A Summer of Floods !

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Dr Rob Thompson, University of Reading

11October2017

A summer of Floods

We’ve entered autumn (or maybe winter given how it feels!), so its definitely time to look back on the summer we’ve had (or not as it feels). This summer has certainly seemed to have been quite convective, without being warm after June.

This article though will focus on the day of the most high profile UK event, on July 18th. This day the headlines were floods in Coverack, Cornwall… but they were far from alone, my home of Reading also got hit by a really very intense storm, and one that produced a huge amount of rain and statistics of note. Coverack suffered as a consequence of a very intense thunderstorm, combined with local orography leading to a lot of water pouring through the village. People had to be airlifted to safety and a huge amount of damage was done when flood waters flowing through the village exceeded a metre in depth. 

Coverack’s case is a fine example of the current state of forecasting… the morning’s weather forecast for the area that day was

“Thundery showers are expected to push north across southern parts of the UK through Tuesday evening. Although many places won’t see these showers, there is a chance of localised flooding of homes, businesses and susceptible roads. Frequent lightning may be an additional hazard with possible disruption to power networks. Similarly, but very locally, hail may cause impacts.”

And I can’t argue that was anything but a good forecast, the thunderstorms certainly materialised and were very intense, but small and hence very localised, perfect to demonstrate a problem for the scientists involved in research in to events such as this, observations are sparse. None of the weather stations in the area received a significant amount of rainfall. This is where radar comes in, radar provides areal coverage over a large range (250km from the radar in the UK operational system), with the UK network having coverage over the whole country, almost all with 1km resolution, we can see the rainfall falling even without the presence of rain gauges.

We can see from the radar images that the intense rainfall in the area of Coverack lasted for a few hours, storms triggering to maintain the rainfall rates for several hours.

But we can also see what was happening further East… that last radar image at 1900 shows a large convective system over the New Forest, heading towards Reading. The next few hours would be interesting for certain.

Where we do have numbers and gauge data is our own University of Reading atmospheric observatory. The rain in Reading was phenomenal, and I personally had the misfortune of being out when it occurred. It was like rain I’d previously only experienced in the tropics, driving conditions were horrendous, with incredibly reduced visibility and water simply unable to clear the roads quickly enough. Little appeared in media about the storm, yet there were certainly local impacts from flooding, as evidenced by this photo I took myself in South Reading:

The rain was of most interest to me, but the lightning was also impressive, both sheet and fork lightning. More than 100,000 strikes over the UK.

So I’ll focus on the rainfall rates, very high rainfall rates are not that uncommon, but lasting more than a few minutes is very unusual – and this storm was very, very unusual. The university’s tipping bucket rain gauge got 38.6 mm in the storm (which was later closely matched by the manual gauge), and 1.6 mm in the little shower 20 minutes before the big storm. The gauge experienced 35 mm of that total in 45 minutes, a phenomenal amount, averaging 47 mm/hr for three quarters of an hour. The University of Reading campus gets approximately 640 mm per year on average, so we had 5.5% of the annual rain in 45 minutes – that’s pretty incredible.

Events like these two are exactly what the FFIR programme is all about, about getting better warnings to the right places and more specifically.

 

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