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 30.2mm 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

Screen Shot 2017-06-02 at 22.55.05.png

Screen Shot 2017-06-02 at 22.56.22

Streams appear at Heronry pond in Wanstead Park https://twitter.com/wanstead_meteo/status/870660267818070016

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.”

Screen Shot 2017-06-02 at 22.42.07

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.

 

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