July began with heat-wave conditions, but the majority of the month was distinctly autumnal, as an unseasonal southerly track of the jet stream contributed to exceptionally unsettled conditions. July was cool, windy and very wet (especially in the west). Overall, it was the wettest July since 1888 in England and Wales and a third wet July in succession; only pre-1800 records (when data are less reliable) contain a wetter 3-year sequence of Julys. Relatively dry conditions had prevailed through the first half of 2009. Consequently, in western areas, the high rainfall caused a rapid decline in soil moisture deficits (SMDs) and a brisk increase in river flows, leading to exceptional monthly runoff totals and new July peak flows in some index rivers. Fluvial flooding was generally localised in extent and impact, but flash flooding was frequent and widespread. The wet weather resulted in substantial inflows to many reservoirs; only Rutland, Bewl and Ardingly had below average stocks, and overall stocks for England & Wales are 10% above average for the start of August. Soils in aquifer areas were still dry enough to inhibit recharge, so groundwater levels remain largely within the normal range. With the outlook for the remainder of the summer suggesting further unsettled weather, and early indications for above average winter rainfall, it is likely that seasonal recovery in runoff and recharge will begin considerably earlier than usual.
A prolonged arid episode, which had persisted from mid-June in some areas, continued into the first few days of July. Associated with the heat-wave, thunderstorms brought torrential downpours to some localities, particularly to western Scotland and northern England, with 53mm falling in 40 minutes at Copley (Durham) on the 2nd. From the 5th, a series of slow-moving low pressure systems crossed the British Isles, bringing prolonged and heavy rainfall which triggered widespread summer spates. Notable 24- hour totals were registered in localities as far apart as Hastings (90mm on the 7th), Cardinham (Cornwall) and Chillingham (Northumbria) which received 94mm and 84mm respectively on the 16-17th. The sustained frontal rainfall contributed to exceptional monthly totals; all regions received substantially above average rainfall, and totals were twice (three times in parts of the west) the average across upland England, Wales and eastern Scotland. South West and North West regions registered their highest July totals in records from 1914, whilst, provisionally, Bude (Cornwall) had its sixth wettest month since 1880. Correspondingly, short-term (3-5 month) rainfall accumulations were appreciably above average everywhere except the South East; Scotland experienced its second wettest March - July total (after 2002) in a 141-year series. Rainfall for the year so far is now above average across most of upland Britain, further underlining the exceptional nature of the recent rainfall; the late spring - early summer was generally dry in England and Wales, so these accumulations reflect a disproportionate contribution from July.
Seasonal river flow recessions were terminated abruptly in early July in a majority of responsive index rivers, as soils wetted up following successive waves of frontal rainfall. Flood alerts were common as river levels rose rapidly. A majority of incidents were caused by flash flooding, frequently associated with convective cells, and often focused on urban areas, including parts of Glasgow, Belfast and London (where flooding from overwhelmed drainage caused transport disruption on the 7th). High river levels caused localised flooding in western England, south Wales and north-east England - provisional data suggest the Wear reached a new maximum level in a >50 year record, causing flooding in Durham. New maximum peak flows for July were recorded in some northern and western index catchments (e.g. the Teifi, Taw, Dove and Whiteadder), whilst many more approached previous maxima. The Warleggan (Cornwall) registered its second highest peak flow (for any month) in a 40-year record. With the exception of south-east England and north-west Scotland, monthly runoff totals were notably high (exceptionally so in some catchments in Cornwall and south Wales), leading to above average runoff accumulations in the short term. Over the longer-term (> 3 months), runoff is generally in the normal range - below average accumulations in some catchments reflect the dry antecedent conditions. For the third year in succession, high river flows have been a feature of the summer months, although in 2009 the runoff has (so far) been concentrated in July in much of the country, whereas in 2007 & 2008, it extended over a much longer period.
Following the relatively dry spring, above-average soil moisture deficits (SMDs) extended across much of the UK at the end of June. The exceptionally wet July has substantially altered this picture; deficits were largely eliminated in western areas, which experienced the brunt of the rainfall, and were reduced in some outcrop areas to the east. SMDs for July for England & Wales as a whole were comparable to those in 2008, but much higher than in 2007. There is no close modern precedent to the 3-year sequence of wet end-of-July soil conditions. SMDs were still higher than average across much of eastern England, and sufficient to limit the potential for recharge from the sustained July rainfall. Furthermore, the most exceptional rainfall fell away from the main aquifer areas. The groundwater outlook is therefore broadly similar to the situation in June; levels in most aquifers are close to their seasonal average although, in the southern Chalk and some limestones, levels are below average where rainfall accumulation has been lower. Any groundwater response to the July rainfall is likely to take several weeks to become apparent, although the sustained rainfall contributed to rises in level at Killyglen (Chalk, Northern Ireland) and in some western Permo- Triassic Sandstone boreholes; Bussels No 7a is likely to be at a record monthly high in August.