The outlook for May is for low river flows in south-east England, with flows elsewhere most likely to be below normal. Over the next three months, river flows are likely to be below normal or lower in south-east England and normal to below normal for the rest of the UK. The one- and three-month outlooks for groundwater levels are similar. Below normal or lower levels are likely in the Chalk of south-east England, particularly in the far south-east. Elsewhere, levels are likely to be normal to below normal, with the exception of above normal levels expected in southern Scotland.
April was an exceptionally dry month. Most of the UK received less than half the long-term average rainfall, and large parts of southern England registered less than 20% of average. Rainfall deficits now span the last nine months for the whole of the UK, and are particularly acute in the far south-east.
The rainfall outlook for May (released by the Met Office on 27th April 2017) suggests that below-average precipitation is considered more probable than above-average. For May-June-July, above-average precipitation is considered slightly more probable than below-average, on balance. Overall, the probability that the UK-average precipitation for May-June-July will fall into the driest of five equal categories is 20% and the probability that it will fall into the wettest of five equal categories is around 25% (the 1981-2010 probability for each of these categories is 20%).
Average river flows in April were below normal or lower for most of the UK, notably so in Northern Ireland, central and eastern Scotland, and parts of northern and southern England. A number of rivers in the English Lowlands registered no more than half their average flow for the time of year.
In May, river flows are likely to be below normal or lower for most of the UK. The signal is particularly strong in parts of the English Lowlands, where flows are very likely to be low. The outlook for the next three months is similar, with below normal flows more likely than above normal flows for most of the UK. The signal is strongest in the south-east of England, where average flows over the next six months are likely to be below normal or lower. In this region, average flows over both the one- and three-month timeframes are unlikely to be above normal even for the wettest rainfall forecast scenarios.
Groundwater levels in the Chalk aquifer of southern England were below normal in April. In the Permo-Triassic sandstones, April levels were below normal in south-west England but above normal or notably high in southern Scotland.
For May, the groundwater outlook is for normal to below normal levels for England and Wales and above normal levels for southern Scotland. Notably low levels are likely along the south coast and in the Carboniferous Limestone of the Peak District. In north-east England, levels are expected to be below normal or lower. The outlook is similar over the three-month timeframe. Below normal to notably low levels are likely in the Chalk aquifer, particularly in the far south-east, though in parts of central southern England levels are likely to return to normal. These outlooks are robust to the wide range of rainfall forecast scenarios.