Category Archives: UK snowfall

February 2018: cold, very sunny, snowy end

February 2018 saw the start of the first decent cold spell since 2013 with thick (by modern standards) snow cover, deep cold air and bitter winds.

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February 2018 is marked by the steady downhill temperature trend from the 18th.

It was the coldest February since 1991, the mean temperature of 2.8C was just over 2.5C below average. The month was ‘cold’ though it was the 19th onwards that really dragged the value down.

Precipitation was just below average; just over 34mm is 87% of average.

It was a sunny month, the sunniest February since 2012 and the 12 sunniest in a local record going back to 1881. Some 87.9 hours were recorded, 120% of average. Of that top 12, eight have occurred since 1988! A mix of changes in weather patterns, industry and council action?

Air frosts: 16. Ground frosts: 19. Snow falling: 6 days. Snow lying: 3 days (greatest depth 8cm 28th)

Full stats for February here: http://1drv.ms/1rSfT7Y

settled snow 28th
The 28th dawned sunny with fresh snowfall after an hour long shower at 3am. Within hours more showers moved in.
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Record cold pools and snowfalls

This week has the potential to see new temperature records set or matched as very cold air moves in off the continent.

Whilst amounts and location of snow are very difficult to estimate at more than 24hrs to 48hrs away there is no doubt that the incoming air is very cold indeed.

In the early hours of Wednesday one weather model is showing extremely cold air (496-504 DAM ie very low thickness) just off the coast of Scotland. In the last 60 years there have been only three occasions where air approaching this thickness (500 DAM and lower) has been recorded in the UK:

February 1st 1956: Hemsby, Norfolk
February 7th 1969 at Stornoway, Outer Hebrides
January 12th 1987 at Hemsby.

thick
Radiosonde (weather balloon) ascents will make very interesting reading this week

With the deep cold air in place the potential for snowfall comes once the air starts to become unstable. East London, and much of the east coast, best falls come where convergence lines ‘streamers’ form.

streamer
One example of a streamer is forecast to occur on Tuesday

If persistent, these ‘Christmas tree’ features are capable of producing snowfall accumulating at the rate of 5–7cm per hour in especially cold
outbreaks, albeit often very locally. The steep thermal contrast between the very cold air and the current warm anomaly in the North Sea could make any snowfall very heavy indeed.

Streamers during the cold spell of January 1987 saw 30cm fall widely with some up to 65cm in Kent and 45cm in south Essex. Parts of Cornwall saw up to 40cm.

During a cold spell in February 2009 thundersnow was recorded – the favoured spot this time being parts of Surrey which saw 30cm.

Personally the most snow I have recorded during a cold spell was in February 1991. A very deep cold pool, not unlike what is forecast this week, covered much of the south. in air approaching 500 DAM. Days and days of snow followed dumping knee-deep powder in my local park in suburban East London. Reported depths included 20cm at St James’ Park in the centre of London, and 38cm at Rettendon, Essex.

There is a very good paper on cold pools and snowfall here.

beast

 

 

 

The most potent cold spells since 1960

With met models now coming into reliable range it now looks odds on that very cold air from the continent will be in place across most of the UK from early next week.

The big question is how long will the cold last and how much snow will fall? While the latter looks likely at some point once the air is in place it is impossible to pinpoint where and how much any given place will receive at this range.

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The 00Z op run of the ECMWF model has the really cold air arriving at 00Z on Monday 26th.

In terms of longevity latest data shows the spell could have real staying power though my experience with models over the years has shown that they can overcook the potential of a cold spell.

I’ve lost count of the number of times when excited enthusiasts proclaim that an incoming cold spell is going to last as least three weeks; the reality being that the intensity of the cold has gone after four or five days. Cold spells since 2008 often arrive as a ‘blob of cold air’ from the continent that eventually gets ‘warmed out’; it’s been a very long time since we had a cold spell that’s been fuelled by a continual feed of air off the continent.

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By Saturday 00Z the really cold air aloft has gone. A typical 4-5 day event?

To illustrate my point I had a look back at every cold spell in this area of suburban east London since 1960. I weeded out the feeble efforts of the last few years by only considering spells where the maximum didn’t exceed 2.8C. The results spanned from the most recent cold spell of March 2013 to the mammoth 31-day Siberian blast that began on Boxing Day 1962.

In another blog I remarked how similar the recent pattern was to February 1962. This cold spell began on the 26th and lasted 9 days. Some 7cm of snow fell, this drifting in the wind, possibly making it seem worse with only 4 hours of sunshine which would have maintained any snow cover.

One of the snowiest cold spells happened in February 2009, eight days after an SSW event that lead to a polar vortex split. This four day spell saw a total of 26cm of snow fall.

February 1991 was even snowier, the intense cold lasting some 11 days.

Looking at other February cold spells a 7-day spell occurred in 1985 about a month after an SSW event.

Overall the median length was 5 days with an average of 6cm of snow and 8hrs of sunshine.

*A survey of winters ranked for temperature and snow can be found here.

