With above average rainfall in the south-east in May after a prolonged period of below average rainfall, the outlook is for normal to above normal river flows across the UK for June. However, this period is expected to be short-lived, and river flows over June-July-August as a whole are likely to return to being normal to below normal. Groundwater levels in the south-east of England are likely to be below normal to notably low over the next one to three months, whilst levels in southern Scotland are likely to be above normal or higher over this period.
Rainfall across the UK has been average in total for May. However there has been a strong north-west to south-east gradient, with the south-east seeing above average rainfall and the north-west seeing below average rainfall. In parts of North Wales, Northumberland and Scotland rainfall was as low as 30% of the 1981-2010 average, whilst parts of East Anglia, the south coast and the Humber estuary saw over 150% of average.
The rainfall outlook for June (released by the Met Office on 25th May 2017) is for above average precipitation being moderately more likely than below average. For June-July-August the chances of above and below average precipitation are fairly balanced. The probability that UK-average precipitation for June-July-August will fall into the driest of five equal categories is between 15% and 20% and the probability that it will fall into the wettest of the five categories is around 20% (the 1981-2010 probability for each of these categories is 20%).
Following above average rainfall across the south-east in May, river flows in many catchments increased from notably low levels in April. Normal to notably low flows were observed across England and Wales and notably to exceptionally low flows were seen in north-eastern England and East Scotland, with record lows for May recorded in the English Tyne and the Spey.
Despite below average rainfall in the north-west in May, the above average rainfall forecast for June with heavy rainfall already seen in early June across northern England and Scotland, indicates that June river flows are likely to be normal to above normal for the whole of the UK. This is likely to be a brief interlude however, as the prolonged period of below average rainfall that has predominated since last autumn has depleted soil moisture and groundwater storage. Therefore, the outlook for the next three months is for river flows to be normal to below normal across the UK.
Groundwater levels in the Chalk aquifer of southern England were below normal in May, with some notably low levels observed. In the Permo-Triassic sandstones, levels were below normal in south-west England, normal or below in central England but above normal or notably high in southern Scotland.
For June, the groundwater outlook is for below normal levels in most of the Chalk. While Chalk in the south-west may see a slight trend to normal levels, areas of notably low levels are likely to persist in the north of London and the south-east. This pattern is likely to continue over the next 3 months. Permo-Triassic sandstones will stay in their current range. In other aquifers there is considerable variability, but normal to below normal levels are expected in the limestones of Wales and the Midlands.