Category Archives: weather pattern

How cold will November end?

Twitter and other forums have been at fever pitch with excitement for a ‘nailed-on’ cold spell. 

But with many getting carried away over tales of a repeat of March’s ‘Beast from the East’ it is important to remember that snow in November is extremely rare in the south-east; despite predictions of the incoming upper air being very cold the ground is still very warm and will take time to cool.

However, as seen in November 1993, a week of hard frosts were enough to chill the ground enough for a decent week-long cold spell. Similarly, although the snow didn’t arrive in London until December, November 2010 was also cold enough.

The weather of late has been in an ‘average mood’ with the mean temperature in September and October both finishing at 0.2C below. The mean for the this month is currently (on the 15th) running 1.8C above average, therefore for the mean to finish the same as September / October would require a big cool down, as hinted by the models.

Will we see another an average, 1993 or a 2010 end to the month. My hunch is it will be something between the three.

Below is a graph that shows this November so far with average, 1993 and 2010 ends.
2018: 0.4
1993: -0.8
2010: -1.4

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November: an Indian summer before turning cold?

With the mean temperature of both October and September finishing 0.2C below average it is probably safe to say that the weather is in an average kind of mood.

Conditions during the first part of November look changeable, according to the Met Office’s 30-day forecast . After mid month, however, the agency says the forecast is uncertain.

Now that much of the UK has had its first frost any warm spell in November will, correctly, be called an Indian summer. A singularity called the St Martin’s Summer occurs in 66 per cent of years, occurring between 15th and 21st and peaking on the 18th.

And, as if by magic, the GFS model today has this chart for the 16th, an Atlantic ridge of high pressure with daytime temperatures about 6C to 8C above average. Though warm during the day I would imagine there being a risk of fog forming at night

16th

Beyond that there could be a tendency for much more unsettled weather at the end of the month. The early December storms singularity occurs in 98 per cent of years, starting between November 24th and December 14th, often peaking on December 9th.

November, the last autumn month, can often surprise with its extremes, though it can also often be characterised by days of anticyclonic gloom. The warmest, coldest and wettest November conditions in London back to 1959 can be found here.

 

 

Perfectly average months are extremely rare

Readers of this blog know that I often described a month as fairly average – a look at local statistics shows that many months come in very close to the monthly mean.

But when you combine mean temperature with rainfall that is 100 per cent average it becomes very rare and non-existent with 100 per cent sunshine hours considered.

Of the 2650 months since 1797 just one, February 1972, saw a mean temperature and rainfall precisely average. However, sunshine hours were just 40 per cent of the 1981-2010 average.

To extend the data I included monthly rainfall totals that were between 90 per cent and 110 per cent of average. Even then just five more months were revealed: September 1806, February 1876, January 1883, October 1886 and March 1972.

Although on paper February 1972 was perfectly average a look at the weather for the month reveals typically variable weather. The website London Weather explains:

The maximum temperature was only 3C on the 1st as mild Atlantic air slowly displaced the very cold continental air eastwards. There were outbreaks of rain, and during the remainder of the first week, although mild, it was often dull and wet. During the second week, active frontal systems crossed the country bringing strong winds but with sunshine between the rain bands. After mid month, east winds returned, and although not cold, it was frequently dull.

Considering the Met Office forecast the mean for this month, October 2018, is set to finish precisely average. However, rainfall looks set to finish well under the average for the time of year.

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 decline through the month, though around the 8th and 20th 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.
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

 

 

 

Wild equinox brings memories of childhood

Wild weather in the run up to the autumnal equinox was frequently a staple of the Septembers of my childhood but it has been absent in recent years.

Since 2013 I have been recording the ‘wind run’ data on my AWS, a stat generated by the amount of times the anemometer spins.

wind run

The results show there has been nearly two-and-a-half times more wind than the 5-year average.

Stormy weather over the equinox is one of the less frequent recurring singularities. The meteorologist Philip Eden a few years ago noted that ‘Mid-September storms’ during the period of 17th to 24th September had a frequency of 60 per cent.

20th
The wind run was the highest on 20th

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

Repeat of March 24th 2013 only 10C colder

The weather this coming weekend reminds me of a March weekend in 2013 when large parts of the south-east woke up to a covering of snow.

Conditions in Wanstead were very knife-edge, the cover being very slushy. I remember going for a bike ride on that Sunday morning. We left Aldersbrook in sleet, the cover grew thicker as we pedalled into Essex. By the time we reached High Beach near Epping there was 8cm of fresh cover. Conditions on the 2-hour plus ride were inclement to say the least – the temperature didn’t rise above 0C!

This weekend we have a similar set-up, only this time the upper air at 5,000ft is a good 10C colder – no knife-edges this time and, if the streamer gets going, many places should see 10cm of cover. Don’t put away that sledge just yet!

 

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.

Screen Shot 2018-02-22 at 09.01.04
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.

Screen Shot 2018-02-22 at 09.02.23
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.

spells

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

Screen Shot 2018-02-16 at 13.54.17

march 1962 summary

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

feb1962feb19622

march 1962