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
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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

temp trace 10122017
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/

 

Snow for day after Boxing Day?

The models are showing another knife-edge situation for snow – similar to the ‘rain turning to snow’ event earlier this month. But there are a number of factors working against it this time round, at least for most of us who live between sea level and 30 metres.

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The highest resolution model AROME shows snowfall rate but I would expect that only high ground (100m +) will see any accumulation

The low level supply of air off the continent is much milder this time round. Hamburg and northern Germany on the 9th was some 5C colder than currently.

The air pressure, crucial to bringing that snow line down lower, is forecast to be around 10mb higher this time.

And even if we see settling the soil temps, after the recent mild spell, are still 5C – 10C down to 10cm… Screen Shot 2017-12-26 at 11.46.19

wet bulb

chilterns

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.

Wanstead fairs well in London cold rush

Since the early hours of Sunday I’ve been tracking official sites around London to see how Wanstead fairs in terms of retaining cold temperatures and snow.

temp trace

The results, comparing hourly obs over 62 hours, show that Wanstead was a degree cooler than St James’s Park, half a degree cooler than Heathrow and marginally cooler than Northholt. Only Kenley, with its whopping 170m of altitude, was cooler.

While half a degree doesn’t sound much it can make all the difference, as we saw at the weekend when some areas recorded inches of snow while others barely a centimetre.

Though the snow has pretty much vanished now it was evident yesterday that the area was far more wintry neighbouring Forest Gate, Leyton and Stratford which had pretty much lost all their cover.

A report on the snow event can be found here.

* For anyone who has a fascination for the weather and whether their back garden in the Counties Estate is colder than their mate’s in the Warren Estate I notice that weather stations are now even cheaper and you don’t need to connect to the net via a dedicated PC. A unit from Ventus (the W830) allows you to connect to the amateur weather network Wunderground. The internet dealer Weatherspares has them on offer. (NB. I am not on commission but would be happy to offer anyone advice who wants to buy and set one up.)

wanstead
Wanstead, Aldersbrook, the Flats and Wanstead Park are all part of a ‘green island’ in east London that tends to hold the cold better than surrounding built up areas

Dreaming of a white Christmas?

I’m not. And it is not because a glimpse at the latest GFS operational run at T+324z (on the 12th) reveals that many could be sitting down to their festive lunch in mild Atlantic air, with possible frontal rain bringing a miserable afternoon with heavy rain in the evening.

Cold spells in early to mid December often end around the 20th, a momentary change to a more mobile regime. Even the cold winter of 1962/63 saw this mid-December warm up with the snow not arriving until Boxing Day.

I will be keeping a daily eye on the GFS and adding to this blog to watch the daily twists and turns, adding to this blog from here.

Just over 24hrs away now on Sunday 24th and the weather tomorrow is again looking mild,in London, about 11C and breezy at lunchtime with rain spreading in very late evening. Best chance of any flakes over the high ground of Scotland.

Screen Shot 2017-12-24 at 08.00.37

On Saturday 23rd, just 54hrs away now, and the GFS op run is again showing the same change to cooler air albeit slightly delayed to previous runs. Temps in London about 10C / 11C, breezy – any rain not arriving to well into the evening. Best chance of any UK snow is Glasgow, into the evening as the cold front clears – and obviously flakes on high ground in the north.

Screen Shot 2017-12-23 at 09.53.24

On Friday 22nd, less than 90hrs away, the GFS is again showing the UK on the cusp of a change. Conditions in London look very mild 11C – 13C before dropping back to 5C after a spell of rain in the evening. Best chance of any snow over the highest points of Scotland.

Screen Shot 2017-12-22 at 08.42.07

Down to 108hrs to lunchtime on the 25th and the situation still shows the weather regime on the cusp of change with the anticyclone over France pulling away south. High temps in London about 9C or 10C with rain late in the evening. Snow risk across the Highlands as colder air digs in. Screen Shot 2017-12-21 at 08.35.34

On Wedesday 20th, the synoptic situation on 25th looks on the cusp of a big change to something more unsettled as the high pulls well south of its current position. A heavy band of rain is approaching from the west though it would be well into the evening before it affects London. Best chance of any snowfall if you fancy a flutter is Glasgow.

