Nature experts say spring is already on the way - but that heavy snow could now "halt its march".
Early signs of spring have been spotted after the warmest year on record for the UK.
Sightings of snowdrops, hazel catkins and even ladybirds and butterflies have been submitted to the Woodland Trust's Nature's Calendar scheme, which asks the public to report natural events which mark the changing seasons.
So far in 2015, there have been reports of snowdrops at 120 locations across the country, hazel catkins at 74 spots and even sightings of ladybirds, small tortoiseshell, peacock and red admiral butterflies, the Woodland Trust said.
In November, the Trust received a record of frogspawn on the Lizard Peninsula in Cornwall, the earliest such incidence for nine years.
A warming climate has seen spring coming earlier, and three of the last four years have seen some of the earliest spring averages the Trust has compiled from its monitoring of the seasons in Nature's Calendar for the last 15 years.
Research looking at nature records stretching back to the 18th century has found flowers over the last 25 years blooming up to 12 days earlier than previously.
The current freezing conditions may halt the march of spring, but conservationists say that causes less of a problem than a late cold snap, as happened in 2013, when more species are likely to be awake from hibernation or already flowering.
Dr Kate Lewthwaite, Woodland Trust citizen science manager, told the Press Association: "People still seem to be surprised to see snowdrops and butterflies in early January, but our warmer climate in recent years means this is now pretty common.
"Records added to Nature's Calendar by the public have enabled scientists to learn how trees, plants and wildlife adapt to our changing climate.
"If we can ensure our natural environment is diverse, resilient and interconnected it will make it easier for wildlife to adjust too."
The Trust said it would be working in partnership with the British Science Association to estimate the rate spring progresses this year.
National Trust's national specialist on nature and wildlife, Matthew Oates, said: "Spring has been calling for some time, due to a mild autumn, December and early January. The signs are all around us.
"Hazel catkins were quite prominent in southern England before Christmas and are now well in flower, such that some hay fever suffers - like me - are already experiencing symptoms."
He added: "Spring's advance has now been checked by the arrival of the first cold snap since March 2013. Perhaps spring needed slowing down, as early springs are risky and almost always end in tears."
But he urged the public to look out for the signs of spring and report them to the Woodland Trust's Nature's Calendar.
So, weather-wise, what does this week have in store exactly? Leon Brown, meterologist at The Weather Channel, told Aol Travel: "After a few days of mild weather another seven days of colder weather will arrive during Wednesday. There will be quite a drop in temperature during the day behind a cold front on Wednesday.
"Between this Wednesday and next Wednesday, snow will be in the forecast for just about everywhere, but amounts will vary quite a bit, even locally.
"The weather pattern from Wednesday will be very similar to two weeks ago with a very unstable polar maritime air mass dragged all the way from North West Canada.
"There'll be a lot of showers over the west on Wednesday night to Thursday, turning readily to snow inland and some thunder around coasts.
"There'll be some moderate to heavy snowfalls on Wednesday night to Thursday over West Scotland, Northern Ireland to North West England, and snow showers also moving over the higher parts of the south west on Thursday morning with 5cm to 10cm across Exmoor and the Mendips. Mid and North Wales may also catch 5cm to 10cm.
"The main snow risk areas on Thursday night to Friday look like Wales again with another 5cm to 10cm, and more over the higher mountains. Some snow showers moving over the western Midlands towards the London area may give 2cm to 4cm.
"Later on Friday and Friday night it will be the turn for North England and then East Anglia to see some snow as an occluded front moves south.
"By next weekend the UK will be in a northerly Arctic flow with hail, sleet and snow showers around coasts and over northern Scotland, with just a few showers inland over southern Britain.
"Before the cold snap is pushed away, it looks like there will be another snow risk next Monday as an area of low pressure tracks southwards in the Arctic flow. This may bring 5cm to 10cm quite widely over northern Britain, and a lot more over the higher hills in Scotland and Pennines. As much as 5cm also is likely as far south as the Midlands.
"The lowest temperatures in this cold snap look likely on Sunday when winds will drop out for a time. We could see -8C to -10C over parts of the north on Sunday morning."
Love spring? Spot the signs:
National Trust's ten signs of Spring
Heavy snow could hamper early arrival of spring
“The popular spring bulbs are valued for their brilliant flower colours and shapes,” says Matthew.
Top tulip spot: Hidcote, Near Chipping Campden. Every year 15,000 tulips burst into bloom providing a beautiful splash colour against the mellow cotswold stone of this magnificent formal house and gardens.
Other great tulip spots include: Sizergh Castle, Cumbria; Dyrham Park, South Gloucestershire; Upton House and Gardens, Warwickshire.
“Although they can be spotted throughout the year, these bushy-tailed creatures come to life as winter recedes and can be spotted foraging on the ground or roaming through the treetops.”
