Review and Experimental Long Range Forecast for SW England. September 2020.

Published 19 September 2020.

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1. Influences.

Changes in sea temperature

Atlantic equatorial sea temperatures remain warmer than climatology as is the area area around the UK, Norwegian Sea and Baltic. Higher than average sea temperatures in the Gulf of Mexico, apart from the cool area near the NW coast of Florida (due to storm Sally) may aid the development of current potential storm “Beta”

Colder sea temperatures in the East Pacific just south of the equator show the La Nina has strengthened.

The tropical north Atlantic is forecast to remain slightly warmer than average through to at least February 2021 (See Met Office graphic above).

Wikipedia states that “The 2020 Atlantic hurricane season is an ongoing tropical cyclone season that has featured tropical cyclone formations at an unprecedented rate. So far, it has featured a total of 24 tropical cyclones, 23 named storms, eight hurricanes, and two major hurricanes.[nb 1] With 23 named storms, it is the second most active Atlantic hurricane season on record, behind only the 2005 Atlantic hurricane season. It is also only the second tropical cyclone season to feature Greek letter named storms, with the other season again being 2005.

Wikipedia track map
Images and time line from Wikipedia

In the Pacific sea temperatures continue to trend towards cooler / La Nina conditions which may now last until mid to late Spring 2021 before returning to neutral conditions.

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) turned positive at the start of August and heralded the drier and warmer weather. The index turned negative in mid August a forerunner of more unsettled and cooler weather. The positive phase is forecast to come to an end by in the last week of September as Atlantic mobility returns.

Recent Climatology –  SW England

September to the 19th: Temperatures have been averaging around 16 Celsius or between 1.5 and 2 Celsius above the 30 year average. There has been above average sunshine and rather low rain totals – typically less than 10mm recorded so far.

This follows an average to slightly warmer summer – June and August were warmer but July was slightly colder than average. It was also a very wet summer in terms of rain total, although July was drier in SW England.

River flows in August 2020 were above normal across the SW and groundwater levels were also above normal in the eastern parts of SW England. Details can be found in the August 2020 Hydrological summary PDF 

The reservoir levels in the SW of England (16th August) were above average as of the 13th of September.

Global Flood Awareness System. August forecast has above normal flood risk for the UK except perhaps inn the south. (Orange below normal Blue above).

Atmosphere: Predictions.

A:  1: Stratosphere

Stratosphere is transitioning towards winter mode as shown but the ECMWF 50hPa height and temperatures for 18th 1200UTC and forecast chart for 28th September.

A: 2: Upper Troposphere

CFSv2 200hPa contours for October to December 2020  shown in top row along with the anomalies from CFS (middle row) and the NMME anomalies (lower row). Forecast of above normal heights more especially in the over western Europe in November and indication of stronger contrast in the north Atlantic in December.

B: Lower Troposphere:

A selection of  October to December solutions using September 2020 data are shown below. Three month season and individual months, top row temperature anomaly and lower row precipitation anomaly. 


Selection of WMO data including the WMO super ensemble. Full set from

WMO super ensemble probability
BoM Australia
BCC China

For the Winter period December 2020 to February 2021.

WMO super ensemble

For Spring 2021, March 2021 to May 2021.


C: Comparing the output of seasonal models for May June July 2020, based on April  2020 data.

For graphics and details see Verification review 2020 Jun Jul August

Comment: 040920 The idea that June and August would have the higher warmer anomalies in the south was correct, but the rain forecast was poor except perhaps for August in the south.

Scoring attempts to state good, fair, poor or no signal for the three month as a whole.

Comment 040920: Monthly data did not imply the correct temperature sequence in any model – several models got the idea of less rain in the south in July.
Season: Temp: To be correct near normal but above across Midlands and E/SE England and parts of W Scotland. Rain: needed to show above average rain except N Scotland and SE England.
1. Russia: Temp poor. PPN fair.
2. USA – CFS2 : Temp fair. PPN poor .
3. UKMO Contingency: Temp good . PPN fair .
4. UKMO : Temp fair . PPN Good for N and SE elsewhere poor . PMSL poor
5. USA – IRI : Temp good . PPN fair .
6. KMA APCC : Temp good . PPN poor/no signal .
7. JMA : Temp good . PPN poor . PMSL poor
8. NMME : Temp good . PPN poor .
9. WMO : Temp good . PPN poor .
10. BCC : Temp good . PPN poor .
11. NASA – Temp good . PPN fair .
12. Brazil:
13. CanSips : Temp good . PPN poor .
14. SAWS: :
15. Copernicus Temp good . PPN poor . PMSL poor
16. ECMWF Temp good . PPN poor . PMSL poor
17. MF Temp good . PPN poor . PMSL poor
18. JAMSTEC: Temp good . PPN poor .
20: CMCC Temp poor . PPN poor . PMSL poor
21: DWD Temp fair . PPN fair . PMSL poor

2. Forecast. SW England.

Remainder of Autumn 2020 (October November) 

Temperature are likely to be near or slightly above the 30 year average (1981-2010) October could see values closer to the average than in November.

Rainfall forecast lately have been poor and continue to be split for this period between drier and wetter solutions roughly 40% wetter/drier and 20% near normal. The start of October seems likely to be unsettled and rainfall for the month may well be near normal (wetter in the N of UK). November also probably near or a little above normal rainfall.

Autumn climate: 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. Autumn 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. October and November have similar number of rain days and totals range from around 70mm east of the moors to over 200mm over the moors.

Winter (2020 December 2021 January February).

A milder than average Winter in indicated as well as each month being near or milder than average. Caution models tend to be poor at picking colder types.

Mixed rainfall indications but Winter probably having above average rainfall and below average snowfall. There is a stronger signal for December being wetter than average compared to other months but February could also be wetter than average which is at odds with the data a month ago and illustrates the uncertainty. Below average snowfall seems likely – snow most likely confined to higher 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  (2021 March April May) limited data.

Milder than average temperatures seem likely for the season although March and April could see values closer to normal. Rainfall possibly below average in March but above in other months, hence above average for the season.

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 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 or Click here for the Teignmouth and Dawlish summary

4. References.

SST anomaly NOAA Remote Sens. 20146(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:

NMME information:

CFS2  info

GLOFAS Acknowledgement: Data were provided by the Global Flood Awareness System – GloFAS ( 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

International seasonal monthly data from WMO

Stratospheric Diagnostics from Japan Met Agency and ECMWF via  Free University Berlin

Climate data from The Met Office UK and NCEP USA

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