Drought in eight areas of England

Rain possible but how much?

By Brian Gaze

Drought is declared

A drought has been declared by the Environment Agency in Devon and Cornwall, Solent and South Downs, Kent and south London, Herts and north London, East Anglia, Thames, Lincolnshire and Northamptonshire, and the east Midlands. More areas will probably be added later this month unless there is a major and prolonged change in the weather.

A parched looking Regents Park in London

As well as leading to the possibility of water rationing in parts of England the impact goes much further. For example, food security is threatened with this season's crops failing and farmers considering whether to plant ones for next year. Also, the threat of wild fires is adding to the woes and making areas of farmland potentially unusable.

Is rain on the way?

The short answer is that more changeable weather should bring some rain to southern Britain in the coming days. However, the longer answer is more nuanced. 

The chart below shows forecast rain totals for the ten days from Saturday 13th August. It is generated by using data from one computer model run and suggests totals of between 2mm and 28mm in the south.

Local variations are large and when looking this far ahead it is not sensible to rely on one computer model run. In the UK, confidence quickly begins to fall when forecasting more than a few days ahead and it is particularly true regarding rain amounts.

Nonetheless, if the predictions shown in the chart were correct then large parts of southern Britain would have only small amounts of rain. Certainly not enough to alleviate the drought, although it could help farmers and growers, so at least reducing demand for water a little.

How much rain in the south?

To gain a better insight into how much rain is likely in the short to medium term, ensemble computer models are preferred to one off snap shots or deterministic runs. An ensemble model is run many times to show a spread of outcomes and it helps to identify the most probable one.

The data table below is generated from the GEFS ensemble model. It shows the rain forecasts from all of the individual runs with each column representing a six hour time period between Monday 15th and Thursday 18th August. (Note: You can view the latest and full GEFS data tables for London and other locations at any time) 

In the data table different colours highlight the percentage of runs forecasting a given amount of rain. For example, the light grey is used to indicate completely dry conditions and the pink, the washout ones with over 20mm of rain.  

The data table suggests that the wettest conditions in the London area during this time period are likely to be on Wednesday 17th August. Most of the runs expect at least a little rain and about 30% of them are going for more significant amounts.

However, it isn't looking particularly wet and the suggested amounts would do nothing to ease the drought. Could there more be significant rain in the following week?

Settled weather to return in late August?

There are signs of high pressure building back from the southwest during the last third of August. If it happens the southern half of Britain could expect drier and warmer than average weather to return. In fact, there is a possibility of it turning very warm or even hot again. 

The ensemble chart above shows the forecast pressure anomaly for the period between August 23rd and 28th. The orange shading over the UK indicates higher than average pressure with the biggest anomaly in southern and eastern England.

The drought continues

Taking all of the data into account the most likely scenario at the present time is for the drought conditions to continue. Significant rain is possible in parts of southern Britain in the short to medium term, but much of it will come from thundery downpours which lead to big local variations. The baked ground and short but intense bursts of rain mean that a lot of run-off will happen with less water being absorbed than would be the case with prolonged and steady outbreaks of rain.

The key question will probably be how weather patterns develop through the autumn. If high pressure declines and more unsettled conditions become established the rain shortfall may start to be reduced. However, in recent decades September has often become more an extension of the summer than the start of the autumn, so the omens may not be good.

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