More rain approaching after this temporary colder respite

September is shaping up to be a varied month and, increasingly, a month notable for substantial rainfall events, especially in the south of Britain. From 9am on 22 September to 9am on 23 September, Oxford’s Radcliffe weather station recorded 77mm of rain, which is well over a month’s worth of rain in just 24 hours, and the highest daily rainfall total there since 10 July 1968. This has led to substantial flooding problems, particularly in Oxfordshire, and sewage spills, particularly along the River Thames.

Further rain through Wednesday and Thursday has exacerbated the situation. Some areas of England have had over 150mm of rain in the five day period from 22 to 26 September, which is two to three times the normal September rainfall.

September rainfall totals so far.

At present the UK is in a cold snap, following the second northerly blast of the month, the first having produced some notably low temperatures for September during the second week. Some areas have seen another notably cold night last night. Despite this, September’s mean temperature is looking set to be close to or slightly above the 1991-2020 long-term average, and up to a degree above the old 1961-1990 average. This reflects the underlying climatic warming trend - even in a warmer climate, individual periods can still be colder than average, but on average the warm spells are becoming warmer, and the cold spells are becoming less cold. However, for some regions of the UK the current cold snap does have potential to push September’s mean temperature slightly below the long-term average.

Potential for rain and flooding through next Monday and Tuesday

This weekend has got off to a cold and sunny start for most of us, and for much of the UK it will stay dry and sunny during the rest of today. However, cloud will increase from the south-west on Sunday, ahead of another southerly tracking low pressure system that is forecast to bring rain into the south-west of England and south Wales late in Sunday, and then to a large area of Britain during Monday and Tuesday.

There is some disagreement between the forecast models regarding where the heaviest and most persistent rain will end up, which will be a critical detail for the areas that have already been affected by flooding. For example, the UKV model run this Saturday morning has the rain mostly concentrated near the English Channel, whereas the GFS model has the largest rainfall totals affecting the south of Ireland, north Wales and an area of northern England and the north Midlands. The GFS model is forecasting 30 to 50mm of rain in two days across a large area.

Thus, it will be one of those events where we probably won’t be able to be sure of where the most heavily affected area will be until close to the time. While none of the forecast models are suggesting anything as extreme as what some regions saw last Sunday and Monday, there is still potential for some areas to see up to a month’s worth of rain in just 36 to 48 hours, which would be enough to exacerbate flooding in areas that are already affected by flooding, plus areas where water levels are currently running high.

The longer term outlook

After the wet weather early next week, an area of high pressure is forecast to build over or close to Britain, promising some drier weather and some relief for the areas affected by flooding, but there is potential for this to break down by next weekend with more low pressure coming in from the west: some forecast models are pointing towards another southerly tracking low crossing England and Wales next weekend which has potential to bring further persistent rain and flooding to the already flood-hit areas. 

By around 6 to 8 October, ex-hurricane Isaac will be entering the North Atlantic Ocean, and some forecast models have its remnants eventually affecting the British Isles, but there is considerable uncertainty over whether it will reach Britain - it could dissipate completely in the mid-North Atlantic or its remnants could end up heading north-eastwards to the north of Britain.

There is currently a lot of uncertainty over the weather patterns that may affect Britain as we head towards mid-October, hence the uncertainty over the track of this ex-hurricane, although it looks probable that it will remain wet at times in the south. However, if this ex-hurricane does affect Britain, it will most likely turn into a normal extratropical depression, making it unlikely that we will experience hurricane force winds from it, though it may contain extra warmth and moisture which could lead to substantial rainfall totals.