The crux of the article is that certain weather types can repeat at the same time of year with one having a 100 per cent probability!
The Christmas storm singularity occurs in 84 per cent of years.
While 84 per cent certainly doesn’t mean ‘nailed on’, in the current set up of models being evenly spread between settled and stormy it can be safely guessed at this range that there will be unsettled weather around on the 25th. Whether it will affect the whole country remains uncertain.
The last big snorter of a Christmas storm I can remember in London was 2013. The entry for the 23rd into the 24th reads.
“Cloudy and breezy start grew steadily duller with rain just before noon. Rain grew heavier with some really strong gusts into evening culminating at 2am. Cloud at one point was 10kms thick. Three deaths related to weather.”
There was chaos nationwide with flooding and power cuts.
This winter is most likely to be on the colder side of average with near normal rainfall.
While the modelled prognosis for the first half of December looks unsettled with an Atlantic influence evolving from the current chilly NW’ly to a mild SW’ly, local analogues of the climate of London suggest the season could be a bit of a rollercoaster with spells of wet, windy and mild weather alternating with dry, calm and cold.
A large factor to consider this winter is the presence of a slight La Nina that is forecast to evolve cooler.
Given the uncertainties involved with the influence of ENSO I’ve decided to stick with analogues found in local data that stretches back to 1797.
Overall then the probabilities for the next 90 days are.
Average (5.1C – 5.9C)
The above table doesn’t reveal a great deal in that extremes can be hidden in a month or season that finishes broadly average. So I decided to look closer at the winters that were revealed in the analogues.
The first month, as already mentioned, looks like it will be on the mild side with possibly a notable storm off the Atlantic before things calm down over Christmas – the period between Boxing Day and New Year’s Eve possibly presenting the best chance of any lying snow in this region.
The below graphs are a smoothed representation of the years revealed in the analogues most similar to this autumn.
What is probably most interesting is that the analogues that shared a similar ENSO / La Nina index to this autumn tended to ‘turbo charge’ any yo-yoing in the weather type, be that super-mild or abnormal cold.
December probabilties for maxima: Mild: 48% Average: 19% Cold: 33%
And precipitation. Wet: 33% Average: 29% Dry: 38%
So, the month overall will be mild and slightly dry
The first month of 2022 probably represents this region’s best chance of lying snow this winter. A mild start perhaps with a falling off of temperatures in the final third of the month and a cold spell of a week or so. As with December the influence of La Nina could tend to boost the swings in the pattern.
January probabilties for maxima: Mild: 33% Average: 10% Cold: 57%
And precipitation. Wet: 19% Average: 43% Dry: 38%
Greatest chances for January, then, are cold with average precipitation.
The second month may see a slight return of the cold spell in January before temperatures recover for a mild and wet spell in the second half.
February probabilities for maxima. Mild: 33% Average: 15% Cold: 52%
And precipitation. Wet: 38% Average: 38% Dry: 24%
The stats suggest on the cold side overall with average to above average rainfall. Perhaps the depth of the cold skewing any very mild second half of the month?
The extremes that no-one can forecast
As well as the very mild winter of 1989/90 the analogues also revealed the very cold winter of 1978/79. There were others but their occurence makes the probability of a repeat at either extreme at less than 10 per cent.
Compiling a list of sunless, rainy days revealed some interesting spells of wet weather – the most miserable runs of June days in the capital since 1959.
First up was a three-day spell starting on June 25, 1974. Some 34.3mm of rain was recorded.
Next was a three-day spell starting on June 23, 1991. Some 26.3mm of rain was recorded.
Another three-day spell started on June 25, 1997. Some 36.2mm of rain was recorded.
Finally, and most recently, a two-day spell this month that began on June 17th. Some 28mm of rain was recorded.
The above spells all happened around the date of the ‘June monsoon’ singularity which has a probability of 77 per cent. Though the fact that these occurred 47 years, 30 years and 24 years ago shows that these extreme cases happen a lot less than three years in every four the singularity would suggest.
Comparing the current Northern Hemisphere pattern with 1974 suggests that while there’s just as much heat around at 850mb as there was 47 years ago, including an extreme heatwave over some Nordic countries, the air above Greenland appears colder.
The weather of late has been in stark contrast to the mostly dry, sunny (if a bit chilly) spring many enjoyed. Indeed the first half of June saw more of the same and, locally, was the warmest start to the first meteorological summer month since at least 1959.
My memory of summers years ago was that it was often hot and sunny but I also remember countless days of staring out the window for hours waiting for relentless rain to let up.
Looking back at sunshine stats to 1959 there has been over 4,400 days where no sun was recorded, roughly a one in five chance of a totally cloudy day.
Considering the months where the absence of sun is most noticed, May to October inclusive, the probability decreases to just under one in ten.
