text: Recent weeks might have suggested an early summer, but Britain is now in for a reality check as ice, rain and snow arrive with -5C (23F) temperatures. The colder weather is set to last the first half of May - and the forecast comes less than a fortnight after the mercury breached the 25C (77F) mark. Conditions this week are set to be average for April, with showers on the way and temperatures barely in double figures for most of the country. Sleet and snow are forecast for today and tomorrow from the Pennines northwards, and there were small accumulations yesterday in the Cairngorms. Meanwhile the Met Office has a severe yellow ‘be aware’ ice warning for north-east Scotland, warning motorists of a disruption risk until 7am today. Scroll down for video . Getting colder: Visitors to Alnwick Garden in Northumberland enjoy the weather yesterday, with conditions expected to turn this week . Going for a walk: Visitors to Alnwick Garden admire the spectacular blossom on Prunus serrulata 'Taihaku' trees in the Cherry Orchard . Polar plunge: This graphic from Netweather shows the temperature at 850hPa (5,000ft) and the pressure at sea level as of midnight today . Forecast: Conditions this week are set to be average for April, with showers and temperatures barely in double figures for most of the UK . It comes after Britain enjoyed what appeared to be one of the warmest Aprils on record, with a 2014 high of 25.1C (77.2F) recorded in Kent on April 15. And Pershore in Worcestershire had the UK's top temperature of 14.8C (58.6F) yesterday, while Morecambe saw 14.1 hours of sunshine. But the dramatic change is being brought in by a ‘polar plume’ - a cold mass of air moving down from polar regions. Met Office meteorologist Charlie Powell told MailOnline last night: ‘It will feel cooler - temperatures are going to be around average. ‘But because we have been used to things being so warm in recent weeks, we are going to notice the change.’ More than 38,000 marathon runners in London yesterday were caught on the brink of the cold front, as cloudy, damp weather greeted them at the start. Surface pressure: A cold front (in blue triangles) can be seen moving away from Britain at midnight today after arriving over the weekend . Cloudy, damp weather at the start: More than 38,000 marathon runners in London yesterday were caught on the brink of the cold front . Better weather: Lily, 11, and Jack Manning, six, enjoy the sunny weather in Portsmouth, Hampshire, only six days ago on Monday, April 20 . Warmer days: People enjoy the warm weather at Jubilee Gardens next to the London Eye in the centre of the capital on Saturday, April 18 . The mercury is expected to fall as low as -2C (28F) early this morning in built-up areas, while rural parts could see -5C (23F), this week’s expected low. This morning will see a frost, Mr Powell said, and showers are expected to arrive from the west today, with eastern parts forecast to be the driest. And today Britain is even expected to be colder than the Norwegian capital Oslo, which will reach around 13C (55F). Temperatures will be around 12C (54F) for most, although the far South East could see up to 15C (59F) - but that will come with a few showers late on. Tomorrow and Wednesday will also be showery days, with overnight temperatures around freezing and highs of 14C (57F) in the daytime. A similar high could be seen on Thursday and Friday, but there is expected to be more sunshine, fewer showers and lighter winds by this point. Conditions will marginally improve for the beginning of the Bank Holiday, with dry and mild weather predicted for the Friday and Saturday. Early forecasts for the Bank Holiday weekend suggest the North East will see the best weather into Saturday - but could still be hit by showers. How it works: This Met Office moving graphic shows how the arrival of an Arctic Maritime air mass is affecting temperatures in the UK . A pair of dolphins arrived on the south coast of England yesterday, with hundreds of residents enjoying the spectacle. The common dolphins, normally found offshore in deep waters, spent several hours swimming around Weymouth Harbour in Dorset. And 48-year-old resident Madeleine Santonna told the BBC: 'I've lived by the harbour all my life and I've never seen anything like it.'. Spotted at the harbour: Two dolphins swam around Weymouth in Dorset for several hours yesterday, causing excitement among onlookers . Unusual appearance: The common dolphins seen at Weymouth Harbour yesterday are normally found offshore in deep waters in large groups . Unprecedented sighting: One local resident said she had lived by the harbour for her whole life - but had 'never seen anything like it'
target: Colder weather set to last this week and into first half of May - less than a fortnight after temperatures hit 25C .
  Snow from Pennines northwards today, while much of Britain will see showers and Scotland has ice warning .
  Maximum temperatures this week of around 15C in far South East, with -5C expected in rural areas overnight .
  Early Met Office forecast for Bank Holiday weekend suggests North East will see best weather into Saturday .