September 2013


Period: From September 2013    Issue date: 06/09/2013

Summary

September river flows are likely to be in the normal range across most of the UK, but with a possibility of below normal flows in southeast England. Below-normal groundwater levels are also likely in parts of the southern Chalk, contrasting with other aquifers where levels are likely to be above average.  The three month outlook is for a continuation of this broad situation, but with a possibility of more widespread below-normal groundwater levels in southern aquifers, as a result of dry late-summer soils.

Rainfall

During September uncertainty is large, with above- and below-average rainfall both equally probable. For September-October-November as a whole the forecast signal is similar to climatology, but with slightly higher probabilities for near-average rainfall. The probability that UK precipitation for September-October-November will fall into the driest of our five categories is around 15% and the probability that it will fall into the wettest category is around 15% (the 1981-2010 probability for each of these categories is 20%).

River flows

August river flows were in the normal range across the majority of the UK, but moderately above average in some catchments in south Wales and northern England, and below average in parts of northern Scotland and southern and eastern England. The one-month-ahead outlook suggests that September river flows are most likely to be in the normal range across the UK. There is a marginally higher likelihood of below-normal flows in southern and eastern England, although the normal range remains the most likely outcome for this region. The three-month outlook is suggestive of stability in river flows over the next three months, with model projections continuing to favour flows being in the normal range across much of Great Britain; however, persistence of below-normal flows can be expected in some groundwater-dominated southern catchments. 

Groundwater

August groundwater levels were normal across much of the Chalk aquifer, but below normal in some boreholes in the southern Chalk. Groundwater levels in the Permo-Triassic sandstone were above average, and notably so in the northwest. Groundwater levels for September are likely to be normal in most aquifers, but below normal in some southern Chalk boreholes, and above average in northern Permo-Triassic Sandstones and Magnesian limestones. The three-month outlook is indicative of this broad pattern continuing – however, reflecting the dry late-summer soils, the outlook suggests below-normal levels will be more widespread across the southern Chalk and may extend to some southern Permo-Triassic boreholes, contrasting with the higher levels further north.