Experimental Long Range Forecast for SW England

Published 19 March 2016 (layout change – Forecast then influences) .

The remainder of Spring (April and May) 2016:

April is likely to have unsettled/changeable weather with spells of rain or showers and shorter drier intervals, hence above normal rainfall accompanied by near or a little above normal temperature. For May a less clear signal, though rainfall is again more likely to be above normal than below normal. This does not rule out some drier spells and parts of Dorset and SE Devon probably near normal rainfall rather than above normal. The temperature in May probably a little above normal.

(1981-2010 average mean temperature roughly Apr 8 or 9°C  May 11 or 12°C but a few degrees cooler over the moors. Average 1981 to 2010 monthly rain April and May 60 to 80mm locally up to 150mm over the moors but 40-60mm around Exeter and widely across Somerset levels.)

Summer  (June July August) 2016:

Probably a typical summer with rather mixed weather, with no strong indication for a long dry summer despite some signs of above average pressure. Rainfall near normal or a little below normal in June and July  (a little more likely to be below normal in July than June) but above normal in August. There is chance that July may see some longer drier spells.

Temperature near or a little above normal in parts of E Devon, Somerset and parts of Dorset but cooler than normal Atlantic sea temperatures may result in near or below normal temperatures elsewhere. This could be due to reduced day time maximum temperature rather than overnight minimum temperatures. July may be a slightly warmer month than June or August compared to the average, which is not unusual.

(1981 to 2010 average mean temperature 14 or 15°C in many areas to 16 or 17°C in main urban areas also locally east of the moors and more widely in Somerset. Maximum temperatures average 19 to over 21°C in similar areas. July often warmer than August.  Average 1981 to 2010 rain 300mm over the moors, typically 200 to 250mm in many coastal and eastern areas. June often drier than July and July drier than August)

Autumn  (September October November) 2016 with limited data:

Overall temperature for the season near normal, perhaps a little below at first then becoming above normal. Rainfall near normal for the season but in September a higher risk of above normal rain than below normal.

La Nina may conditions may become established in the Pacific by Autumn which may start to influence global weather patterns. For the SW of England this implies drier than normal conditions may develop. The onset of this potential impact is very uncertain.

 (1981 to 2010 average mean temperature 11 or 12°C but nearer 10°C in rural areas. Maximum temperatures average 14 to 16°C. November normally colder than October which is colder than September. Average 1981 to 2010 rain 300 to 400mm, 600 to 800mm over the moors but east of the moors and in lowland Somerset more like 200 to 300mm . September often drier than October or November)


Experimental Long Range Forecasts do not have a good success rate. The data used for the above forecast summary and brief verification of previous Long Range Forecast summaries can be seen at   http://www.weatherservice.co.uk

The attempt at a Regional Forecast for SW England aims to test whether such a forecast of temperature and rainfall variation from average can be made using numerical model data available on the internet. The forecast should not be used for any other purpose. A brief verification summary for the UK and Eire is routinely published at http://www.weather-info.co.uk/wxsvc/Verification.html

Potential influences that could affect the forecast.

Sea temperature.


NOAA/NESDIS night time sea surface temperature anomaly 17 March 2016

Typically the coldest time of the year for sea temperatures around the UK but note the further cooling of North Atlantic temperatures relative to normal. Long range forecast systems tend to retain a colder than North Atlantic right through the summer months, much like last year. The impact of this is a likely reduction in temperatures, especially maximum temperatures, in at least the western half of the UK – assuming winds from a westerly direction predominate.

The strong El Nino in the Pacific continues but forecast systems tend to weakening this by summer with potential La Nino conditions for the Autumn and Winter 2016/17 as shown by the graphic below.


Nino 3.4 Sea Surface Temperature Anomaly prediction comparison

Rainfall probabilities linked to El Nino.


IRI rainfall probabilities during El Nino for April May and June (Dry Wet Normal left to right)

The El Nino linked probabilities for April May and June suggest above normal rainfall most likely but nearer normal in parts of Devon, roughly Torbay and eastwards, also in Dorset.A graphic for the summer months is not included due to the weakening El Nino.

There is a chance that La Nina conditions develop in the Pacific later this year. The graphic below shows the Autumn rainfall probabilities based on IRI research for a well established La Nina.


IRI rainfall probabilities if strong La Nina develops in time for Autumn – not very likely.

Changes in the Stratosphere.

The anticipated Sudden Stratospheric Warming arrived too late to influence the winter pattern with a major warming event starting during the first week of March, roughly at the same time as High pressure developed across the UK. The 50hPa analysis for the 17th March is shown below for comparison with earlier publications.


ECMWF 50hPa analysis by FU Berlin

200hPa predicted anomalies.


200hPa CFS2 mean and anomaly

NMME mean not available. CFS2 suggests above normal heights over UK area except perhaps in April. A slightly enhanced westerly jet is predicted in the NW Atlantic in May but with ridging over the UK area.

Conflicting model output.


NMME March 2016 1 deg GRIB data.

The North American Multi Model Ensemble (NMME) which includes CFS2 data, suggests slightly above normal temperatures for SW England and a drier than (model) normal June and July. CFS2 is not so clear cut, some output did suggest drier in SW England in July but the set shown below implies near normal rain although better agreement for April. The data for CFS2 used in the NNME had June drier in the SW. Temperature anomalies show below normal across SW England in the summer month otherwise near average +/-0.5C which is not far from the NNME anomalies. Other models to indicate a risk of cooler summer temperatures include UK Met Office and Beijing Climate Centre and these centres also suggest that slightly wetter than normal is more likely than below normal.

Many long range forecast models seem fairly poor at picking drier than normal months.



CFS2 averaged 6 to 16th March. Left temperature anomaly. Right precipitation anomaly.



SST anomaly NOAA Remote Sens. 2014, 6(11), 11579-11606; doi:10.3390/rs61111579

IRI statistics: Mason, S.J. and L. Goddard, 2001: Probabilistic precipitation anomalies associated with ENSO. Bull. Amer. Meteor. Soc., 82, 619-638.

UK climate details see: http://www.metoffice.gov.uk/public/weather/climate/

Stratospheric images prepared by Freie Universität Berlin using European Centre for Medium Range Weather Forecasting data supplied by Deutscher Wetterdienst

NMME information:   http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-12-00050.1

CFS2  info