To decant these to ‘washout days’ I’ve only included those sunless days that were also ‘wet days’ where 1mm or more of rain was recorded. The probability further decreases to just under one in twenty.
All very interesting but were there more washout days decades ago or is the memory playing tricks?
Looking overall shows an increase since 2013. Out of all the months the most notable change has been August.
Air pressure is probably the least celebrated meteorological statistic, most probably because unless you’re staring at a barometer all day you can’t ‘see’ it.
High pressure brings mostly fine weather while low pressure usually brings wind and rain – though it is the gradient of the isobars that can measure the impact each system can bring.
Steep falls and rises in pressure bring the most memorable events, in my case the October 1987 storm.
During the next week there will be a notable fall in pressure – but how will it compare with the past recent years? Looking at my own stats, a relatively small but complete set of 9am barometer readings back to May 2013, it does look like it will be notable.
The table below shows the numbers to beat.
As with many of these events the most impactful weather was found in the north of Scotland. The first period coincides with Storm Caroline. The Met Office blog says Caroline brought a max gust of 93mph at Fair Isle and transport disruption and closed schools across the Western Isles.
Of course what goes down also comes up and pressure rises can be even more notable. In February 2016 the pressure rose 50mb in just three days, a week or so after Storm Imogen.
According to the Met Office FAX charts the 9am pressure in London on March 11th will be around 988mb, which would represent a fall of 48.6mb from 9am on March 6th.
The best chance for lying snow this winter looks like being at the end of the second week of January.
A combination of local analogues and global atmospheric factors including the El Niño-Southern Oscillation suggest that the coming season will be colder than recent years. Though that’s not saying much considering just how mild winters of the past decade have been.
Meteorological autumn was the warmest for 5 years and slightly drier than last year’s wet autumn. Though it shares some similarities with 2015 the external influences are thankfully different to that season which produced the warmest December on record. Considering data back to 1797 I was able to make the following suggestions on how the next 90 days may unfold.
December is most likely to be around average temperature-wise with rainfall also about average. Possibly stormy at the end of the first week. Any snowfall events are likely to be marginal – bad news for anywhere below 70m above sea level. In terms of Christmas a white one in London looks unlikely. There may be interest in the week running up to the big day but I wouldn’t be surprised to see that inexplicable warm up that often happens just as the 25th arrives. Mean: 5.8°C (5.6°C 1981-2010 average) Rainfall: 57.1mm (53.2mm 1981-2010 average)
January is the month most likely to see any lying snowfall, particularly during the first half of the month, with the mean temperature about 1C colder than average. Rainfall is likely to be above average. Mean: 4.1°C (5.2°C 1981-2010 average) Rainfall: 63.8mm (53.2mm 1981-2010 average)
February looks wet and mild overall. Mean: 6°C (5.3°C 1981-2010 average) Rainfall: 55.6mm (39.2mm 1981-2010 average)
Overall the mean for winter: 5.3°C, a little below average. And rainfall about 120 per cent higher than average.
Looking in closer detail reveals that the coldest period is most likely to be between 13th and 19th January, with anomalies sufficiently low enough for lasting lying snow.
The extremes that no-one can forecast
As well as the extreme December 2015 the analogues also revealed the severe season of 1822-23 which saw ice on the Thames by late December. February 8th saw a great snowstorm in northern England where people had to tunnel through the snow.
Another was 1950-51 which was very snowy at high levels. There were 102 days of lying snow at Dalwhinnie (1000ft), exceeding the 83 days set in 1946-47. December 15th saw 15in of snow in Shanklin, Isle of Wight in 3.5 hours.
Last November was on the cold side prompting me to investigate whether we were about to record a third month in a row below average. December turned out to be mild and wet, the lack of snow especially stark in Scotland.
The findings of that study showed that any sustained period of colder than average months was more likely to happen during months of March, April and May, nothing unusual there, especially considering H.H Lamb’s weather types.
I decided to scrutinise further all the colder than average months in this area, considering the 1981-2010 average, back to 1981. This gave the below results.
The dataset covers 399 months, of which 200 were colder than average.
The overall picture shows that negative anomalies are becoming more and more rare, though with notable exceptions being March 2013, December 2010 and January 2010.
The only month that has showed any sort of consistent general decrease in negative anomaly is November.
*For good snowfall at this station needs a negative anomaly of 2C during the months of November, December, January and February.
Damage from Storm Ciara was a lot less notable than further north though the relentless wind saw three records broken locally. Though the gusts were nothing like the St Jude storm in October 2013 the sustained wind blew at its greatest 1, 2 and 3-day rate since this particular automatic station was reset in November 2012.
Over the 3 days the wind direction was locked in a south-westerly, from 199 to 203 degrees.
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