The worst storm in years?

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

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

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?

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.

Is this stormy weather unusual for June?

We’ve seen some particularly stormy weather across parts of the UK in the last 24 hours as heavy rain and strong winds have brought disruption particularly across the south of the UK.

The strongest winds were recorded in the early hours of Friday morning where the Needles on the Isle of Wight recorded gusts of 82 mph. Otherwise the strongest winds were across South West England with gusts around 60 to 70 mph.

UK MAX GUST SPEED 7TH TO 8TH JUNE 2012
DATE/TIME	SITE NAME	MAX GUST SPEED MILES PER HOUR
08/06/2012 05:00	WIGHT: NEEDLES OLDBATTERY	82
07/06/2012 21:00	MUMBLES HEAD	67
07/06/2012 16:00	PLYMOUTH, MOUNTBATTEN	62
07/06/2012 15:00	CULDROSE	60
07/06/2012 16:00	SCILLY: ST MARYS AIRPORT	58
08/06/2012 06:00	ISLE OFPORTLAND	58
08/06/2012 07:00	AVONMOUTH	58

UK RAINFALL 7TH TO 8TH JUNE 2012
SITE NAME	PRECIP        AMOUNT ( MM)
TREDEGAR, BRYN BACH PARK	53.4
USK	49.4
DUNKESWELL AERODROME	45.6
OKEHAMPTON	42.4
LISCOMBE	42.0
CARDIFF,BUTEPARK	38.6
SENNYBRIDGE	38.4
MUMBLES HEAD	37.6
GLENANNE NO 2	35.4
KILLOWEN	34.6

Whilst this type of weather may seem unseasonable for early summer you only have to take a look through the Met Office observations records to see that stormy weather in the summer is not all that unusual.

The most comparable storm we have seen recently was only in 2010. The 15^th July saw a complex area of low pressure over the UK bring wet and windy weather across South Wales and South West England. The highest gusts recorded were 84 mph at Aberdaron in North Wales. Elsewhere, winds gusted to between 60 and 70 mph widely across the south west of the UK.

Before this, the 23rd June 2004 saw another area of low pressure bring stormy weather across the UK. The strength of the winds during this storm were very similar once again with the strongest winds being seen at the Needles on the Isle of Wight where gusts of 84 mph, whilst gusts of 60 to 70 mph were recorded across South West England and South Wales.

You can get the latest weather forecast and weather warnings on the Met Office website

Related articles

• Gale-Force Gusts And Flooding Set To Batter UK (news.sky.com)
• Strong winds and rain batter UK (thisislondon.co.uk)
• Strong winds and rain batter UK (express.co.uk)
• Strong winds and rain batter UK (thisislondon.co.uk)
• You: Torrential rain and winds batter UK (guardian.co.uk)
• 70mph winds set to batter Britain (thisislondon.co.uk)
• Forecasters warn of 65mph gales (thisislondon.co.uk)
• Wales News: Summer on hold as Wales braced for strong winds and heavy rain (walesonline.co.uk)
• 70mph winds and heavy rain set to batter Britain (itv.com)
• Gales poised to batter South West (thisislondon.co.uk)

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

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.