Published 17 June 2016.
Potential influences that might affect the forecast:
Although North Atlantic sea temperatures remain below normal the lack of westerly winds in the last month have allowed sea temperatures to rise above normal around the UK. A return to a more normal westerly type may reverse this trend.
RAINFALL PROBABILITIES LINKED TO LA NINA.
For the summer months the impact of El Nino is reducing hence the rainfall graphics have not been included. One impact of neutral Pacific sea temperature conditions is the likely increase in the number of Atlantic tropical storms and hurricanes compared to last year. Because El Nino conditions work to inhibit Atlantic Tropical storm formation this year tropical storms probably occurring at the normal rate but potentially developing stronger hurricanes due to warmer than normal equatorial sea temperatures. Several centres suggest above normal numbers of Atlantic tropical storms in 2016.
For the Autumn of 2016 a moderately strong La Nina is expected to develop, see the spread of solutions in the graphic above. The rainfall probabilities based on IRI statistics may be worth considering again for the Autumn and Winter and these are shown below
The probabilities are less clear cut during the Winter but still suggest wetter in the North and normal to drier in the South.
Text summary of the seasonal model output from the UK, various USA, Canada, China, Japan, Korea, Brazil and South Africa can be seen in the UK and Eire summary at www.weatherservice.co.uk and gives a quite mixed output for the remainder of summer but a clearer signal for Autumn.
200hPa heights predicted by CFS2 and NMME to be above normal but hints at enhanced zonal pattern in October perhaps steering rain systems further north and away from SW England.
Remainder of Summer (July August) 2016:
Pressure is forecast to be higher than average across SW England hence June and July may be drier and warmer than average although still with rather mixed and weakly unsettled weather patterns. The signal for a drier July is less clear cut with hints now of a drier August. These details are fairly unreliable but at least some drier periods seem likely, perhaps a little longer than those of early summer.
The main risk of heavier rain may still be in August although summer thunderstorms can give locally high rain totals in any summer month. Despite this there is a chance that the S and E of the region may have below normal rainfall compared to near normal elsewhere.
In parts of East Devon, Somerset and Dorset, the temperature for July and August is most likely to be above normal with other parts of the area nearer normal.
(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 for the season 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:
Average temperature for the season probably above normal. September probably a little warmer than normal, October nearer normal and November in particular is expected to be mild.
Rainfall probably below normal for the season with October and possibly September drier than normal but with a wetter than normal November.
La Nina conditions expected to 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 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 – (limited number of models available):
A mild winter is most likely, although February may see temperatures nearer normal. Impact of La Nina is not strong for the winter season but there is a hint that N Devon and Somerset could see above normal rainfall. Indications from the models however suggest near or below normal rainfall across the SW is more likely, especially in S and E Devon and across Dorset. Consequently below normal snowfall is more likely than above normal which means 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