September 2015


Period: From September 2015      Issued on 11.09.2015 using data to the end of August 2015

Summary

The outlook for September is for river flows to be in the normal range across much of the UK, although the far south of England is likely to see above normal flows, mainly reflecting the influence of the exceptionally wet August in this area. Normal flows are the most likely outcome over the next three months. For groundwater, normal levels predominate in the southern Chalk, again partly reflecting the wet August, while below-normal levels persist in parts of north-east England. The three month outlook suggests mostly normal to below-normal groundwater levels. Autumn rainfall will be influential in determining the longer-term outlook but the recharge season will thus commence from a normal or moderately below normal baseline.

Rainfall

For September and September-October-November as a whole, the forecast for UK precipitation has a wide spread and its distribution is indistinguishable from climatology. The probability that UK precipitation for September-October-November will fall into the driest of five equal categories is around 20% and the probability that it will fall into the wettest of the five categories is between 20 and 25% (the 1981-2010 probability for each of these categories is 20%). 

River flows

August saw below normal flows in some catchments in central England, but normal to above normal flows dominated elsewhere, with above normal flows concentrated in the far south of England, Wales, central Scotland and Northern Ireland. For the one month outlook, across the majority of the country the most likely outcome is normal flows, with the exception of some isolated catchments in central England where below normal flows may persist, and the far south of England where above normal flows are most likely; in south-west England there is an increased likelihood of flows being notably high or higher. The three month outlook is for normal flows across most of the UK, and normal to above normal flows in south-west England. 

Groundwater

Groundwater levels in August exhibited a very similar pattern to recent months: below normal levels in parts of the southern and eastern Chalk contrasted with normal levels elsewhere in the aquifer. Levels in other aquifers were mostly normal, with above normal levels in some western aquifers, and variability in the Permo-Triassic boreholes where exceptionally high levels persist in Scotland. The one month outlook indicates a broadly similar pattern but suggests that levels in the southwestern chalk will return to normal. The three month outlook suggests this may be a shortlived effect, and a more mixed picture of normal to below-normal levels emerges across the southern Chalk. However, below normal levels in the north-east Chalk are likely to persist. Levels in other aquifers are likely to be normal or below-normal, with above-normal levels in parts of the Permo-Triassic sandstone.

Further information - September 2015

Comprehensive further information that supports the production of the Hydrological Outlook UK is available here.