During the second part of March most of the British Isles experienced exceptionally warm and summery weather. At the end of the month and during the first week of April temperatures dropped and skies became overcast. Newspapers which had been reporting record-breaking temperatures and showing pictures of scantily dressed people enjoying the sun, showed pictures of lambs shivering in the snow.
Possibly this change could have been anticipated by country weather forecasters because blackthorn (Prunus spinosa) was coming into bloom, and it is widely recorded that a spell exceptionally cold weather coincides with this.
A letter from Thorncombe, Dorset, dated 1 April 1974, reported:
‘The weather here is very dull and really cold with a north-east wind, but the blackthorn is out, so I suppose it’s the blackthorn winter.’
One of the press stories during the warm spell was how plants, particularly bluebell (Hyacinthoides nonscripta), were flowering much earlier than usual, possibly due to climate change, but, judging by the 1974 letter, it appears that blackthorn produced its flowers in 2012 at much the same time as it did almost 50 years ago.

Update, 16 March 2019:  Blackthorn has been in flower in the London area for about a week, and, although the weather was bright and spring-like earlier in the month, it is now cold, though temperatures are not dropping below freezing point.

Image: South West Coast Path between Lynmouth and Combe Martin, Devon; March 2014.