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rank cold

 

1960

 

 

 

 

 

 

 

 

 

 

 

 

 

The cold spell of February / March 1962

The last week or so has been agony for model watching coldies wishing for a snowy end to winter.

27021962
The Synoptic chart for February 27th which saw 4 inches of snow in Stratford, east London

Solutions have often flip-flopped between a mild and cold outlook. Even this morning at 6 days out the GFS and ECM model temperature outcomes differed by some 20C, a choice between spring and deepest winter!

Sometimes you have to try and second guess what is going to happen by looking at previous patterns from years past.

Yesterday at Philip Eden‘s funeral I spoke to Woodford Green resident Ron Button. He pointed out how much this February reminded him of February 1962, a month that was non-descript for the first three weeks before turning very cold and snowy. Ron, who has kept a meticulous record of the weather ever since his interest was prompted by the severe winter of 1947, produced diaries of 1962 when he was living in Stratford. The entry for 26th / 27th read: “4 inches of snow with drifting”!

The March that followed was the coldest of the 20th century, ranking 10th in my list back to 1797, and 0.3C colder than March 2013. The fact that the monthly sunshine in 1962 was only slightly below average is testament to how cold the source of the air must have been. March 2013 was very dull by comparison!

An entry in London-weather.eu reads that March was colder than any of the previous 3 winter months: “The first three weeks were mostly mild and dry. It was often breezy which resulted in fewer than average night frosts. Frontal systems off the Atlantic passed through the London area, most of them weak though on the 12th, nearly 8mm of rain fell with southwesterly winds gusting to 57 knots. During the last week of the month, it became much colder, and on the 26th snow fell with the temperature not rising above -0.3C all day.”

There was no stratospheric sudden warming that winter and ENSO was neutral.

The winter of 1961/62 ranks only 14th in my list of worst winters, mostly because the core of the cold happened in March which is considered spring in meteorological circles

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march 1962 summary

The Snow Survey of Great Britain also makes interesting reading with these entries for February and March.

feb1962feb19622

march 1962

The severe cold spell of February 1991

The cold spell of February 1991 saw unusually deep snowfall in central London. The 20cm recorded at St James’s Park on the 8th was the greatest cover recorded at the site since the severe winter of 1962/63.

summ821991
Courtesy of the Met Office

My own memory of the event was that the synoptics evolved fairly quickly. I was away at university at the time and had to be back for a family event that weekend. After seeing a forecast predicting that a foot of snow was on the way I jumped on a train a day earlier than planned and returned to London. The following morning all hell had broken loose as deep snow paralysed public transport.

Snow fell on the following 6 days with no thawing as the temperature remained below zero until the 10th. The maximum of the 7th was -3C. By the 9th there was widely 20cm of level powdery snow lying. Getting around was difficult – I remember some drifts during walks into town were thigh high.

The month saw the three coldest February days of the last 60 years in central London.

By the end of the 19th all of the British Isles were snow free.

February 1991
February 1991 in suburban east London

The nine charts below show how a strong ridge of high pressure from an anticyclone over northern Sweden on the 5th brought very cold air and heavy snowfall over the following days.

These significant weather charts show the snow depths at noon from 6th to the 13th.

 

summ1021991
Courtesy of the Met Office

Ian McCaskill’s late evening BBC forecast on February 6th.

Francis Wilson’s breakfast telly forecast on February 7th 1991: “Temperatures rising from -11 to -5C. Depths in excess of a foot. It’s all downhill from now.”

Harlow, Essex, during the cold spell.

A 100% guaranteed snow risk for rest of winter

This weekend marks the halfway point through winter. Though December brought some snow January has been broadly average – really cold air has been absent with only one air frost recorded this month.

The cloudy, anticyclonic type weather is about to be replaced, however, with cold, polar maritime air this week set to flood down from the north-west.

Since December 1st, Wanstead has recorded 12 air frosts – about average. The coldest night was just -3.7C. The current mean temperature this winter to January 14th is 5.7C with rainfall 111mm – statistics that are remarkably similar to the winters of 2012/13 and 1990/91. Both those seasons were followed by cold late winters, February 1991 saw some of the deepest snow that I’ve ever seen in the south-east; the mean temp for that February finished 1.6C, the 14th coldest in the local series going back to 1797.

Using my method for finding patterns stretching back over 50 years to forecast this winter I picked out years that were +/- 10% of the 2017/18 total rainfall. From these I then weeded out the seasons where the average temperature was +/- 10% of the 2017/18 mean.

This gave a list of just two other winters with similar temperature and rainfall. Both winters had above average ‘snow lying’ days, the long terms average for this area being six.

Before any readers accuse me of going all Daily Express with the title of this blog I would emphasise that this piece doesn’t echo the latest long-term output from the models, which are in a state of flux, caused mostly by the evolution of the explosive cyclogenesis expected midweek. It is simply a reflection of what the local data is telling me.

A 100% guarantee of snow isn’t such a fantastical claim as, during the past 10 years, there is only one winter when no snow fell or was lying at 9am!

