Review and Experimental Long Range Forecast for SW England. November 2018.

Published 17 November 2018

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1. Influences.

Changes in sea temperature.

151118sstanom

The sea temperature near and to the East of the UK remains above normal but is likely to cool as strong easterlies develop over the next few days. The northern North and Northeast Atlantic is below normal as are the Great Lakes given how warm it was in this area at the end of October.

SST171118

The tropical Atlantic is trending to near normal and in the Pacific the El Nino is established as can be seen in the above plots.

The dominant IRI statistical predictor for UK rainfall based on the Pacific sea temperature anomalies for the winter season is shown below. Probabilities shown are for “near normal” precipitation, which is the highest category.

elninonormal

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) had been mostly in a positive phase as forecast turned sharply negative near the end of October and after a recover is forecast to again turn sharply negative: see below.

171118nao

Conditions applicable to a persistent negative NAO are shown by the Met Office graphic below:

nao_lo_winter_pr_tas2

However the persistence of a negative NAO is not yet certain. For NAO background information see the Met Office NAO information.

Recent Climatology

Temperatures during September and October in SW England were near average following an unusually warm summer and so far November has been well above average but due to the cold spell about to start may well end up near average. Rainfall in September and October was mostly a little below average and followed a drier than average summer. November is currently near or a little above average rainfall so may well end up slightly above average.

171118climate

Despite recent rainfall soil moisture remains below normal across SW England.

171118soilm

Rover flows in October reflect the longer term drier period.171118riverflow

Groundwater levels in Eastern parts of SW England are near normal and the November outlook (below right) suggest the main problem areas are further east.

171118groundwater

The October Hydrological Summary can be viewed here – October 2018 summary PDF 

The reservoir levels in the SW of England (11th November) show 55% storage which is a slight increase since last month due to the wetter start to November.

171118res

The experimental product from the Global Flood Awareness System (Version 2) based on early November ECMWF seasonal forecast for November to February.

141118glofas

Atmosphere predictions.

A:  Stratosphere and Upper Troposphere

The N Pole stratosphere is cooling in line with normal variations.

50mbnhlo

The polar vortex formed in October and has migrated SW of the pole. Forecasts show a splitting of the vortex and reduced stratospheric flow over Europe for a time before reforming again after about a week. This is illustrated in the 50hPa forecast sequence shown below.

16111850hpa

FS2 200hPa data (as supplied to NNME data set October 2018) for the December to February 2019 period continues to suggest above normal heights across the UK. for both December and January there are signs of enhanced jets suggesting stormy spells at least for the north of the UK.

CFS2 200hPa 7th Nov 2018 data (missing from NNME data set) for the December 2018 to February 2019 period continues to suggest above normal heights across the UK during the winter period. Anomalies imply enhanced Atlantic jet flow suggesting disturbed weather for northern UK in particular during December and the far north in February.

200z

CFS2 mean 200hPa contour and anomaly forecast Dec Jan Feb.

B: Lower Troposphere:

The Copernicus multi model ensemble has been upgraded with the addition of models from Italy and Germany adding to ECMWF, UKMO and MeteoFr. The winter forecast is shown below.

Capanoms112016

Copernicus Temperature mean anomaly forecast 

CapanomsPPN112016

Copernicus Rainfall mean anomaly 

CFS2 (E3) 10 day mean of model output for the winter months (December January and February). Oranges/red are above “model normal” blues are below.

E3DJF1118L

NMME mean of model output for the winter months (December January and February). Oranges/red are above “model normal” blues are below.

NMMEDJF1118

ECMWF mean of model output for the winter months (December January and February). Oranges/Purple are above “model normal” blues are below.

ECDJF112018

There is good agreement for above normal temperatures in December and an indication than anomalies could be lower or negative in February. Rainfall is much more variable but a signal for wetter December may offset less wet month to follow.

For Spring graphics see seasonal latest.

C: Recent results:

A review of how useful forecast have been can be seen at the review page.

For the period August to October using July 2018 data the following marking was given (good/fair/poor/no signal) for the UK.

Original text Summary – Good – Slightly optimistic temperature but idea of the west trending to normal was good as was the idea of higher than average pressure. Rainfall trend towards normal was fair and the idea of the of the north becoming above normal was good. 
1. Russia: Temp fair . PPN poor .
2. USA – CFS2 : Temp good . PPN fair .
3. UKMO Contingency: Temp fair . PPN good .
4. UKMO : Temp poor . PPN poor . PMSL poor .
5. USA – IRI : Temp no signal . PPN good .
6. KMA APCC : Temp fair . PPN fair .
7. JMA : Temp fair . PPN good . PMSL fair
8. NMME : Temp fair . PPN good .
9. IMME – Temp fair . PPN fair .
10. BCC – Temp fair . PPN fair .
11. NASA – Temp poor . PPN fair .
12. Brazil: Temp fair . PPN good PMSL fair .
13. CanSips : Temp fair . PPN poor .
14. SAWS: not available
15. Copernicus Temp good . PPN no signal . PMSL good
16. EC Temp good Temp fair . PPN no signal . PMSL poor
17. MF Temp fair. PPN fair . PMSL fair
18. JAMSTEC:
19: KMA: Temp fair . PPN fair. PMSL poor
20: ECMWF (monthly) Temp: fair PPN: fair

Last winter most seasonal forecast output based on November data, was poor.

2. Forecast.

Winter 2018/9  (December January and February).

Main consensus is for near or above average temperatures for the season but starting off milder than average and ending less mild and perhaps even slightly below normal.

Rainfall likely to be near or above normal for the season with the wetter month compared to average being December.

Below average snowfall is likely with the highest risk of snow possibly late in the winter and chiefly over 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 2019  (March April May) based on less data.

Possibly near normal or a colder than average March then recovering to milder than average temperatures for the season.

March may be wetter than average with increased snow risk. April and May probably near normal or a little wetter 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.

Summer 2019  (June July August) based on very limted data.

Another warmer than average summer with above normal rain although the number of wet days may be below average.

Summer climate: 1981 to 2010 average daily 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 can be seen at  here.

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

ECMWF seasonal monthly data from weather.us

Stratospheric Diagnostics from Free University Berlin

Climate data from The Met Office UK and NCEP USA.

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