Atlantic depressions bring above-average rain for some

During the first half of the month, a succession of low-pressure systems tracking close to the UK brought rain to most places as fronts swept from west to east. After a cool, showery interlude around mid-month, a more autumnal pattern emerged with some foggy nights and sunny days interspersed with further bouts of rain.

Following August’s trend, provisional statistics for the UK for September so far suggest it has been slightly cooler than average and rather unsettled, although frosts have been reported by very few stations up until 28 September. The cool trend isn’t apparent everywhere though, as parts of northern Scotland, including Shetland, Orkney and Caithness were the warmest spots relatively in Britain, when compared with average temperatures. Shetland enjoyed mean temperatures during the month more than 1 °C above the long-term September average. In contrast, the maximum daily temperatures in some parts of southern England were around 1 °C cooler than the September average.

Dr Mark McCarthy is the head of the Met Office’s National Climate Information Centre. He said: “The UK receives weather influences from all points of the compass. However, the weather during September has been largely dominated by an influence from the Atlantic. This is evident in the above-average rainfall in parts of western Britain and Northern Ireland. The relative lack of northerly winds will have been partly responsible for keeping the temperatures of parts of northern Scotland above average.”

Up until 28 September, most places had already exceeded their average rainfall for the month, with a UK figure of 108.1mm with two days to go (UK September average is 96.4mm). Some locations, such as Cornwall and Plymouth, have had well above their September rainfall average.

Dr Mark McCarthy added: “Cornwall has experienced it wettest September since 2000 and there is more rain to come before the month concludes.” Other areas with well above average rainfall include parts of northwest England, northern and western Wales, Northern Ireland and northeast Scotland.

In contrast parts of southeast England, Herefordshire and Central and northwest Scotland have been rather drier. Provisional figures for Shoeburyness, in Essex, reveal that it has only received 26.8mm  of rain; just under two thirds of the long-term September average.

For temperatures, values have been close to or slightly below average so far, with daytime temperatures for the UK 0.5˚C below average at 16.0˚C. Some counties across southern England have had an average daytime maximum of 1˚C below normal.

With rainfall amounts generally above average, it’s no surprise that sunshine hours are below or close to average for most areas, with the exception of parts of southeast England near the Thames Estuary. UK sunshine amounts are 102.6 hours, 82% of average (124.7 hours) up until 28 September.

The forecast for the last days of September is changeable, with some persistent and heavy rain forecast for many areas as we head into the final night of the month. This means that rainfall amounts may increase substantially for some areas, while there is unlikely to be a surge of warmth to counteract the cool feel so far.

1-28 September 2017 provisional figures Mean temp (°C) Rainfall (mm) Sunshine (hours)
Actual  Diff to avg (°C) Actual % of avg Actual % of avg
UK 12.5 -0.2 108.1 112 102.6 82
England 13.4 -0.3 84.3 121 114.3 83
Wales 12.6 -0.2 149.5 128 96.4 75
Scotland 11.1 0.2 131.0 96 86.0 82
N Ireland 12.0 -0.3 137.6 150 95.6 84

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3 Responses to Atlantic depressions bring above-average rain for some

  1. xmetman says:

    I notice that daytime maxima so far this September in Central England have been around 0.8°C below average – which explains the cool feel to the days – whilst night time minima have been close to the September average.
    I know its the same thing, but I always thought the phrase was “difference from average” and not “difference to average”.

  2. Hello,
    I always enjoy seeing your monthly weather summaries, but I wonder whether you have any means of recording overall light levels, rather than just hours of sunshine? We have had a PV installation for many years now, and interestingly the inverter’s annual output which records, of course, all received, useable light, has been amazingly constant every year (within a couple of percent), though the distribution of that light throughout the months has been very varied.
    However this year, the readings look like they will be nearly 10% down. Combined with very few dry days, since the beginning of July, even if rainfall totals haven’t been excessive, this seems to have had dramatic effects on local plant, and particularly tree, growth and fungal diseases. If this is the shape of things to come, I fear landscape vegetation changes could be dramatic. I live in upland North Carmarthenshire,
    best wishes
    Julian

    • Hello Julian

      We passed your question onto Mark McCarthy, our climate specialist and he says: There are people who have investigated using renewable power generation as an alternative source of meteorological data so they are definitely thinking along the right lines. We do have routine measurements of total irradiance within our present day observing network, but our climate records are based on bright sunshine hours by virtue of them having been available over a much longer period of time historically because the Campbell-Stokes sunshine recorder which is still used in many places today was invented in the 1850s.

      Sunshine hours have indeed been below average for our observing site in Aberporth for each of June, July, August and September this year with only 87% of average for that location, which is broadly speaking consistent with your readings being 10% down.

      However the disappointing summer this year is not an indicator of things to come, but part of the natural variability in the climate of west Wales. For example we could contrast this summers 549 hours of bright sunshine at Aberporth with the miserable summer of 2012 at 424 hours followed by the sunnier 2013 with 733 hours. The upland areas where they live are likely cloudier than the coastal site at Aberporth but it indicates the general variability.

      Hope this helps
      Helen

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