December was an extraordinary month in both meteorological and hydrological terms, with some of the most widespread and severe flooding witnessed in the UK. It was remarkably mild throughout the UK, and the warmest December in the Central England Temperature series (from 1659) by a wide margin, ~5°C above the 1971-2000 average. Exceptionally stormy and wet conditions across the north contributed to the wettest calendar month on record for the UK (in a series from 1910). Slow-moving low pressure systems (including the named storms ‘Desmond’, ‘Eva’ and ‘Frank’), driven by a sustained moist south-westerly airflow, brought prolonged heavy rainfall to northern and western areas. Several major flood episodes caused widespread and severe impacts: early estimates indicate 16,000 homes were flooded in England alone, while Scotland also suffered major impacts. ‘Desmond’ established new UK rainfall records over 24‑hour (341.4mm at Honister Pass, Cumbria) and 48-hour (405.0mm at Thirlmere, Cumbria) timeframes. Saturated soils resulting from substantial November rainfall exacerbated the fluvial flooding which followed in December. The spatial scale of sustained very high flows was remarkable; many large catchments in northern Britain recorded their highest ever peak flows and/or monthly mean flows. The three largest flows ever registered in river flow records for England occurred on the Eden, Lune and Tyne. The wettest parts of the UK were away from the main aquifers, although groundwater levels increased sharply in some boreholes in the north. Reservoir stocks in the Northern Command Zone (north-west England) doubled since the end of October, and end of December stocks for England & Wales were appreciably above average. With the exceptional wetness and flooding continuing into January in parts of northern Britain, the risk of further flooding in early 2016 remains high.
The full Hydrological Summary can be found here.