this week is certainly unusual in that it crossed central parts of the UK, some parts of the UK have seen pressure systems of this kind of intensity many times before at this time of year. In fact, Met Office records show some 31 occurrences of pressure below 975mb being observed in the UK in September, but the vast majority of these were confined to north and west Scotland, Northern Ireland or the far west of England.

For example a deep low affected the northwest of Scotland with pressure as low as 972mb as recently as 12 September 2011, whilst the Isles of Scilly and part of Cornwall saw pressure as low as 966mb on 7 September 1995. So, with regard to the system which has recently affected the UK, the key to what makes it remarkable is that it has tracked over a wide area of the UK rather than those areas which are more used to storms of this intensity.





What’s bringing the stormy weather to the UK?

24 09 2012

The UK has seen some very wet and windy weather since the early hours of Sunday morning and that is set to continue in places for the next couple of days – but what has brought these disruptive conditions?

As is the norm, a low pressure which moved in from the Atlantic is to blame, bringing bands of heavy rain and strong winds (as you can see from the tightly packed isobars on the image below).

Forecast synoptic chart for 12:00 on Tuesday 25 September showing the low pressure over the UK.

Despite some reports to the contrary, this low is not what’s left of tropical storm Nadine, but is a completely separate entity – the remnants of Nadine are currently sitting close to the Azores far to the south of the UK.

Some warm tropical air dragged over by Nadine was sucked up into the low pressure, however, giving it some extra energy – essentially increasing its potential for strong winds and rain.

This isn’t unusual though, virtually every weather system we see will have had some input of sub-tropical air during its evolution.

There are two more notable features of this low pressure, however. Firstly, it has remained unusually active as it sits over the UK, leading to the strong winds and heavy rain.

This is due to the fact that, as the low pressure system moved north across the UK, it has also pulled in cooler polar air from the north.  This cold air has come up against the warm sub-tropical air, re-invigorating the depression and allowing it to continue to deepen over the UK.

Secondly this low pressure is lingering for longer than we would often see. The reason for this is down to the position of the jet stream, a narrow band of fast moving winds high up in the atmosphere which ‘steers’ weather systems.

Normally the jet stream runs fairly directly from east to west and pushes weather systems through quite quickly. Similar to earlier this year, the steering flow of the jet stream is currently in a meandering mood – looking much like a river, curving north and south as it heads west across the Atlantic (we call this a meridional flow, with the more linear west to east flow being called a zonal flow).

When it meanders, weather systems can get stuck in the ‘peaks and troughs’ it creates – so they get stalled in one spot rather than moving on. The below picture of the jet stream as at 12:00 today shows with the steering flow of the jet over France and the UK in the resulting trough.

The weather system will move on during the day on Wednesday, but that still means the UK will have had three days of unsettled weather.

Like our weather, the jet stream can change rapidly and it’s difficult to forecast precisely what it will do for more than a few days ahead – so there’s no reason to expect it to continue to behave in this way and there’s plenty still to play for in terms of our autumn weather.

The low pressure system that is affecting the UK is unusually deep for September, with the lowest air pressure recorded so far being 973mbs. To find a similarly intense low pressure system in September you need to go back to 1981, when pressures below 970mb were reported over a period of 24 hours. Like this week’s, this low pressure system brought unsettled weather as it crossed the British Isles – starting in the Isle of Man and tracking east and then north to cover Cumbria, Northumberland, eastern Scotland, Orkney and Shetland. There have been other times when pressures as low as 970mb were recorded in some parts of the British Isles in September, such as in the Isles of Scilly in 1995 and others across the far north or west of Scotland or Northern Ireland, however none were as widespread as the low that pushed across the UK in 1981.





A challenging forecast for the weekend

20 09 2012

This weekend’s weather forecast is proving more challenging than usual as we see signs of much more unsettled conditions developing for all parts of the UK over the next few days.

Much of the UK is set to have a fine and dry weekend with sunny spells, light winds and temperatures in the mid to high teens Celsius after some cold nights.

There is now increasing confidence that southern parts of the UK (roughly south of a line from south Wales to Ipswich) will see wet and windy weather on Sunday.

This wet and windy weather is not the remnants of tropical storm Nadine – this stays close to the Azores. However, we are expecting a new area of low pressure to develop to the west of Iberia on Saturday which will move northeast, pulling some warm air from Nadine with it. It is this that would bring wet and windy weather to the far south of England and Wales for Sunday and other parts of the UK next week.

The challenge for forecasters is to pinpoint how this low pressure area will move. The weather forecast models available to Met Office forecasters are giving slightly different answers to this problem. As Anthony Astbury, Met Office Deputy Chief Forecaster, explains: “One model brings the low over Brittany giving rain and gales in the south of England, while another brings the low further west, with the risk of wet and windy weather for southwestern England and south Wales.”

There is still uncertainty about how the low pressure area will develop and move on Monday, and therefore which areas of the UK will see the worst of the weather early next week.

However, this heralds a spell of very unsettled weather for the whole of the UK for next week, with all parts seeing unsettled and windy conditions with showers or longer spells of rain.

Keep up to date with the forecast and warnings for the latest information.





What’s behind the stormy weather so far this winter?

3 01 2012

So far this winter the UK has seen some very strong winds associated with a series of Atlantic storms. This has included some near-record strength gusts of wind in places, with Scotland particularly badly affected.

On 8 December a gust of 165mph was recorded at Cairngorm Summit (1245m above sea level) during the first of the winter storms – that gust was just 8mph off the strongest ever recorded in the UK (set at the same spot in 1986). Even at low level there were strong winds, with 105mph at Tulloch Bridge in Highland.

Another powerful storm affected Scotland on 28 December, and today we saw another – this time notching up winds of around 100mph in parts of Scotland. The 102mph gust at Blackford Hill in Edinburgh is the third highest recorded at that station, and the strongest since 1998.

So has this winter been particularly stormy? We actually expect stormy weather in the UK at this time of year. In normal circumstances, low pressure systems develop over the Atlantic and track from west to east – eventually moving over the UK bringing strong winds and often heavy rain with them. This year we’ve seen nothing to disrupt that westerly flow, leading to the succession of low pressures systems we’ve seen so far.

While the general weather pattern is what we expect to see at this time of year, the strength of the storms and winds has been unusual. This is down to the jet stream – the high altitude winds which blow from west to east across the Atlantic and brings us our traditional changeable weather.

Over the past few weeks the jet stream has occasionally been particularly strong and some of the low pressure systems have interacted with that as they have tracked over the Atlantic, boosting their strength. This has led some of the lows to develop into powerful storms which have brought very strong winds to the UK.

The weather is set to remain changeable for the rest of this week, and beyond that it’s not possible to say whether we will see any more particularly strong storms this winter. However, we’ll continue to keep everyone up to date with the latest outlook in our forecasts and warnings.








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