May was sunny, warm and largely dry. The mean temperature was 1.3°C above the long-term average (the fifth warmest in a record from 1884) and, despite downpours in the last week, the May rainfall was 72% of the long-term average for the UK. May rounds off an exceptional spring, with sustained warmth and rainfall deficits that have, in turn, led to extremely dry soils. Extensive wildfires were reported (e.g. across western areas on the 5th‑9th) and there were wider impacts on agriculture and terrestrial ecosystems. Average river flows for spring were the lowest on record for many rivers across northern and western Britain, with localised environmental impacts (e.g. fish rescues and algal blooms) as well as reported impacts on navigation. Groundwater levels for May were in the normal range or below normal across southern and eastern areas but exceptionally low in some northern and western boreholes. In much of northern Britain, the dry spring adds to longer-term rainfall deficiencies, and these are reflected in the end-of-May reservoir stocks. Combined stocks for England & Wales were the lowest on record (a series from 1990) while stocks were more than 20% below average in some impoundments (e.g. Derwent Valley, Bradford Supply, Celyn & Brenig, Elan Valley) and 33% below average for the Northern Command Zone. Drought status was declared in north-west England on the 29th, with many other regions on alert (‘prolonged dry weather’ in England and Wales; ‘moderate scarcity’ in Scotland). The late May rainfall and a wet start to June has brought some respite. Nevertheless, summer begins with depleted river
flows, groundwater levels and reservoir stocks. This heightens the risk of further dry weather impacts, particularly as climbing evaporation rates limit the effectiveness of rainfall through summer. The latest Hydrological Outlook indicates that below normal or lower river flows and groundwater levels are likely to persist through summer across large parts of the country, although there is high uncertainty in seasonal forecasts at this time of year.
