The sub-Arctic weather conditions of December continued into January with average temperature for the winter thus far being extremely low: Scotland reported its coldest Dec/Jan in a series from 1914; Northern Ireland its coldest since 1962/63. The impact of the snow and ice was widespread and severe (e.g. on transport, commerce, education and agriculture). The provisional UK precipitation total for January is around 70% of average but with a clear east to west gradient reflecting the dominant synoptic patterns. Limited rainfall and frozen headwater catchments restricted inflows to some reservoirs, particularly early in the month but reservoir stocks in index reservoirs generally remained above or within the normal range. In a few areas stocks declined (e.g. in north-west England and at Farmoor - where poor water quality curtailed abstractions). Nonetheless, overall stocks for England & Wales exceeded the average. After steep recessions early in the month, river flows generally increased briskly, snowmelt being a contributory factor in many catchments and flood warnings were widespread in mid-month. Frozen ground limited infiltration during an appreciable part of January but generally groundwater levels continued to rise (in some cases reflecting healthy early winter recharge), exceeding the monthly average in the majority of index wells and boreholes.
With the Jet Stream much further south than usual, synoptic patterns were conducive to northerly and north-easterly wind directions through much of January. Correspondingly, temperatures were exceptionally low and the UK registered its coldest January since 1987; there was a modest respite in mid-month when the passage of Atlantic frontal systems produced significant rainfall (e.g. 35mm at Spadeadam, Cumbria on the 16th). A few daily totals in January exceeded 50mm but in many cases they probably included melting snow from earlier falls. As in December, much of the January precipitation fell as snow (in a few localities in southern England accumulations reached 20-30cm by the end of the first week, whilst depths exceeded 50cm in parts of the Cairngorms). The high proportion of snow implies that the initial national and regional 'rainfall' totals should be treated with caution; they are likely to be underestimates. The spatial distribution of the January precipitation reflected the predominance of northerly and easterly airflows. A few western areas (e.g. parts of Northern Ireland and western Scotland) registered less than half the January average rainfall whilst some catchments exposed to the prevailing winds (e.g in north-east Scotland and the North York Moors) reported well above average precipitation; much of central southern England was also relatively wet. In the three- month (Nov-Jan) timeframe, parts of western Scotland have been notably dry but all other regions have registered above average rainfall - parts of southern England being particularly wet.
Most UK rivers reported a wide range of flows during January. Early in the month, frozen headwaters contributed to notably low flows in Scotland; in the 2nd week, the Earn recorded its lowest January flow (provisionally) in a series from 1948. Many northern headwaters were frozen and ice flows on rivers were common (note: such conditions increase the uncertainties associated with river flow measurement). Towards the end of the 2nd week frontal rainfall, augmented in many areas by snowmelt, triggered rapid recoveries in runoff rates - spate conditions (and Flood Watches) were widespread in mid-month. Near-bankfull flows and moderate floodplain inundations were common and some notable flows were reported. On the 16th theTyne (Lothians) registered its 2nd highest January flow in a 45-yr record. Later in the month, the slower-responding Yorkshire Derwent reported its 2nd highest January peak since 1982. This high runoff interlude ensured that monthly runoff totals were generally close to, or above, average throughout much of eastern, central and southern England. In north-east Scotland the Deveron catchment reported its highest January runoff in a 50-yr series. By contrast, runoff in much of western Scotland was only 30-40% of the January mean; the Carron and Nevis both recorded their 2nd lowest January runoff in records of around 30 years. Across the UK, runoff accumulations in the 3- and 12-month timespans are generally within, or above, the normal range but relatively low in a few, mostly Anglian, index catchments.
Over almost the entire country soil moisture deficits were close to zero throughout January and precipitation across most major aquifer outcrop areas was in the 70-130% range (but lower in the west). Whilst, as in December, frozen ground limited infiltration for much of the month, accumulated recharge totals since October are generally very healthy particularly in north-eastern and southern England. This is reflected in the notably steep rise of groundwater levels at Chilgrove (and other boreholes in the South Downs) where late-January levels were exceptionally high - standing around 40 metres above the 2009 minimum level (registered in the autumn). Elsewhere in the Chalk, levels are mostly a little above the mid-winter average but somewhat below in parts of East Anglia (see the hydrographs for Dial Farm and Washpit Farm) where the seasonal recovery didn't begin until very late in 2009. Most index wells in the Permo-Triassic sandstones and the limestone aquifers are also near to, or a little above, average for the time of year; a particularly brisk recovery was recorded in the Magnesian Limestone (of the North York Moors) at Swan House. With soils still close to saturation, the groundwater resources outlook remains generally healthy but further late-winter replenishment would be particularly useful in some superficial deposits (e.g. in western Scotland) where they can be significant in a local water supply context.