Britain faces more storms and rain this autumn and winter caused by an 'unprecedented' El Niño event, forecasters warned today.El Niño is declared when sea temperatures in the tropical eastern Pacific rise 0.5C above the long-term average, impacting global rainfall patterns and temperatures.The natural warming of sea temperatures is forecast to reach more than 3C in the coming months, according to Met Office calculations – while other climate groups have projected something even stronger, perhaps even 4C above average. The Met Office said this year's event will be 'the largest since the 19th century' – with one expert even claiming it could be the strongest 'in 500 or even 1,000 years'. El Niño events also drive an increase in global average temperature, and the Met Office added that it is now 'very likely' 2027 will be the hottest year on record.The event creates a heightened risk of drought in the tropics, southern Africa and north-east Australia, while north-western Europe – including the UK – is likely to experience wetter and stormier conditions in the coming autumn and early winter.Professor Adam Scaife, head of long range forecasting at the Met Office, said: 'We should be clear that this is an unprecedented event. I have never seen an El Niño signal this intense in our forecasts.'He added: 'A typical El Niño leads to an equatorial Pacific sea-surface temperature rise of between one to 2C, with 2C being a really big event.'This implies that if the forecast is accurate – and other forecast systems show similar extreme values – then 2026 will far exceed our recent experience of El Niño and its worldwide climate influences.' A group of racegoers brave wet and windy conditions at Cheltenham in November last year El Niño begins when sea surface temperatures in the region known as Niño 3.4 reach a certain threshold. El Niño conditions are deemed to have developed when ocean temperatures remain 0.5C above normal for five consecutive three-month periods (Graphic by Murat Usubali)Met Office science fellow, Dr Nick Dunstone, said: 'During large El Niño events significantly more heat is released from the ocean to the atmosphere leading to an increase in global average temperature, particularly in the calendar year following the winter peak of the El Niño.'Hence, 2027 is very likely to replace 2024 as the warmest year on record and be another year to temporarily exceed plus 1.5C above the pre-industrial period.'Professor Stephen Belcher, the Met Office's chief of science and technology, said: 'This summer we have experienced heat waves across Europe reminding us of the effects of climate change, this large El Niño is a reminder that natural variations in the climate system are also at play, adding to the stresses of climate change.' What is El Niño? El Niño is the warming phase of a natural cycle in the tropical Pacific known as the El Niño-Southern Oscillation (ENSO).It sets in when winds that blow from east to west temporarily ease up, allowing warm water which normally piles up in the western Pacific Ocean to slide back east and rise to the surface.When those warmer-than-average surface temperatures last for a few months, they cause more storms and clouds over the region, which cause ripple effects across the world.It essentially means the weather is hotter and drier conditions in some places; and cooler and wetter in others – including Britain. Dr Zeke Hausfather, a climate scientist at US research group Berkeley Earth, told BBC News: 'We have many months to go while this event continues to develop and that's one of the reasons that we have increased confidence that this will be a record-setting event.'On the possibility of a 4C rise, Mr Hausfather added: 'It's not beyond the realm of possibility to say this might be the strongest El Niño event in 500 or even 1,000 years.'However, it is impossible to substantiate such a claim given records do not go back that far.The unprecedented string of heatwaves and lack of rainfall in recent months has brought the driest July in 190 years and has pushed almost three-quarters (71.3 per cent) of England and all of Wales into drought, putting severe pressure on the water system, wildlife, farmers, emergency services and infrastructure.Many areas of the country are facing water shortages as reservoir levels have fallen to 62.6 per cent – 16.3 per cent lower than is expected for this time of year, according to Defra.Following an emergency meeting to discuss the heatwave last week, Prime Minister Andy Burnham warned: 'We might be in for a wet autumn and winter straight after this, because of El Niño and some of the issues there, which was reported to Cobra.'Parts of the country were hit by heavier showers on Wednesday and Thursday, with the north-west of England seeing the wettest conditions in the last 24 hours, with 52.5mm of rain falling over the seaside town of Morecambe in Lancashire, according to the Environment Agency (EA).This represents a third of the region's rainfall totals recorded over the period between June 1 and August 18 this year, which reached up to 156mm on average, the Met Office said.