Hot spell is minnow among historic heatwaves

Heatwave hype has been building over the past few days and, for once, the media has been justified in its coverage of the current spell.

Temperatures over the past few days have been enough for the Met Office to trigger its Heath-Health Watch whereby the mean temperature (an average of the maximum and minimum) for two consecutive days reaches 23.5C.

A couple of years ago I compiled a list of heatwaves for this area back to 1852 where the mean temperature reaches at least 23.5C three days running. This spell, the only other June heatwave (the other being 1976!), only just scrapes into the list.

Although it felt hot this spell, in terms of Premier League quality, was more Middlesbrough than Chelsea.heatwave table

june heatForecasters say today is likely to be the hottest June day for 40 years though I’d be surprised if we exceed the 34.2C recorded on June 27th 2011. It would be the fifth day running that 30C has been reached in this area, enough to fulfil the World Meteorological Organisation’s (WMO) definition of a heatwave.

Whatever happens today the fact that the heat has been produced by a tropical maritime airmass is remarkable. With France and Spain recording some very high temperatures of late the same pattern in July and August could put more records at risk of being broken.

This week marks the 41st anniversary of the start of probably the finest British summer in history, but anyone hoping that yet another parallel with the 1970s is about to come true should take a look at the latest model runs.

june low
The latest GFS operational run suggests a much cooler, more unsettled end to June – a polar opposite of 1976…

 

 

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Poor start to June? That’ll be the Euro monsoon

Widespread heavy rain and gales across the UK have made the start of summer feel very unseasonal. But the conditions, which follow an extended period of mostly dry weather, are very typical for early June.

june rainfall with record
The pattern for June over the past 10 years reveals the month often starts unsettled before drying up in the final week

The ‘NW European monsoon’ is one of the most reliable ‘singularities’ on the annual weather calendar. Though it sounds very unscientific that the atmosphere can remember how it behaved on a certain date in previous years much statistical work over the past 170 years highlight tendencies for unusual weather at particular times of the year. These tendencies were first identified by the German climatologist, A.Schmauss, in 1938.

While the pattern isn’t set in stone statistics show that the probability of the euro monsoon occurring between June 1st and 21st is 77 per cent.

GFS rain to 15th
The GFS operational run suggests an unsettled outlook

One of the most notable inclement spells of weather in June happened during the D-Day landings in 1944.

With the changes in ice at the North and South Poles, together with the massive positive temperature anomalies last winter,  it would be thought logical that this would have some bearing on the general pattern this year. But polar ice is only one variable to consider when trying to predict the world’s climate.

 

 

May 2017: rather warm, average rain

The mean temperature for May 2017 of 14.7C was 1.6C above average; the warmest for six years and the 11th warmest in a local series going back to 1797.

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Sunset on May 1st 2017

Despite the first ten days of the month continuing the dry spell that has plagued London for months the total recorded finished above average: 62.5mm rainfall (122% of the 1981-2010), the wettest May for three years.

Though the month finished warm with some cracking sunny days the first two-thirds of the month were relatively dull: 157.1hrs of sunshine were recorded which is 86% of average, the dullest for 13 years.

I published my summer forecast on June 1st. It’s looking average.

Here follows the full weather diary for May. To view full stats follow this link:http://1drv.ms/1kiTuzv

