Category Archives: UK climatology

London winter forecast 2020/21

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.

Hours of frost

Frost is a rare beast this year, even in the Wanstead Park frost hollow.

So far this year I’ve recorded just 82.5 hours where temperatures at 4ft (1.2m) were -0.1C or below. That’s just 37 per cent of what is recorded in an average year. And would suggest that frosts in December will be above average.

The lack of frost this year is even more remarkable given that the past seven years, especially the winters, have been so mild.

A closer look at the data shows that winter months are no guarantee of seeing temperatures fall below freezing. The record mild December 2015 and February 2014 are testament to that.

The fact that January 2017 looks the most remarkable month for frost reveals just how mild recent years have been.

Two warm April days 128 years apart

The synoptic pattern on Sunday, April 5th, was very similar to the pattern on Tuesday, April 5th 1892.

With so few planes in the sky because of the coronavirus lockdown it offered an ideal opportunity to compare temperatures and sunshine totals between now and then.

Sunday dawned sunny and clear and stayed that way until dusk, some 11 hours of sunshine recorded, exactly the same as 1892!

The temperature in Wanstead reached 22.3C, 0.9C cooler than what was recorded at the Royal Observatory, Greenwich, in 1892. This maximum was reached after an overnight minima of 5.3C, the same as the 41.5F recorded at Greenwich all those years ago.

Looking further afield, and at the spell over 3 days… markmpcc

4th

5thww

6th

 

 

 

 

London’s colder than average months

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.

neg anoms

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.

Winter 2019/20 forecast review

For those interested my winter forecast this year was way out. A prediction of a mean of 4.7C was 2.3C too low. The chief culprit for the 7th mildest winter back to 1797 was most probably the strong polar vortex which has often been at record strength over the past three months.

Rainfall prediction was also over 100mm too low. The wettest winter for 25 years with 248.1mm recorded places it 13th in wettest winters. The only crumb of comfort I can take is that the stats indicated an uptick in precipitation in February! The 90.8mm recorded made it the wettest February for 10 years, just short of 2010. Before that you have to go back to 1951 to find a wetter February. It places 9th in wettest Februaries since 1797.

Over the past months I have looked into the method of seasonal prediction and found some interesting results. For example, the winter of 1989/90 – a winter that was very similar to this one – led to a prediction that was 2.7C too low. Similarly the winter of 2013/14 was predicted 2C too low.

Rainfall looks to be far more random. Though it is obvious that we are now in a wetter than average spell experience shows that it is impossible to tell how long this will last.

Pattern matching and singuarities can be helpful in long distance forecasting. The spanner in the works, however, can be a sharply positive or negative ENSO (El Nino / La Nina) or, as in this year, a very strong polar vortex.

You can find the original forecast here.

anom
The coldest anomaly was at the start of winter.

 

The relentless wind

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.

wind run

Over the 3 days the wind direction was locked in a south-westerly, from 199 to 203 degrees.

9th10th11th

Dynamic early final warming and spring

In my search for some winter weather a tweet by Amy H Butler about dynamic final warmings piqued my interest.

According to the atmospheric scientist a winter where there was no major disruptions of the polar vortex (SSW) we are more likely to see a dynamic early final warming. A table published by Wiley shows the final warming dates.

FW
* means there was a SSW, bold means late FW (after the mean date of April 15).

So what could this mean for the weather in the London area? Considering all the above years with no SSW gives an average date of April 19th for a dynamic final warming.

I then looked at the TMax anomaly for those years for 60 days following a DFW and came up with the following graph.

june cold

The results suggest temperatures in April will be heading down in the final week for a  cold end. The average to cool theme continues into May before temperatures lift in the final week for a warm end, with anomalies up to 5C above average. June, however, looks shocking with temperatures nearly 6C below average by the 16th.

This winter has so far been very similar in type to 1990. The dynamic final warming that year was among the latest in the list and led to a cool and dull June with anomalies in the second week nearly 7C below average!

1990

2019 Wanstead annual weather review

This year finished as the 17th warmest on record – some 0.4C warmer than the 1981-2010 average. Although mild it was the coolest for six years.

After a dry start a wet autumn saw annual rainfall finish above average, the wettest for five years, 105 97 per cent of average.

Sunshine was down on 2018 though the total was just above average.

2019

2019 temp

2019 rain

2019 sun

For a review of each month, click JanuaryFebruaryMarchAprilMayJune, JulyAugustSeptemberOctoberNovemberDecember

You can read the national review of weather events at the Met Office blog here.

 

Summary for 2019

Temperature (°C):
Mean (1 minute)  11.7
Mean (min+max)   11.7
Mean Minimum     7.3
Mean Maximum     16.1
Minimum          -6.5 day 30/01
Maximum          36.8 day 25/07
Highest Minimum  19.6 day 23/07
Lowest Maximum   1.3 day 23/01
Air frosts       43
Rainfall (mm):
Total for year  667.4
Wettest day      20.6 day 19/07
High rain rate   65.8 day 09/08
Rain days        158
Dry days         207
Wind (mph):
Highest Gust     36.5 day 10/03
Average Speed    2.5
Wind Run         22211.5 miles
Gale days        0
Pressure (mb):
Maximum          1053.4 day 09/08
Minimum          969.8 day 12/12
Days with snow falling         5
Days with snow lying at 0900   2
Total hours of sunshine        1451

December 2019: wettest for 17 years

The December monthly mean finished 6.3C, that’s 0.7C above average, making it the ninth month of 2019 where the monthly mean temperature finished above average.

The wet theme of October and November carried on into December; the 111.1mm collected was 209 per cent of the 1981-2010 average, the wettest since 2002.

Some 51hrs of sunshine were recorded, 126 per cent of average, the sunniest for three years.

There were seven air frosts.

To view full stats follow this link:http://1drv.ms/1kiTuzv

december t max

November 2019: coldest for nine years

The November monthly mean finished 6.4C, that’s 1.6C below average, the coldest November since 2010. It was the third month of 2019 where the monthly mean temperature finished below average.

The wet theme of October carried on into November; the 68.5mm collected was 116 per cent of the 1981-2010 average, slightly drier than last years.

Some 59.7hrs of sunshine were recorded, 102 per cent of average, the dullest for four years.

There were six air frosts.

You can find my thoughts on how winter is going to pan out here.

To view full stats follow this link:http://1drv.ms/1kiTuzv

nov t.PNG

Summary for November 2019 – to follow