Experimental Long Range Forecast for SW England. December 2016 issue.

Published 18 December 2016.

1. Potential influences.

Changes in sea temperature.

anomnight-12-15-2016

The temperature of the sea around the UK remains slightly above the seasonal norm with the pattern of slightly cooler temperatures SE of Greenland and SW of Iceland being very similar to mid November.

decnino

ENSO 3.4 Early December probability plot and Mid December model comparison. 

The weak El Nina is expected to become neutral and then in late summer trend towards another El Nino. Consequently any statistical relationship between ENSO conditions and rainfall becomes even more uncertain by Spring.

The IRI produced graphic for January to March rainfall probability during La Nina (shown below) suggests near normal totals but hints at wetter in the NW of the UK and drier in East of England

jfmnina

Rainfall probability for dry/normal/wet during La Nina January to March

Numerical (model) output.

Most of the model output for the Jan to March period also implies near normal rainfall but with a chance of above normal in more northwestern parts and less than average in the South or East of the UK. (Text based summary of model output for the UK and Eire). Much like the CFS2 output in November the data from mid month suggests a drier than normal January, suggesting that February or March might well be wetter in order to get a near or above normal season average.

NMME data illustrates this quite well with mean rain rates (shown on the lower part of the graphic) showing below model norms in Jan and Feb then above normal March to May. February has the least positive temperature anomaly being not far from normal.

nmmejfm1216

NMME: Top row – temperature anomaly (+/-1C). Lower row – rainfall rate indicative anomaly blue below and orange above average rain rates

Rain rate anomalies might reflect the intensity or the number of rain days but implies a wetter Spring for many. Skill in the output for Autumn was not good – see verification Autumn 2016.

NMME 200hPa mean values not available but the CFS2 mean and anomalies (below) imply developing enhanced jet/zonal flow for part of February and more especially for March.This replaces higher heights shown in January. Above normal heights are also evident in the forecast for April and May.

z200dec2016comps

CFS2 data from 8th December 2016. 200hPa  January to May. On top row month mean heights and on lower row the month mean anomaly.


2. Forecast.

Remainder of Winter ( 2017  January and February) :

 

Temperatures in early January may be colder than average followed by a recovery to milder weather and then near normal values in February. This means at least some frost, especially early in the period, although the overall temperature for Winter (Dec to Feb) may well be near or milder than the long term 1981 to 2010 average.

Rainfall may also be lower than average in January, especially the early part of the month, before returning to near average rainfall figures. The snow risk remains lower than average which means some snow accumulation over the moors but not much elsewhere.

 

Winter climate: 1981-2010 average temperature values for lowland areas 7°C in the West and 5°C in the East. (Jan and Feb similar to season values) 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. (Jan 60 to 100mm to lee of moors, 100 to 200mm elsewhere but higher totals over the moors. February, slightly lower values than in January)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) :

 

Above average temperatures are most likely for the season, as well as for each individual month. Rainfall probably near normal but perhaps above in the more exposed western parts of the Southwest but less likely in places to the East of the Moors, e.g: in parts of South and East Devon due to pressure being higher than normal in the south of England

Spring 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.

Summer 2017  limited data (June July August):

Warmer than average summer temperatures with below average rainfall  in terms of number of dry days if not the rain totals. Drier types more especially for June and July rather than August. Note a drier June and/or July and wetter August is not unusual.

Summer climate: 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.


3. Caution.

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

4. References.

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

CFS2  info

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