Published 19 July 2016.
Potential influences that might affect the forecast:
The cooler than normal North Atlantic sea temperature area has expanded towards the UK again due to more frequent W or NW winds. This may impact on day time temperatures in some western parts of Eire and the UK. Note though that sea temps to the SW of UK remain above normal so winds from SW or S probably warmer/milder than average. (Sea Temperature in the west Atlantic including the tropical areas are above normal and this may impact rainfall amounts through late summer and Autumn).
El Nino/La Nina conditions are neutral at present with La Nina conditions predicted to develop over the next few months.
The duration and strength of the La Nina is uncertain consequently the potential impact on the rainfall as shown by the IRI statictical relationship may be unreliable at the start of Autumn. Consequently the drier than normal signal could be much weaker than shown in Graphic 1: below.
For the winter months the relationship is not so clear as in Autumn, slightly favouring normal to below normal rainfall across much of SW England apart from N Devon and Somerset which have similar probabilities of each category..
A text summary of the seasonal model output from the UK, various USA, Canada, China, Japan, Russia, Korea, Brazil and South Africa can be seen in the UK and Eire summary at www.weatherservice.co.uk
200hPa heights predicted by CFS2 and NMME to be above normal but hints at enhanced zonal pattern in September and October but not in November. This may steer rain bearing weather systems further north and away from SW England. There is some support for this from other models.
The anomalies suggest lower than normal heights during February, perhaps hinting at slow moving low pressure centres across or near the N of UK.
Looking at the NMME near ground level output for Autumn and Winter a trend to mild conditions is shown but with a hint of less mild temperatures in February. Rainfall anomalies favour less wet conditions in parts of the S and E of England compared to elsewhere.
Remainder of Summer (August and possibly into September) 2016:
In summer the main risk of heavier rain can come from showers or thunderstorms. These can give locally high rain totals but may not be widespread across a region. Despite this risk it seems more likely than not that August and parts of September will see some long dry spells. The S and E of the Region in particular may have below normal rainfall compared to nearer normal elsewhere.
In most parts the temperature for August and for a time in September is probably going to be above normal although some western parts may be nearer normal.
(1981 to 2010 average mean temperature 15°C to 17°C or more 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. Average 1981 to 2010 rain for August 100-150mm over the moors, typically 60-100mm but locally E of Dartmoor and inland in Somerset 40 to 60mm)
Autumn (most of September, October and November) 2016:
The temperature for the season probably above normal. September probably a little warmer than normal, October nearer normal and November is expected to be especially mild.
Rainfall probably near normal for the season as a whole but parts of SE Devon and Dorset below normal. September and some of October possibly drier than normal but with a wetter than normal November for Cornwall, N and W Devon but perhaps not in SE and E parts of the Region.
La Nina conditions expected to become established in the Pacific in the Autumn which may start to influence global weather patterns. For the SW of England this implies drier than normal conditions may develop through the Autumn which is consistent with at least some of the model forecasts.
(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)
Winter (2016 December 2017 January and February – :
A mild winter is most likely, although February may see temperatures nearer normal. Near normal rainfall across the SW is more likely, especially in the N and W but with a chance that SE and E parts of Devon and Dorset might be a little drier than normal for the season as a whole. There is a chance that February may see above normal precipitation totals across the region. Below normal snowfall is more likely than above normal which means just a little snow accumulation over the moors but not much elsewhere.
(1981-2010 average temperature values for lowland areas 7°C in the West and 5°C in the East. Rainfall; Dec and Jan typically wetter than February. 1981-2010 Autumn average 300-400 mm lowlands but 200-300 mm areas to E of Dartmoor. Snow climatology less than 5 days lying snow over lowland areas 5 to 10 hills, say hills around 200 metre elevation – one in three years have no lying snow)
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
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