Alistair Grant and Fiona McKay

SCOTLAND is on course for the coldest spring for 39 years - with more snow expected in the coming days and sub-zero temperatures threatening to extend until June.

The Met Office's three-month April to June forecast said there would be colder-than-normal spells during the first half of the period.

This rare "polar plunge" phenomenon was caused by "Sudden Stratospheric Warming" which has disturbed the jet stream.

It makes air above the North Pole incredibly warm, pushing bitter conditions southwards

This caused the historic cold weather at the end of February -- dubbed the Beast from the East -- and the brief but intense wintry spell in March.

Forecasters said this also means that cold spells could be ongoing for a further two more months.

The Met Office April-June forecast said: "The Sudden Stratospheric Warming event in February very likely gave a strong impetus to recent cold spells and its influence is expected to continue in the early part of the outlook period.

"This implies increased chances of weather patterns bringing colder-than-average conditions."

This March, average temperatures were 1.7C colder than usual, according to Met Office records.

Average temperatures across March to May in Scotland are set to be 1.5C below normal, under 2013’s 4.8C, and would make this spring the coldest since 1979.

The Weather Outlook forecaster Brian Gaze said: "Spring looks cold for Scotland.

"This may turn out to be one of the coldest springs for a number of years.

"Further chilly spells are possible. Colder air could be pulled in from the northeast during the next few weeks."

Snow is forecast in coming days on mountains, with cold periods and frost expected throughout a mixed April.

A Met Office forecaster said: “From Monday, it will be showery at times but with some drier spells. As winds strengthen, eastern areas will become rather cold and cloudy.

"From April 21 to May 5, signals suggest temperatures around or a little below average, with frosts likely to be confined to northern areas."

The Mountain Weather Information Service said: “Snow on the higher mountains in Scotland will probably be followed by easterly winds towards the middle of next week. Snow loss on the higher Munros will be slow."

Scotland’s ski resorts said that favourable conditions meant the season would extend into the summer, which begins June 1.

All five resorts -- CairnGorm Mountain, Glencoe Mountain, Glenshee, The Lecht and Nevis Range -- are still open with good snow cover.

Experts said: "Scotland’s ski season continues to be one of the best in years."

Meanwhile, Scotland’s transport chiefs ordered an extra 100,000 tonnes of road salt to prevent running out last month, it has emerged.

Figures from the Scottish Government-backed Salt Group Situation Report, which covers salt held by Transport Scotland, councils and in reserve, show a record 720,535 tonnes of salt were spread this winter, beating 2012-13’s 691,000 tonnes

Extra salt orders had to be placed after Scotland had only 622,723 tonnes of salt on December 12 - 508,504 tonnes in stock and 114,219 tonnes spread by that point, Salt Group reports reveal.