Experimental Long Range Forecast for SW England. November 2017 issue.

Published 18 November 2017.

1. Influences.

Changes in sea temperature.


Positive temperature anomalies (shown in the image above for the 16th November) are most marked in the North Atlantic south of about 50 degrees North but are less marked towards the equator. Consequently winds from a generally SW direction are likely to be milder than average, at least for the next month or so.

La Nina is evident in the Pacific and forecast to remain as a fairly weak La Nina through to Spring or Summer 2018.


NMME ENSO area 3.4 ensemble and (right) UKMO tropical N Atlantic ensemble

IRI statistical relationship between La Nina and UK rainfall for the winter is shown below. La Nina is weak so influence may not be strong but many of the seasonal model forecast hint at wetter in north and drier in south –  see models section below.

La Nina

Winter season precipitation probabilities during La Nina events.

Other statistical relationships may play a part in the Winter and Spring seasons, for example according to the Met Office late October Seasonal Forecast; “the Quasi-Biennial Oscillation (QBO), an oscillation of the equatorial winds in the stratosphere, is in an easterly phase. The QBO is linked to conditions over Western Europe during late autumn and early winter through an influence on the phase of the North Atlantic Oscillation (NAO) at the surface. An easterly phase of the QBO tends to moderately increase the chances of a negative phase of the NAO, which in turn increases the chances of below-average temperatures.”

QBO monitoring data is shown below along with the 50hPa ECMWF analysis on the 17th. Minimal warming is shown during the subsequent 10 days in the forecast at 50 and 30hPa.


Left QBO monitoring (left )shows negative phase descending in the stratosphere. ECMWF analysis at 50hPa shows the well established winter polar vortex and minimal warming.

North Atlantic Oscillation monitoring using 500 hPa is shown below and includes a forecast for the next season. NCEP ensemble forecast suggest NAO may move into negative phase later in November and for much of December. Consequently increased chance of blocking patterns which could be cyclonic or anticylonic over the Uk area.

For more information see Met Office NAO information.


Analysis and Numerical (model) output.

A: Recent Climatology.

Following on from a couple of unsettled wetter months, October / early November was drier and milder than average across the SW of England. Recent reservoir water levels remain above normal for the time of year, although Wimbleball was only at 55% of capacity at the time of writing. River flows in the region show western areas above normal and eastern areas below with groundwater in eastern parts of Devon and Dorset at notably low levels.. The  summary is available from the Hydrological Summary  –  October 2017 summary PDF 

A new experimental product from the Global Flood Awareness System suggests the flood risk areas for the next 4 months based on ECMWF seasonal forecast. Hint at below normal river flows in Somerset and high flows in the Exe estuary which looks odd.


B: Upper Troposphere:

CFS2 forecast for 200hPa suggest above normal heights across the UK but with below normal heights to the north suggesting increased zonal types (unsettled systems from the west) with February heights below normal across the north of the UK.


200hPa CFS2 forecast mean for 2017 December, 2018 January and February

C: Lower Troposphere:

Only BCC China seasonal forecast suggest below normal temperatures this winter and it is often a little cooler than other centres. The main signal from almost all seasonal forecasting centres is for above normal temperatures this winter. Highest anomalies appear to be in the S and E of the UK.  NMME and CFS2 winter season graphics are shown below.


NMME temperature anomaly (top) and rain rate indicator (lower row)


CFS2 mean of 10 days output. Monthly mean temperature and rain rate anomaly (winter)

Probably worth noting that the multi-model ensemble tends to be a little warm and that rainfall forecasts in general are not good with month to month detail. Despite a warm signal colder spells are still possible – see for example temperature plot from a single CFS2 model run.


Temperature forecast for Exeter area to August 2018

NMME plots for spring 2018 continue with a mild theme but note the much drier signal.


D: Comment:  Seasonal temperature and rainfall forecast data could have been better recently but 2016 data suggets data in November might fare better Verification summary.

2. Forecast.

Winter (2017 December, 2018  January and February) :

There remains a strong indication for a mild winter in the SW of England which continues to be signalled by almost all model output. This does not rule out some colder spells but suggests that they will be short lived.  The suggestion that temperature anomalies could be lower early in the winter and higher later can again be seen in the latest model data. This suggests that the temperature in December may be nearer normal but with milder months to follow.

Rainfall is uncertain month to month but the suggestion that mobile westerly patterns may bring cloud and rain suggest above normal  rainfall especially in Cornwall and to the west of the moors. Elsewhere near normal or even slightly below normal rainfall seems likely, perhaps with January being the wetter month. The snow risk is reduced compared to average with little if any snow for lower ground but some snow for the moors.

Winter 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 (2018 March April and May):

Temperature probably slightly above normal for the season but close to normal in April. Rainfall near normal in west elsewhere drier than average.

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.

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

GLOFAS Acknowledgement: Data were provided by the Global Flood Awareness System – GloFAS (http://www.globalfloods.eu/) of the European Commission Joint Research Centre and the European Centre for Medium-Range Weather Forecasts.
Reference: Alfieri, L., Burek, P., Dutra, E., Krzeminski, B., Muraro, D., Thielen, J., and Pappenberger, F.: GloFAS – global ensemble streamflow forecasting and flood early warning, Hydrol. Earth Syst. Sci., 17, 1161-1175, doi:10.5194/hess-17-1161-2013, 2013.

‘Copernicus Products’  as listed in the C3S or CAMS Service Product Specification or any other items available through an ECMWF Copernicus  http://climate.copernicus.eu

Stratospheric Diagnostics from http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/index.html