Archives for July 2012

Jet stream set to spoil summer again

Paul Hudson | 16:32 UK time, Monday, 23 July 2012

After a fine and warm weekend across much of the country, temperatures today have reached 80 degrees Fahrenheit in Bridlington in East Yorkshire for the first time this summer - although similar values were reached in places for a time during late May.

It's certainly come as a welcome relief after a what's been a dreadful summer so far, which, according to Philip Eden writing for weather online, has been the worst first half of summer on record (summer for climatological reasons is June, July and August).

But in typical British fashion it looks like the summery conditions will prove to be short lived, with much of the country by the end of the week again under the influence of low pressure, bringing cooler and unsettled conditions, as the jet stream once more heads south.

For our region, wednesday will be the transition day, with more cloud in general and possibly the odd light shower. After that the more unsettled conditions will slowly take hold from the north.

And with the jet stream likely to remain further south than normal as we head into early August, unsettled & cool weather will once again dominate our weather.

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What's causing our extreme weather? Is it unprecedented?

Paul Hudson | 16:19 UK time, Wednesday, 11 July 2012

The awful weather which continues to affect the UK led yesterday to the Great Yorkshire show being cancelled (because of the weather) for the first time in its 154 year history.

And on Monday, serious flash flooding devastated parts of the Calder valley for the second time in as many weeks.

In fact as we all know our weather, with a notable exception during the second half of May, has been dreadful since the end of March.

And research conducted in part by scientists at the University of Sheffield has concluded that declining Arctic ice could be to blame.

The Arctic has been the fastest warming area in the world and this has reduced, according to the research, the temperature contrast between the Arctic and the Tropics.

And it's this temperature contrast which determines the strength and position of the jet stream - and could be responsible for the weak, slow moving jet stream which has caused the long period of cool, unsettled weather.

If the research is correct, it suggests that the swings we have experienced in the last few years could become the norm, although it doesn't suggest we would always have poor summers.

We could also become 'stuck' on the warm side of the jet stream in summer, leading to heat wave conditions like parts of America has been experiencing.

But there are some climate scientists who say that the weak solar activity that we have observed in recent years is altering the position of the jet stream.

One of the problems with this theory is that no-one understand the precise mechanism as to how solar activity could lead to changes in the position of the jet stream - and until we do, it can't be described mathematically, and so can't be modelled by computers.

Looking back, the early 1800s was a period of unusually weak solar activity, so much so that it was named after British meteorologist John Dalton, the so called 'Dalton solar minima'.

And data shows that from 1809-1819, after what was described as a 'relatively benign period with several warm summers and less cold winters', the period saw a return to 'often harsh winters, and cold, wet summers'.

The decade from 1810-1819 was in fact the coldest since the 1690's.

One complicating factor to this is that in 1815, a huge volcanic eruption in Indonesia depressed temperature levels because of the amount of volcanic ash in the atmosphere, and contributed to the well documented 'year without a summer' in 1816.

But this example shows that the weather patterns we have experienced in the last few years are not unprecedented.

And even from a specific extreme rainfall perspective, as high as local rainfall figures have been this summer, none have been anywhere near the levels reached in Sheffield on 15th July 1973 when 119mm fell in just one day (and which incidentally led to severe flooding despite much less development on the flood plain, a subject for another day perhaps).

Whether you favour the theory of melting Arctic ice and the link to global warming, or weak solar activity, both suggest that more extremes are likely in the future.

And of course it's possible that both could be working together at the same time.

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Parts of UK at risk of renewed flooding

Paul Hudson | 13:54 UK time, Thursday, 5 July 2012

UPDATE at 6.30pm

Lastest information from American GFS model shifts emphasis for heavy rain towards the earlier ECMWF solution detailed below, i.e with much of Yorkshire and Lincolnshire at risk of some of the highest rainfall totals.

ENDS

Amber alerts are in force for parts of the UK tomorrow, with heavy prolonged rain likely to lead to renewed flooding in places.

This is on top of possible localised flooding from scattered thunderstorms today, and a risk of flooding on saturday across parts of Northern Britain.

Computer models have in the last few days consistently developed an area of low pressure over the near continent tonight, bringing the centre across East Anglia and the Midlands tomorrow.

This feature will be very active, driven by high humidity air, leading to prolonged heavy, thundery rain.

The precise northern and southern boundary of this area of rain is open to some doubt at the moment.

The chart below, from the UK Met Office, gives an indication of the highest risk area (yellow) of the heaviest rain (50mm or more in total on Friday), with East Anglia, the Midlands and Wales in the firing line.



The area fringes into parts of Yorkshire and Lincolnshire too. This is considered to be the most likely outcome at the moment.

But the European Centre for Medium Range Forecasting (ECMWF) model, shown below, has the risk of disruptive rainfall further north (Red), with parts of Yorkshire and the Humber at highest risk.


The European model arguably offers the most consistent and reliable guidance for weather forecasters, and gives an insight into the on-going uncertainty regarding the positioning of tomorrow's heavy rain.

Whichever is correct, with much of the land now saturated, another month's worth of rain in less than 24 hours tomorrow will inevitably lead to renewed flooding across parts of the UK.

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Poor summer weather to continue well into July

Paul Hudson | 14:42 UK time, Monday, 2 July 2012

UPDATE at 5PM

According to the Met Office, averaged across the UK, June 2012 has been the wettest since records began in 1910, the coolest since 1991, and the second dullest since records began (record for lowest sunshine in June is still 1987).

ENDS

It will come as no surprise that June in Yorkshire and Lincolnshire has turned out to be one of the wettest on record - in fact second only to the incredibly wet June of 2007 which saw some of the worst widespread summer flooding that the region has ever seen, with Sheffield and Hull hit particularly hard.

Bingley, in the Pennine hills of West Yorkshire has recorded 212.8mm this month, compared with the average which is 70mm. But this is quite a bit short of the 283mm which was recorded in June 2007.

Sheffield is also well short of what was recorded in 2007, when 286mm (463% of average) fell; not only was June 2007 the wettest June on record in the city, but their wettest month ever in 125 years of records.

It's also been the second wettest June in Lincolnshire. Coningsby recorded 141mm in the month, compared with their average of 50mm.

The reason for the on-going poor weather has been the unusual positioning of the jet stream, which continues to be too far south than normal.

This point is graphically illustrated by the fact that June has been in the top 3 most cyclonic (low pressure) Junes in 140 years of records.

And there's still a chance that when official Met Office statistics are published later today or tomorrow, that averaged across the UK as a whole, June 2012 has been the wettest on record.

It's also been cold - with June the coolest since 1991 - and one of the dullest on record too.

Of course June has just seen a continuation of the cool and wet weather which began at the end of March.

At Linton-on-Ouse in North Yorkshire, for example, April, May & June combined have been the wettest on record (data back to 1939), with the wet spell only punctuated by one spell of fine warm weather towards the end of May.

And for those of us desperate for a change to more settled weather, there is little to suggest an improvement anytime soon.

Current indications are that low pressure will dominate our weather until at least the middle of July, if not beyond.

This means that one or two fine days are possible, in an overall unsettled and at times wet picture.

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