1st: Cloudy all day with just a few bright intervals.
2nd: Sunny and clear morning, cloud bubbled up at noon. Cloud thickened with very light drizzle at 8pm.
3rd: Cloudy, dull with occasional very light drizzle.
4th: Cloudy and feeling chilly all day. Odd drizzle.
5th: Sunny start though cloud thickened quickly to leave an overcast though bright, breezy afternoon.
6th: Cloudy all day with a couple of bright spells.
7th: Cloudy and cool start, cloud decreasing after noon, eventually clearing to leave long sunny spells at 5pm.
8th: Cloudy and cool start – the cloud sticking all day and felt chilly.
9th: Cloudy and cool start, cloud decreased late afternoon but did not completely clear until 5am, saving area from frost. -2.8C in Benson!
10th: Sunny and mostly clear into the afternoon with patchy cirrus.
11th: Sunny and humid to start with much high-level cirrus. Breezy too. Showers overnight.
12th: Cloudy start, one or two light showers before a brief heavy one at 2pm. Sunny spells developed late afternoon.
13th: Cloudy start before cloud decreased to long sunny spells in afternoon. Cloudier overnight with showers between 7am and 8am.
14th: Sunny spells and breezy. Cloudier overnight with shower at 7am.
15th: Light rain and showers through the morning, clearing in the afternoon to sunny intervals.
16th: Bright start with cloud decreasing. Some very warm sunny spells – felt like summer for first time. Light rain in the evening though it didn’t start falling consistently until 2am with heaviest fall arriving at 4am.
17th: Bright start with plenty of sunny spells. Showery rain arrived at noon, turning more persistent at 4pm, clearing around 10pm.
18th: Bright start with lots of sunshine. Cloud began thickening at 1pm with bursts of light rain. Persistent rain arrived at 3.30pm and was still moderate to heavy at 10pm. Started petering out at midnight but then two further showers at 3am and 5am.
19th: Showery with longer spells of rain. This tended to clear to sunny spells. Thunder in the evening with a brief heavy shower at 6.23pm.
20th: Sunny, warm and breezy morning to 1pm.
21st: Sunny start with cumulus bubbling up. Sunny spells thereafter, growing longer as day wore on. Still 20C at 6pm during first Music in Wanstead Park. Overcast early but cloud started to break around 8am.
22nd: Sunny with decreasing cloud, clear by 1pm with just patchy cirrus.
23rd: Cloudy start with sunny spells mid morning, turning mostly cloudy again though.
24th: Bright start with variable cloud and sunny spells.
25th: Sunny with variable cloud at obs time, the cloud quickly clearing though a convection line could be seen in the west from Essex.
26th: Sunny and clear though breezy. Long sunny spells through the day. Feeling hot. Thunder and brief downpour between 8.30am and 9am.
27th: Sunny though only after front had cleared. Then alternating cloudy to sunny. More cloud late afternoon and not feeling as nice as forecast.
28th: Sunny start with variable cloud though the day. Rain in the evening before storm arrived at 1am, the rain turning heavier at 1.45am and lasting till just gone 3am.
29th: Cloudy start though soon turning brighter. Feeling humid with occasional very light rain.
30th: Cloudy but bright with the odd break. Feeling fresher
31st: Bright start with plenty of holes in the alto cumulus. Cloudier in the afternoon

Thunderstorm: June 2, 2017

It’s not often you experience what it’s like to be right under a cloudburst but June 2, 2017, provided a perfect example.

Some 31.2mm of rain fell in little over an hour accompanied by frequent thunder and lightning. Some flash flooding was reported and there was an 8C fall in temperature in 2 hours. The max rainfall rate was 108.4mm/hr at 3.49pm.

The rain started at 3.14pmScreen Shot 2017-06-02 at 22.52.40.png

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Convergence https://twitter.com/wanstead_meteo/status/870659046038286337
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Streams appear at Heronry pond in Wanstead Park https://twitter.com/wanstead_meteo/status/870660267818070016
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Charts for the storm. https://twitter.com/wanstead_meteo/status/870683164049424384

 

Elsewhere in London there were reports of ‘golf ball sized’ hail in Orpington. Here is a link to the pictures posted on ukweatherworld.co.uk https://1drv.ms/w/s!Au79-nZSg0ihlSXF5dTb08AQB74b

Tudor Hughes, of Warlingham, NE Surrey, said: “Heavy thunderstorm 1525Z..  Torrential rain and a short spell of large hail near the end. The stones were all spherical with a maximum diameter of 12 mm.  Temp fell “vertically” from 22° to 17°.  Just measured the rainfall – 12.7 mm, the bulk of which fell in about 7 mins.  Rain now (1555 Z) ceased but plenty of Cb still around.”

Smartie, a contributor to the Google group Weather and Climate, explained the science behind the storm.

“The cold front was still to the west, these showers were in a prefrontal trough. South-east London was in the inflow region of the convergence zone and strong initial updraughts and unimpeded ingestion of air from the SE likely accounts for the hail. Storms north and north-east of London likely benefited from urban heat.”

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The showers were in a prefrontal trough. Here is the 15 UTC WRF_LAPS objective 10m wind analysis. Red shades are convergence, blue-green divergence. SE London was in the inflow region of the convergence zone and strong initial updraughts and unimpeded ingestion of air from the SE likely accounts for the hail. Storms north and north-east of London likely benefited from urban heat. Chart courtesy of David Smart (UCL)
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This chart is a SharpPy analysis of the GFS analysis 12 UTC for the Heathrow grid point. Courtesy of Smartie
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The synoptic chart for 12Z shows the slack flow over London, the perfect set up for slow moving thunderstorms.