In terms of the rest of the winter, outside of the models, I would expect a couple more snowfall episodes, similar to the ones we had in January and February 2013. A repeat of February 1991, while not impossible, looks unlikely – there seems to be far too much energy coming from the Atlantic to allow the all important Scandinavian / Russian high to form and exert its influence far enough west for long-lasting cold and snow.

Record low pressure of February 25th 1989

The low pressure system that brought widespread rain and snow on Wednesday reminded me of another event where very low atmospheric pressure helped many low-lying areas experience heavy snowfall.

The unusually deep depression ran along the Channel on February 25th 1989.  Its central pressure of 948mb has been unequalled over southern England since, the value was some 13mb below the lowest on record at the Kew observatory.

Though temperatures reached around 6C in the morning they fell away as the day progressed. This and falling pressure helped turn the rain to snow.  Accumulations were fairly wet in the London area but further north over East Anglia some notable falls were recorded.

greenwich

MetO 25021989
Courtesy of the Met Office

GFS 25021989

summary 25021989
Courtesy of the Met Office
summary2 25021989
Courtesy of the Met Office

Hyperlocal forecasting of snow

The knife-edge situation for snowfall across England today set me off wondering if there was a way to forecast snow locally to a given location.

Temperature, dew point and altitude are all crucial to forecasting snow, as is precipitation intensity that can lower the wet bulb temperature, the ‘rain turning to snow’ scenario. Less well known is the role air pressure plays in these events, such as the one on December 10th which caught many forecasters by surprise by its intensity.

While various models continually improve in their skill at warning us of impending rain / snow events that often cause chaos, their resolution can still struggle and especially when the event more or less becomes a nowcast.

In the International Standard Atmosphere (ISA) there is a 1 hPa difference for each 30 feet vertical change in height in the lower levels.

If we take a forecast pressure as 970 hPa, the difference between standard conditions (1000 hPa) and 970 hPa is thus: 30 hPa

30 hPa * 30 = 900ft height variation.

You then consider local altitude: in my case 18m (59ft)

So 59ft – 900ft = 841ft  (256m)

A simple table, therefore, to consider air pressure is thus.

1000 hPa = 0m
995 hPa = 27.6m
990 hPa = 73.3m
985 hPa = 119.1m
980 hPa = 164.8m
975 hPa = 210.5m
970 hPa = 256.2m
965 hPa = 301.9m
960 hPa = 347.7m
955 hPa = 393.4m
950 hPa = 439.1m

Looking at the system today the pressure rose quickly after the centre passed, leading to a 1hr (at best) window of snow falling. As the pressure rose, and the temperature picked up only slightly the snow turned back to sleet and then rain.

pressure
Pressure rose quickly on the 27th, far more rapidly than the 10th – the pressure on this day stayed below 976mb from 8am until 3pm, allowing snow to accumulate. Temperature on the day was also around 0.5C cooler

 

pressure 10122017
Pressure fell to its lowest point 970.3mb at 1159z on the 10th

temperature

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The temp and dew point hardly moved from 9am

wet bulb

chilterns

http://weatherfaqs.org.uk/node/152

http://aeropowerflightschool.com.au/how-to-calculate-pressure-heightaltitude/

 

London Grimness index

The last three months in east London have been sunnier than average – hard to believe as we move into a regime of easterlies that will bring showers and anticyclonic gloom.

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A sunless outlook from the Met Office last December

With no sun and short days  it is no surprise that many are probably already feeling short-changed in the solar stakes.

A blog by XMetman on a ‘grimness index’ got me thinking how London fairs in terms of sunshine, temperature and rain in any given winter season.

Using the same criteria, and assuming that most people welcome sunshine, it can be seen that since the least grim winter of 2007-08, the season, over the past 10 years, has been growing steadily worse.

Screen Shot 2017-12-21 at 21.51.54The grimmest winter, considering statistics back to 1881, was, ironically, the 1978-79 ‘Winter of Discontent’ when, again ironically, ‘Sunny Jim’ Callaghan was in Downing Street as widespread strikes coincided with the coldest winter for 16 years. On looking at the Top 10 of grim winters it is striking how most coincide with depressing world events, the Great War, World War 2 and the Korean War!

grim index

The Tory ‘Crisis? What Crisis?’ campaign was probably my first memory of a general election broadcast.

December 10th 2017: rain turned to snow

Today’s snow came thanks to an area of low pressure that tracked further south than forecast.

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Wanstead Park

‘Xanthos’ brought several hours of precipitation, 21.3mm in all to 3pm. Associated precipitation started as rain in Wanstead at 4am, turning to snow at 8am and quickly settling on all surfaces. By the observation time at 9am about 1cm had accumulated as the snow turned heavier.

Snow continued to fall through the day though, because soil temperatures are still relatively warm, the snow thawed from below and accumulations locally were restricted to around 5cms.

There is still a full, slushy cover on lawns at nightfall.

met office fax 06z10122017

low xanthos

pressure 10122017
Pressure fell to its lowest point 970.3mb at 1159z
ground temps
Had 0-10cm ground temps not been so warm today’s snow would have been a lot less slushy here