Summer forecast 2015: average

This summer is looking an average one. Before you write it off, however, average summers do come with decent spells of warmth and sunshine. But I think the old saying that an English summer consists of three fine days and a thunderstorm will be used more than once this year…

To reach my conclusion on this summer I have used pattern matching of meteorological data from this area for March, April and May stretching back to 1799.

The dry and sunny weather of March and April was tempered by a very average May. The mean for the spring season was 10.5C with 75.8mm of rain and 511 hours of sunshine.

If you take into account all years that were within +/- 10 per cent of these figures, for rainfall and then mean temperature, you get the following table.

The ‘best fit’ years were revealed as 1844, 1870, 1880, 1943, 1995, and 2009. As an average this summer could be expressed as: Mean: 17.4C (about average) Rainfall: 127.3mm (below average) Sunshine: 567hrs (about average)

Or, expressed in probabilities, I concluded the following:

Very Warm ( above 20.4) 0%

Warm (19.4 – 20.3) 0%

Rather warm (18.4-19.3) 17%

Average (16.9C – 18.3C) 50%

Rather Cool (15.9 – 16.8) 33%

Cool (14.9 – 15.8) 0%

Very cool (below 14.8C) 0%

Very Wet ( 3.8 x average) 0%

Wet (2.9  x average) 0%

Rather wet (1.9 x average) 0%

Average 50%

Rather dry (0.7 x average) 33%

Dry (0.5x average) 17%

Very dry (0.25 x average) 0%

So from the above you could deduce that the next three months will be average to rather cool, with average to slightly below average rainfall. Sunshine average.

Trying to predict daily detail over the next 3 months is impossible, but looking at the ‘best fit’ years mentioned above it is probable that the opening 10 days of June will be among the coolest of the summer.

Of interest to most will be when are the hot spells most likely to happen. Considering the median of all rain days a dry spell happened without fail between the dates of June 28th – June 30th and August 15th – August 19th; both these spells likely ending with thundery breakdowns. Another date to bear in mind for a possible two-day fine spell is July 24th-25th.

So, all-in-all, a mixed bag. Looking at the ‘best-fit’ years, however, it is worthwhile noting that although the overall picture looks average there exists the record-breaking dry August of 1995 and the notably wet and thundery July of 1880.

My summer forecast last year was broadly correct. How this one will fair obviously only time will tell. One could argue that what I’m forecasting is just climatology which has a good chance of being correct should no external influences, such as a huge eruption on Mount Etna, have a bearing on the end result.

* Taking into account the fact that temperatures in London are up to 0.66C warmer than they were 100 years ago I have added 0.66C to mean temperatures before 1915.

** Obviously, in the event of a series of direct hits from thunderstorms, my rainfall estimate could be hopelessly short – a symptom of abundant solar energy at this time of year which creates a ‘noisy’ atmosphere compared with winter.

*** The 1981-2010 average mean for summer in this region is 17.6C, with 144.9mm of rain and 564 hours of sunshine