Top spot for red squirrel viewing: BrownseaIsland, Dorset. This island sanctuary, located within Poole Harbour and only accessible by boat, is a haven for wildlife. Thriving natural habitats provide great wildlife spotting potential, particularly for the often elusive rare red squirrel.
Other places to seek out red squirrels: Formby, Lancashire; Mount Stewart, Co. Down; Wallington, Northumberland.
“One of the earliest and finest signs of spring is a rook with nest building material held in its beak. You can spot a rookery in the tree tops or in old houses across Britain, with eggs appearing around early March,” says Matthew.
Top rook-spotting territory: Tattershall Castle, Lincolnshire. Rookery building in the trees at Tattershall means visitors who make the climb to the top of the castle keep can enjoy spectacular local views and down into the nests below too.
Other great places to see a rookery: Tyntesfield, Bristol; Fyne Court, Bridgwater; Crook Peak, Somerset
“After the long winter, migrating birds return from milder climates to spend the summer. Spot chiffchaffs from March, with swallows and house martins usually arriving in April and swifts following in late April or early May,” says Matthew.
Top bird watching spot: Crom, Co. Fermanagh. Escape to 2,000 acres of one of Ireland’s most important nature conservation islands, full of ancient woodlands and picturesque islands. Watch the winter birdlife depart and migrants arrive from the south from our new bird hide on the shores of Derrymacrow Lough.
Other top spots for spring birdwatchers: Stackpole, Pembrokeshire; The Lizard, Cornwall; Blakeney Point, Norfolk.
“Newborn lambs are emerging on unsteady legs at farms all over the country,” says Matthew.
Top viewing spot: Wimpole Estate, Cambridgeshire. This working estate is still guided by the seasons and brims with new life in spring as lambing season arrives. With a substantial collection of rare breeds, Home Farm sees the arrival of approximately 300 new born lambs each year.
See more spring newborns at: Brockhampton Estate, Herefordshire; Scotney Castle, Kent; Llanerchaeron, Ceredigion.
“Fields of bright yellow dandelions are a sure sign that spring is on its way – and a big help to bees and insects as they come out of hibernation, although possibly not for gardeners.”
Top dandelion spot: Attingham Park, Shropshire. The open fields of many National Trust parks and estates are perfect places for dandelions to flourish, such as just north of the mansion at Attingham.
Other great dandelion fields: Clumber Park, Nottinghamshire; Felbrigg, Norfolk; Stonehenge Landscape, Wiltshire.
“Over half of the world’s population of bluebells can be found in the UK and the delightful carpets of blue that cover our woodlands throughout late April and May are one of our greatest spring treasures,” says naturalist Matthew Oates from the National Trust.
Top bluebell viewing spot: Blickling Estate, Norfolk. Visit in April through to May and follow the winding paths through the great wood and pass through swathes of dainty English bluebells.
Other bluebell hotspots: Hardcastle Crags, West Yorkshire; Lanhydrock, Cornwall; The Vyne, Hampshire.
“One of the great signs of the arrival of spring is the so-called ‘mad March hare’ – as these usually shy and reclusive creatures can be spotted ‘boxing’ in open fields throughout the mating season.”
Top hare viewing spot: LymePark, Cheshire. The open spaces here are perfect hare-spotting territory during early spring. Head up to The Cage, an atmospheric hill-top hunting lodge, for great views across the park’s vast acres.
Other good spots for hare spotting include:
Wimpole Estate, Cambridgeshire; Mountain hare can be found on the Dark Peak Moors, Derbyshire
Irish hare can be seen on Divis Mountain, Belfast.
“These bright yellow bulbs are already beginning to burst into life, heralding spring as they spread outwards from the south west to the rest of the country,” says Matthew.
Top spot for daffs: Sizergh Castle, Cumbria. Wordsworth’s famous poem was inspired by a beautiful belt of wild daffodils in Cumbria. The wild variety found here has a particular beauty. Dora’s Meadow at Grasmere, created by Wordsworth in memory of his daughter, will also be open for daffodil-viewing under the National Gardens Scheme on 1 April.
Other top spots for daffs include: Kingston Lacy, Dorset; Chirk Castle, Wrexham; Gibside, Tyne & Wear.
“These astonishing goblet - or star-shaped flowers found on trees or shrubs are much admired seasonal icons, and for good reason.”
Top magnolia viewing spot: Nymans, West Sussex. This spectacular garden is famed for its amazing collection of rare and important plants. In spring the stars of the show are magnificent magnolias, underplanted with drifts of daffodils and grape hyacinths.
Also see magnificent magnolias at: Winkworth Arboretum, Surrey; Glendurgan Garden, Cornwall; Bodnant Garden, Conwy
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Heavy snow could hamper early arrival of spring
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