Category Archives: UK rainfall

London’s October extremes since 1959

October is one of those months that can see both ends of the spectrum; from calm ‘mists and mellow fruitfulness’ and, rarely, frost, to wet and wild systems whistling in off the Atlantic, best known being the 1987 Great Storm and, more recently, the St Jude storm.

I’ve put together a few top 10s of stats for Wanstead, St James’s Park and Heathrow for the month of October.

october extremes

October SJP

Heathrow oct

Some national UK October values according to TORRO

Hottest: 29.4C March, Cambs – 1st 1985
Coldest: -11C Dalwhinnie 28th 1948
Wettest: 208.3mm Loch Avoich 11th 1916

In terms of climatology October maxima, considering the 1981-2010 average, shows a through the month, though around there is often a spike. This would reflect the October singularities; early October storms, between 5th and 12th, peaking on the 9th, occur in 67 per cent of years. St Luke’s summer, between 16th and 20th, peaking on 19th, also has a 67 per cent probability. If you look at the graph there is a spike just after the 19th.
Mid-autumn storms occur between 24th and 29th October, with a 100% probability.

october average graph

The average rainfall graphic shows that downpour amounts are variable through the month. A tendency for dry weather around the 17th and 18th before the wettest days on the 20th and 21st.

October rainfall

 

 

 

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The disappearing deluges of September

While putting together my September extremes blog I noticed that the month was marked by some big rainfall episodes. On a national scale TORRO statistics show that a south-east climate station holds the daily record for September – unusual in that every other month is dominated by stations in the north and west of Britain.

Further analysis of local data since 1959 shows how September has slowly evolved from being dominated by autumnal to summery weather. The wettest period, from the mid Sixties to the mid Seventies, saw 42 per cent of highest daily rainfall events recorded.

Two of these events, in 1968 and 1973, are well noted and appear in RMET’s Weather magazine.

1968

In Wanstead, between 14th and 15th September, a total of 58.4mm fell, a large total though far less than elsewhere.

The rain became torrential overnight in southeast England and continued through most of the 15th. Rainfall totals ranged from three inches in the London area and nearly four inches in southern Essex to approach the quite abnormal level of nearly seven and a half inches in parts of Kent during the 14th/15th.

The area of greatest precipitation was near the Kent, Surrey and Sussex border where violent downpours in the 12 to 15 hour period from midnight on Saturday to the early afternoon of Sunday 15th led to widespread and disastrous flooding. The heavier rain area moved north and east during the night of 15th/16th and Gorleston recorded.

The highest accepted two-day falls were 201mm at two rain gauges at Tilbury
and Stifford in Essex, and a similar fall north of Petworth in Sussex. The highest ‘rainday’
totals (i.e. nominally 0900–0900 GMT) listed in British Rainfall 1968 were 129 mm at
Bromley and 125 mm at South Godstone sewage works in Kent, both on 15
September.

This exceptional event was described by Bleasdale (1974), Salter and
Richards (1974) and Jackson (1977) and the map below (Fig. 1) is taken from Bleasdale’s
paper in British Rainfall 1968 (p. 231). In all, some 575 km2 received more than 150 mm
in 48 hours.

1968 rainfall

1973

In Wanstead, on 20th, a total of 55.4mm fell, the largest daily total recorded in September. Again it was far less than elsewhere.

5pm gmt 20th 1973

There were notable falls of rain in London, Surrey, West and East Sussex, and particularly Kent. At Manston, near Margate, 172 mm fell in 18 hours 40 minutes commencing 1710 GMT; at nearby West Stourmouth 190.7 mm fell in the rainfall day i.e. the 24 hours commencing 0900 GMT on 20 September (source: Met Office internal list of heavy falls of rainfall in short periods in the United Kingdom during the year 1973; Rainfall/heavy falls section listing, available in manuscript/computer printout form in Met Office archives).

The future

Big September rainfall events seem to be becoming rarer in our part of the UK, the last was in 2014, on the day of the Scottish referendum, though this was convective rather than frontal rainfall.

The outlook for the rest of this month suggests yet another absence of a large rainfall event. The ingredients for large rainfall totals in the south-east – blocking high to the north with slow moving low pressure over Brittany – look unlikely to form during the remainder of the month. We probably will see rain but any fronts are likely to move through quickly, with typical totals being around 5mm.

rain rest.gif

When did September become a summer month?

The answer is around 1993. A look back at mean temperature and rainfall statistics for east London over the past hundred years reveals that the ninth month has, since that date, slowly become warmer and drier.

Putting all the arguments of meteorological and astronomical summer aside, many people of a certain age regard September as an autumnal month, but as recent years have shown it can very often be an extension of summer; September 2016 was the second warmest on record in the local area, warmer than many previous summer months!

Looking back even further, over the past 100 years, the September mean has trended upward, though many peaks and troughs reveal how the month has ebbed and flowed from being summery to autumnal.

The prognosis to the end of this month suggests that air pressure will be anomalously high – so the pattern in the south-east for settled, summery weather in September doesn’t look like ending any time soon.

sept means
The September mean trend has crept generally upward.
sept rainfall
A look at rainfall back over the last 100 years shows that wet Septembers have been on a general decline since 1994.

sept chart.gif

England’s heatwave hangover

These superb MODIS satellite images from Nasa show how the bulk of England turned from a vibrant green in June to a parched brown a month later, as the weeks-long heatwave reached its peak.

