May felt like an early start to the summer, with prolonged periods of warm, dry weather which culminated in thunderstorms at month-end. Large parts of the UK registered monthly mean temperatures more than 1.5°C above average. For the UK as a whole, rainfall was around three quarters of average with large areas of northern Britain registering less than half of average. In central and southern England, rainfall was above average and dominated by rainfall from thunderstorms in the last week.
Intense downpours yielded patches of more than 170% of average rainfall in parts of the West Midlands and south-east England. For the UK as a whole, spring (March-May) rainfall was near average, but there was a distinct spatial difference in rainfall anomalies.
The assessment is contained in the latest monthly hydrological summary for the UK, an authoritative analysis of the country's water resources status. The monthly summaries are produced by the National Hydrological Monitoring Programme, operated by the Centre for Ecology & Hydrology (CEH) in conjunction with the British Geological Survey.
With the continued dry weather and above average soil moisture deficits in northern Britain, river flows were generally in the normal range or below, whilst in southern Britain river flows were in the normal range and often above normal in the south-east. Due to the delayed end to the recharge season, groundwater levels were in the normal range or above for the time of year.
Reservoir stocks fell in the majority of index impoundments, substantially so at some in the north and west, although most remained only moderately below average for the end of May (Northern Command Zone and Daer were more than 10% below).
Summary author Nikolaos Mastrantonas from the Centre for Ecology & Hydrology said, "Given the above average spring rainfall, groundwater levels and reservoir stocks, the water resources outlook remains healthy for the summer in the south and east, a significant transformation from the situation at the start of 2018.
"Conversely, a continuation of dry weather in the north and west would accentuate current rainfall deficiencies and increase the potential for low flows and localised water resource pressure in the summer."
The monthly summary is a look back at hydrological events occurring in May 2018. Latest information on flood warnings is provided by the Environment Agency, Natural Resources Wales and Scottish Environment Protection Agency (flood warnings for Northern Ireland are not available).
The Centre for Ecology & Hydrology jointly operates the National Hydrological Monitoring Programme (for the UK) in conjunction with the British Geological Survey. NHMP scientists produce the UK Monthly Hydrological Summary which assesses rainfall, river flows, groundwater and reservoir levels. They also operate the UK’s National River Flow Archive.
The NHMP also has a remit to analyse major flood and drought events in the UK and analyse long-term trends in UK hydrological data. The UK Monthly Hydrological Summary is normally published on, or before, the tenth working day, of the following month. A Hydrological Outlook for the UK is also available, and is updated monthly. The latest Outlook can be viewed here.