In the middle of April 2012, Piers Corbyn of Weather Action made this dramatic forecast for England’s weather:
The coldest or near coldest May for 100 years in Central and East parts with a record run of bitter Northerly winds.
*Confidence of E / SE England mean temps: Coldest in 100yrs 80%
The early part of May seemed to bear him out. It was cold, with frequent frosts. By the middle of the month, Metro was saying.
Central England temperatures so far this month average 8.6C, 1.6C below normal. The last time May was colder was in 1698, at 8.5C over the whole month.
Leaving aside the underhand trick of comparing the first two weeks of May 2012 with the whole month of May in other years, this does indicate that May so far has been unusually cold. But how cold does it have to be to reach Piers’s forecast of the coldest in 100 years, of which he was 80% confident in the middle of April?
The Met Office publishes Central England monthly temperatures going back 300 years, and this shows the coldest May in the last 100 years was May 1996 with a mean temperature of 9.1C (there are 14 colder Mays in the record, but they were all before 1912).
So if the first two weeks this year have averaged 8.6C, and if this is 0.7C colder than for the whole month, which is about the normal difference, the mean temperature for May 2012 will be about 9.3C, which would not quite be the coldest in 100 years; it would be warmer than 1996 and 1923.
I sent a message to Piers on Twitter on 16 May, asking:
Have you revised your 80% chance that May in S/SE England will be coldest in 100 years?
I haven’t heard back from him yet, but I will post again if I do, and also to track how the rest of May turns out.
Update added 1 June 2012
You may be wondering how this all turned out. I never did hear back from Piers. The weather turned much warmer in the second half of May. As of today, the Met Office has only released a provisional result up to 30 May, but this shows a mean temperature in Central England for May 2012 of 12.0C, which is not only not the coldest for 100 years, but is 0.8C above the average for the 1961-1990 period.