Archives for December 2012

How the Arctic may be impacting UK summers

Paul Hudson | 12:48 UK time, Monday, 17 December 2012

We may have to get used to wet summers like we've seen recently across the UK, according Dr Edward Hanna from Sheffield University in an interview which you can see on Inside Out and Look North tonight.

According to Dr Hanna and an international team of scientists, melting summer Arctic ice may be weakening the jet stream, leading it to meander and become slow moving.

This effectively means that weather patterns become locked in for long periods of time.

The jet stream is a ribbon of strong winds high up in the atmosphere, a result of the temperature contrast between northern latitudes towards the Arctic, and latitudes further south.

Because the Arctic is warming faster than any other region on earth, this temperature contrast is getting weaker, leading to a less powerful jet stream in summer.

Crucial to the UK and Northwest Europe is Greenland, a huge mountanous land-mass which can act as a barrier to the jet stream.

If the jet stream is weaker than normal, two things can happen.

It can either split, with one arm going northeastwards, with the other travelling southeastwards towards the UK.

Or the whole jet stream can be deflected southeastwards towards the UK.

The result in both cases would be wet, cool, unsettled conditions as we have seen since 2007.

Not every summer is likely to be poor.

The slow-moving jet stream may become positioned to the north of us, leading to warm settled conditions.

But because of our position relative to Greenland, these summers are likely to be the exception to the rule.

Dr Hanna says if this theory is correct and summer Arctic ice melt continues, there is also likely to be a higher risk of extreme rainfall events such as we have experienced in 2007 and again this year.

The research, which was carried out jointly by experts from NOAA (National Oceanic and Atmospheric Administration), Rutgers University, University of Washington, and the University of Sheffield, was published in the journal Geophysical Research Letters.

You can see more on this on BBC Look North from Leeds, on BBC1 at 6.30pm, (Sky channel 956, Freesat 966) or on BBC1's Inside Out at 7.30pm (Sky 956 & 957, Freesat 966 & 967).

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The Beast from the East is slain

Paul Hudson | 13:40 UK time, Thursday, 13 December 2012

The cold easterly which a large majority of computer models were predicting to develop this week has failed to materialise, with a much more unsettled weather pattern expected to return from the Atlantic during tomorrow.

Last week, 80% of the ECMWF model solutions wanted an easterly 'blocking' weather pattern, with the average of those solutions shown below.



Compare that with the atmosphere this morning (according to the GFS model), below.


Crucially the centre of gravity of the large area of high pressure, which should have been closer to Scandinavia, is further northeast than predicted.

This positional error means that Atlantic weather systems will now be able to make further progress eastwards across the UK.

It illustrates very well just how difficult it is sometimes to forecast general weather conditions a week ahead, even when there's high model confidence.

So after a temporary cold and dry spell, it now seems likely that the rest of December will be very unsettled, with showers or longer spells of rain, some of which will be heavy, with only very brief incursions of colder air.

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The Beast from the East is lurking

Paul Hudson | 13:54 UK time, Wednesday, 5 December 2012

There is now growing consensus between most weather computer models that cold air from the east is likely to spread across Britain next week.

If so, it will be the first time since March that high pressure has properly dominated our weather, and will end a long sequence of at times record breaking wet weather.

And it looks to be a classic winter-time set up, with a powerful anticyclone developing across Scandinavia and into western Russia, pulling in cold easterly winds across a large part of the country, hence the old saying 'the beast from the east'.

The diagram below is what's known as an 'ensemble mean' from the ECMWF model for the middle of next week.



The computer program is run 51 times, each time with slightly different starting conditions.

The solutions are then compared, and give forecasters an indication as to how likely a particular outcome is.

From the midnight run of the computer, 41 out of 51 of the solutions suggest an easterly weather pattern developing next week, with varying degrees of cold.

10 solutions do not agree with this cold easterly outcome, hence there is still some uncertainty. But, there's clearly a large majority in favour of this scenario at the moment.

What is much less certain is how much snow is likely to be associated with this change in the weather.

Quite often in these situations, there's a distinct lack of precipitation apart from wintry flurries which can develop as the air picks up moisture as it heads westwards across the North Sea.

But some solutions are suggesting 'disturbances' in the easterly flow, which would bring the risk of more general snowfall.

And of course there's always the risk of milder air trying to re-assert itself from from the west, which would also bring the risk of snow.

At the moment though, the cold but relatively dry scenario is the most likely outcome.

One way or the other, our weather is likely to become more seasonal in the lead up to Christmas.

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2012 on course to be in wettest top 10

Paul Hudson | 15:21 UK time, Tuesday, 4 December 2012

Only two thirds of average rainfall needs to fall across the UK in December for 2012 to end up in the top ten wettest years on record, according to Met Office data which was first collated over a hundred years ago in 1910.

Should this be the case, it would also mean that nearly half of the years since 1998 would be in the UK's top ten wettest years on record.

2000 (wettest), 2008, 2002, 1999, 1998, and if rainfall is sufficient, 2012, would all make it into the top 10.

1923, 1927 and 1928 are also in the top 10, illustrating that wet years do come in clusters, but the 1920's sequence is nothing like what we have experienced in recent years.

Such a rainfall sequence suggests that over and above any cyclical change to weather patterns that are naturally occurring, other factors are likely to be at work, fuelling suspicions that climate change is playing its part.

November was another wet month.

Across England and Wales, rainfall was 128 per cent of the 1981-2010 average, making it the 8th successive month with above average rainfall.

And in the last 100 years only 20 Novembers had more rainfall, despite the fact it was only the wettest since 2009.

The continued positioning of the jet stream further south than normal is responsible for the very wet weather.

Current computer projections suggest that December is unlikely to be another washout month dominated by the Atlantic.

Although more rain (or snow) is expected at times, high pressure is likely to exert much more of an influence than in recent months, leading overall to colder but somewhat drier conditions to develop.

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