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On Tuesday 19th, Christmas lunchtime appears to be the transition day from the present high pressure dominated weather to something cyclonic. In London a dry, cool and cloudy start will be quickly replaced by a spell of rain that clears eastwards. Max 11C, min 3C as the front clears. Best chance of snow looks west coast of Scotland and Pennines.Screen Shot 2017-12-19 at 08.28.58

A week from today and Christmas day will have already dawned. The GFS 00z op presents an unsettled outlook with a transition from mild to cold; London looks warmest at 9am before a cold front sweeps SE’wards. Best chance of anything wintry falling: west coast of Scotlands and Pennines. Over the last couple of years the pattern has already been locked in to mild but this year’s constant ups and downs suggests the weather type  it is all still to play for. Screen Shot 2017-12-18 at 08.13.28

On Sunday 17th, the weather is looking less settled than yesterday with a weak high pressure much further south over the Pyrenees. A cool NW’ly regime with upper air around -5C, allowing any precipitation to be wintry. A chance of snow west coast of Scotland, Pennines and north-west Wales.

Screen Shot 2017-12-17 at 12.17.56

On Saturday 16th, weather on the 25th is under the influence of high pressure centred over Brittany. Quiet weather with a high of 6C or 7C after an overnight frost. There appears little chance of anything wintry falling as the source of the upper air is all the way from the Azores.

Screen Shot 2017-12-16 at 09.46.47

On Friday 15th, Christmas day lunchtime again looks like a day in between a relatively settled mild pattern and a cold, unsettled regime with potential for slider lows after Boxing Day. The 25th itself again looks chilly and nondescript. A high of 6C or 7C, light winds and potential for some rain later. Things are in a state of flux so I’d expect detail to change again tomorrow.

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On Thursday 14th, midday on 25th is looking fairly non-descript. High pressure sits over the near continent with a declining ridge up to Scotland; the UK sits in a battleground with cold air over the near continent and mild air trying to push in from the Atlantic – most of the UK is just on the chilly side. The best chance of any snow is the east coast of Scotland. Still all to play for but it was this date over the past couple of years that the models started to get a good handle on Christmas day proceedings.Screen Shot 2017-12-14 at 14.35.19

Screen Shot 2017-12-13 at 09.52.12On Wednesday 13th I notice that the Daily Star reckon we are ‘odds on’ for a white Christmas, yet I can’t find a bookie anywhere to ‘lay’ these odds; at 12 days out there’s no way any model could be ‘odds on’.

This morning’s (12th) 00Z operational run of the GFS has London at 10C with a chance of rain. Best chance of any white stuff probably Aberdeen.

Screen Shot 2017-12-13 at 09.57.05

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December 10th 2017: rain turned to snow

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

Screen Shot 2017-12-10 at 17.38.33
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

A repeat of December 1990 this weekend?

The outlook for possible snow over the weekend reminds me of a similar synoptic set-up in December 1990 that left large parts of the Midlands northwards covered in deep snow, but that delivered only cold rain to the London area.

The situation in 1990 is explained on the excellent Booty Weather site.

“A low formed dramatically over central England on the 7th, large quantities of rain, turning in many places to snow, fell on its western and northern flanks. On the 7th and 8th very heavy snow fell over northern England, Wales, the Midlands and south west England, with heavy drifting in gale force winds, causing considerable disruption to traffic and cutting power lines.

coventry“The snow did not freeze, however, but melted very rapidly during the next few days, as the temperature rose a little. By late on the 8th, many parts of the Midlands had 20cm or more of lying snow. Acocks Green, Birmingham, reported 42.5cm on the 8th. The Peak District had 38cm at Middleton and 25cm at Winksworth. Newcastle under Lyme reported 28cm, and many other places had more than 20cm. Drifts up to 60cm on motorways in the Derby area, and at Carlton in Coverdale, near Leyburn, a report of 240cm. Snowfall on the 9th in the Dorchester area in 1990 was the heaviest pre-Christmas fall in that area since 9 December 9th, 1967.”