At the beginning of September the South and North Downs are once again turning green but, after the driest summer for 15 years in many places, the landscape still has a long way to go.

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June 25th 2018
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July 27th 2018
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September 2nd 2018

Summer 2018: second hottest since 1797

The results are in and statistics show that this summer was the second hottest in a local record going back to 1797. The mean temperature of 19.57C was 2.4C warmer than average, just six hundredths of a degree cooler than the summer of 2003.

A sustained period of heat that continued the theme of the hot late spring was enough to see a new entry into the hottest-ever list of heatwaves.  But the hottest weather was over by the end of July, August being very disappointing compared with the first two months of summer.

It was also the driest summer for 15 years. Just 87mm were recorded, that’s 60 per cent of average and just 7mm wetter than the historic summer of 2003, the year the UK’s highest temperature record was set.

It was the sunniest summer for 5 years: 663 hours of sunshine were recorded, over 100 hours less than 1976, the 24th sunniest summer since 1881.

Hottest day: 34.7C (26/7)
Coolest day: 17C (9/8)
Warmest night: 19.7C (26/7)
Coolest night: 6.4C (12/6)
Wettest day: 13.9mm (16/8)

summer max 2018

summer 2018 rain

Summary for period 01/06/2018 to 31/08/2018

Temperature (°C):
Mean (1 minute)  19.4
Mean (min+max)   19.6
Mean Minimum     13.7
Mean Maximum     25.5
Minimum          6.4 on 12/06/2018
Maximum          34.7 on 26/07/2018
Highest Minimum  19.7 on 26/07/2018
Lowest Maximum   17.0 on 09/08/2018
Air frosts       0
Rainfall (mm):
Total for period 86.9
Wettest day      13.9 on 16/08/2018
High rain rate   48.3 day 10/08/2018
Rain days        19
Dry days         73
Wind (mph):
Highest Gust     22.8 on 24/08/2018
Average Speed    2.6
Wind Run         5714.2 miles
Gale days        0
Pressure (mb):
Maximum          1029.9 on 21/06/2018
Minimum          1001.3 on 26/08/2018

London’s August extremes since 1959

I’m running out of superlatives for this summer. There has already been 15 days where the temperature has reached or exceeded 30C, two more than 1976!

However, the mean temperature has not been as intense as that classic summer where two heatwaves of five days and four days length were featured during the season. This summer has so far only seen one similar heatwave of 5 days length.

I’ve put together a few top 10s of stats for Wanstead, St James’s Park and Heathrow for the month of August.

august top ten wans

august SJP

august heathrow

Some national UK August values according to TORRO

Hottest: 38.5C Brogdale, near Faversham, Kent – 10th 2003
Coldest: -4.5C Lagganlia, Highland – 21st 1973
Wettest: 238.8 Cannington, Somerset – 18th 1924

In terms of climatology August maxima, considering the 1981-2010 average, shows a fairly steady decrease through the month, though around the 15th there is often a spike before a steady decrease to month’s end. This would reflect the late August winds singularity which occurs every year around the 20th at 67 per cent probability.

august av max

The average rainfall graphic shows that downpour amounts are fairly random from year to year. The driest days are the 15th and the 30th.

aug av rain

 

July 2018: warmest month on record

July 2018 was the warmest month locally in a record going back to 1797. The mean temperature of 21.7C is 3.2C above the 1981-2010 average and exceeds the previous warmest July (1983) by 0.3C. Indeed, it is the warmest calendar month ever.

jul152018
MODIS image July 15th 2018

Just 16.7mm of rain fell, all of it coming after the 47 day long drought ended with a thunderstorm on the 27th. The amount is 38 per cent of average and is the driest July since 2010 and 7th driest in the local record.

There were 273 hours of sunshine, 142 per cent of average, making it the sunniest for five years and 10th sunniest on record.

As well as many hot days the lack of rainfall was most noticeable with catastrophic results on Wanstead Flats.

The warmest day occurred on the 26th with 34.7C.

The wettest day occurred on the 27th with 7.3mm falling.

 

 

East London: drier than the Sahara desert

This summer has seen less than a millimetre of rain fall across our region, less than some parts of the Sahara desert!

The lack of precipitation has had catastrophic results, not least being a big contributing factor to the huge grass fire that swept through parts of Wanstead Flats at the weekend.

Since May 29th just 0.7mm has fallen in Wanstead, less than the 2.9mm recorded in Bechar, Algeria, and far less than Agadez in Niger which recorded nearly 108mm.

50 days

Of course the fact that the mean temperature of Wanstead over the same period is at least 13C cooler than both Saharan towns means that the dunes won’t be blowing across the Temple any time soon.

The affects of the dry weather can also be seen across the UK in these MODIS images, the lush greens of last year looking much paler and browner.

comparison

 

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MODIS image July 23rd 2014
jul152015
MODIS image July 15th 2015
jul192016
MODIS image July 19th 2016
jul172017
MODIS image July 17th 2017
jul152018
MODIS image July 15th 2018