Published 18 November 2016.
1. Potential influences.
Changes in sea temperature.
The sea temperature around the UK remains above normal and the colder anomaly in the North Atlantic is similar to last month. In the Pacific a large colder than normal area has developed and the La Nina remains fairly week. The La Nina is expected to weaken further during the course of the winter months as shown by the consensus probability forecast below.
The probability plot above suggests a move to Neutral ENSO conditions later in the winter. Until then the statistical relationship between La Nina and UK precipitation may be worth a look, especially as there is support in some climate model output for a drier December (eg: NMME CFS BCC China Cansips).
Numerical (model) output.
CFS ten day mean ensemble precipitation output for December has moved towards a drier forecast, as shown below. Despite this change there remains a weak signal for a wetter than normal winter across parts of SW England in the CFS output.
There is also a corresponding change in the temperatures with a less mild forecast for December and January made up for by a milder February. At the time of writing almost all the available seasonal models predict a normal to mild winter for SW England.
NMME data also shows variation in the precipitation but is fairly consistent with an above normal temperature forecast.
CFS (200hPa) upper wind forecast mean and anomaly suggests enhanced jet over Canada, extending at times across the Atlantic towards UK. This implies stronger Atlantic low pressure systems during the January to March period. Later model runs appear to have maintained a stronger than normal jet.
Winter (2016 December, 2017 January and February) :
Temperatures this winter are most likely to be near or a little milder than the long term average (which does not rule out some frost). The second half of the winter may well be relatively milder than the first half when compared to the average for the individual months.
The early part of winter may be drier than average but this is likely to be offset by wetter weather later. (This would be different from the long term average in which December and January are wetter than February). Overall near normal or above normal precipitation is most likely for the N and W of the Region with parts of the SE and E of Devon, E Somerset and Dorset closer to normal and in places a little below normal. Below average snowfall is more likely than above average, which means some snow accumulation over the moors but not much elsewhere.
Chance of increased number of storms in January and possibly February although the tracks may be to the north of the Region.
Climate: 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.
Spring (2017 March April and May) :
Slightly above average temperatures are most likely for the season as a whole. Rainfall probably above normal overall with near or below normal precipitation early in Spring offset by wetter than normal April and/or May.
Climate: 1981-2010 average mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly Mar 7 or 8°C Apr 8 or 9°C May 11 or 12°C. Average 1981 to 2010 rain 200 to 300mm lowest values in E Devon and over parts of Somerset.
Limted data early look at Summer 2017 (June July August) 2016:
Conflicting temperature forecast perhaps suggesting near normal values. Rainfall, hint at drier June and/or July and wetter August 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)
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 or Click here for the Teignmouth and Dawlish summary
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/
NMME information: http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-12-00050.1