My stats in London suggest that the system was a bit of a non-event here. Another account of December 1990 is covered here.

rain

 

met office
Met Office synoptic charts, general situation and surface obs for December 8th 1990

story

Constable’s fair view of Waterloo Bridge

A sketch of Waterloo Bridge by John Constable sold for £2.3m as auction this week.

The canvas, confirmed as the first prototype for “The Opening of Waterloo Bridge”, the celebrated work by one of Britain’s greatest landscape artists that hangs in Tate Britain. was discovered hanging in the hallway of a private home.

It is an excellent example of how Constable managed to harness the realistic detail of skies of the period, a detail that was overlooked by artists before him.

1817Sketched shortly after his 41st birthday on June 18th 1817, the scene depicts a pleasant summer afternoon, echoing that day’s meteorological observations in Luke Howard’s Climate of London: a high of 26C with light SE’ly winds, the start of a fine spell of weather.

It is thought that Constable, who in 1817 moved from his native Suffolk to London, had been greatly influenced by Howard’s work on naming the clouds a decade or so earlier. It was the same year that Howard gave his Seven Lectures In Meteorology, the subject matter of which was later published as the first meteorological textbook in 1837.

The sketch, which shows ceremonial barges leaving the shore at Whitehall to celebrate the opening of the new bridge, with St Paul’s Cathedral and the spires of Wren’s City churches visible beyond, fetched £2,289,000, far the figure it was expected to receive, between £1m and £1.5m, at Sotheby’s, on Wednesday.

waterloo
“The Opening of Waterloo Bridge seen from Whitehall Stairs, June 18 1817,” oil on canvas, by the English artist John Constable. 51.5″ X 85.83″. Courtesy of the Tate Britain, London.}} |Source=http://www.the-athenaeum.org/

 

November 2017: below average, dry

November 2017 was another dry month. Just over 33mm of rain was recorded, the driest November since 2011 and the 40th driest since 1797.Screen Shot 2017-12-02 at 17.23.28

The monthly mean finished 7.1C, 1C below average and only the fourth cooler than average month this year.

Some 71 hours of sunshine were recorded, 122 per cent of average – the sunniest November since 2007.

Air frosts: 6, Ground frosts: 17, snow falling: 1 (just a few flakes on the 30th, the first November snow in years.)

The start of December looks like it start mainly dry, an anticyclone over the Atlantic that has brought respite from last week’s cold weather will pull away into the continent to be replaced by low pressure. This depression will track across northern England gradually pulling cold, Polar air across the UK on Thursday.

The models go against what has followed dry Novembers in the past – patterns overwhelmingly suggest that December will be average to rather mild and on the dry side. Winter is looking average overall.

Here follows the full weather diary for November…Full stats for the month here:http://1drv.ms/1rSfT7Y

Summary for November 2017
Temperature (°C):
Mean (1 minute)  7.4
Mean (min+max)   7.1
Mean Minimum     3.5
Mean Maximum     10.6
Minimum          -2.7 day 24
Maximum          15.5 day 22
Highest Minimum  12.3 day 21
Lowest Maximum   3.7 day 30
Air frosts       6
Rainfall (mm):
Total for month  33.3
Wettest day      8.5 day 10
High rain rate   12.1 day 22
Rain days        14
Dry days         16
Wind (mph):
Highest Gust     22.1 day 23
Average Speed    2.7
Wind Run         1952.8 miles
Gale days        0
Pressure (mb):
Maximum          1029.6 day 17
Minimum          985.2 day 22
Days with snow falling         1
Days with snow lying at 0900   0
Total hours of sunshine        71.2
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