October 2015


Period: From October 2015      Issued on 13.10.2015 using data to the end of September 2015

Summary

The outlook for October is for normal to below normal river flows and groundwater levels across most of the country, but with normal to above normal flows and levels in southern England. This picture reflects rainfall patterns over recent months; the outlook in southern areas is a legacy of the wet August, whereas the increased likelihood of below normal flows elsewhere reflects the very dry September, which was notably dry in the north-west. The three month outlook suggests a similar contrast between southern England and areas further north, but with a greater prevalence of normal flows and groundwater levels in the latter. At this transitional time of year, uncertainty is high for the three month outlook, as late autumn rainfall will be highly influential.
The three-month rainfall forecast suggests wetter than normal conditions are more likely over this timeframe.

Rainfall

For October, the forecast for UK precipitation has a wide spread and its distribution is indistinguishable from the usual range of conditions at this time of year. For October-November-December precipitation is more likely to be above-average than below-average. The probability that UK precipitation for October-November-December will fall into the driest of five equal categories is 15% and the probability that it will fall into the wettest of five equal categories is 35% (the 1981-2010 probability for each of these categories is 20%).

River flows

River flows in September displayed a contrast between southern England, where river flows were above normal in most catchments (largely following the wet August), and the rest of the country which saw normal to below normal flows, with some notably low flows in many northern and western catchments. The one month outlook is for a broadly similar pattern: flows are most likely to be normal to above normal in southern England, whereas elsewhere flows are likely to be normal or below. For the three month outlook, normal to above-normal flows are likely to continue in southern England. Elsewhere, the normal range is the most likely outcome, although there is high uncertainty, with a wide range in the possible outcomes.

Groundwater

Across the chalk aquifer, levels in September were below normal in central southern England and in much of eastern England, and normal elsewhere. Levels in other aquifers were mixed, mostly normal to below except for above normal levels in parts of the Permo-Triassic sandstone and Carboniferous limestone. Groundwater levels are expected to be normal to below normal over the next month, but with above normal levels occurring in parts of the southern Chalk, mainly in response to the August rainfall. Over the next three months, normal levels predominate, but the outlook is much more uncertain as this is the period where winter recharge typically starts; entering the recharge season with broadly normal groundwater levels, the outlook is highly sensitive to late autumn rainfall.

Further information - October 2015

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