Review and Experimental Long Range Forecast for SW England. March 2021.

Published 18 March 2021.

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1. Influences.

Changes in sea temperature

Atlantic sea temperatures are generally warmer than climatology as are waters of the North Sea. Norwegian Sea, Baltic Sea and the Med. The tropical North Atlantic is forecast to remain slightly warmer than average through Summer 2021 (See Met Office graphic below).

Colder sea temperatures in the East Pacific, just south of the equator, show a weakening La Nina with further noticeable warming having occurred in the East. Forecasts suggest that Pacific Sea Surface temperatures may remain in a La Nina or Neutral state through the remainder of 2021.

The North Atlantic Oscillation index (500hPa index shown below) has been slightly positive recently and is forecast to remain slightly positive for a short time before becoming uncertain.

The positive NAO phase represents a stronger than usual difference in pressure allowing winds from the west to predominate. The position of the jet stream enables stronger and more frequent storms to travel across the Atlantic bringing more frequent milder and wetter weather types for NW Europe which in summer results in cooler temperatures.

For Met Office information about the NAO see https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/north-atlantic-oscillation

Recent Climatology –  SW England

Up to the 17th temperatures averaged out around 7 Celsius which is slightly below the March average. There has been about 40% of normal rainfall and 60% of sunshine. High pressure was over the UK to start the month before becoming unsettled and at times windy in the second week of March, followed again by a spell of High pressure..

CPC graphic 7th to 13th March showing temperature anomalies and total rainfall in this period.

Looking back at the last three months Winter temperatures were a little colder than average across SW England and there was above average rainfall.

River flows and ground water levels in February 2021 were well above normal across SW England. Details can be found in the February 2021 Hydrological summary PDF 

UK based data is available in The Hydrological Outlook which provides an insight into future hydrological conditions across the UK.  Detail can be found at http://www.hydoutuk.net/

River flow from climate forecast

Global Flood Awareness (updated) System. February forecast for the period February to May indicates slightly above normal flood risk for SW England.

Glofas March forecast for March to June 2021. Blue above normal flood risk which probably mostly reflects the current above average ground water levels.

Atmosphere: Predictions.

A:  1: Stratosphere

Major warming in the stratosphere (see N Pole temperatures below) in January 2021 caused a reversal of the westerly flow across North America and a significant re-location of the polar stratospheric vortex towards Norther Russia. The SSW was deemed to have ended on 13th February.

The polar stratospheric vortex has strengthened into normal winter mode. Below the ECMWF 10hPa and 30hPa for 17/1200UTC and 27th March show the strong vortex relocating poleward and remaining stronger than climatology.

A: 2: Upper Troposphere

CFSv2 200hPa contours (top row) and CFSv2 anomaly (middle) and NMME (lower row) for April to August 2021 hint at a less mobile patterns across the Atlantic for April and June but more cyclonic patterns near the UK for July and August.

B: Lower Troposphere:

Selection of April to June solutions using March 2021 data are shown below, top row temperature anomaly and lower row precipitation anomaly.

CFSv2 April to June
NMME April to June
NASA April to June
ECMWF April to June
UKMO April to June
WMO super ensemble April to June

The full set of graphics can be seen at http://www.weather-info.co.uk/wxsvc/seaslatest.html

Selection of Summer (June to August) solutions using March 2021 data are shown below, top row temperature anomaly and lower row precipitation anomaly.

CFSv2 SUMMER 2021 note there looks to be a cold bias in temperature across the N Sea in summer.
NMME SUMMER 2021
NASA SUMMER 2021
ECMWF SUMMER 2021
WMO super ensemble for SUMMER 2021
Beijing for SUMMER 2021
Australia BoM for SUMMER 2021
Germany DWD for SUMMER 2021
Canada for SUMMER 2021
Korea KMA for SUMMER 2021
France for SUMMER 2021
UKMO for SUMMER 2021

C: Comparing the output of seasonal models for 2020 December, 2021 January and February based on November 2020 data for the UK and Ireland.

For graphics and details see Verification review 2020 Dec 2021 January February

Review of details looked for in seasonal forecast.


Temperature. The North, mainly Scotland, was colder than average elsewhere mostly near average. The colder January mostly off set by above average December and February.

Rainfall. NW Highlands ended with below average rain but elsewhere values were mostly above average for the season.

Sunshine Northern areas had above average sunshine but southern areas had cloudier skies.

Pressure. Below normal for the season but above normal in N Scotland in Jan and Feb.

Temperature forecast was a little warm but got the idea of a milder February but not the cold January. Moscow output (WMO) which is often on the cold side got the idea of a colder January with near normal other months and some other models had January with smaller anomalies hinting at less mild. The colder than average season in Scotland was not indicated although the idea of above average snow in the north was correct. Above average rainfall was correct but the month to month detail was poor.

Models affected by the cold January and the lack of ability to forecast cold conditions.

Scoring for the three month season will attempt to state good, fair, poor or no signal. (E.O.E.)

1. Russia: Temp poor . PPN poor .
2. USA – CFS2 : Temp poor. PPN fair .
3. UKMO Contingency: Temp fair. PPN fair .
4. UKMO : Temp poor . PPN fair . PMSL fair
5. USA – IRI : Temp no signal . PPN no signal .
6. KMA APCC : Temp poor . PPN no signal .
7. JMA : Temp fair . PPN fair . PMSL poor
8. NMME : Temp poor . PPN poor .
9. WMO : Temp poor . PPN fair .
10. BCC : Temp poor. PPN poor .
11. NASA – Temp poor . PPN fair .
12. Brazil: Temp poor . PPN poor .
13. CanSips : Temp poor . PPN poor .
14. IMME : Temp poor . PPN poor.
15. Copernicus Temp good . PPN poor . PMSL poor
16. CMCC Temp fair . PPN poor . PMSL poor
17: DWD Temp poor. PPN poor. PMSL poor
18. EC Temp good . PPN poor. PMSL poor
19 UKMO Temp poor. PPN poor. PMSL poor
20. MF Temp poor . PPN poor . PMSL poor
21 NCEP Temp poor. PPN fair. PMSL fair
22 JAMSTEC: Temp poor. PPN good .
23 ECMWF (monthly) Temp: poor. PPN poor.

2: Forecast.

Remainder of Spring  (2021 April May)

The overall indication for the April and May is for slightly above average temperatures with the strongest indication for above average values is in April.

Rainfall uncertain, probably near average overall but with some hints that April could be drier.

Spring climate: 1981-2010 average mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly Apr 8 or 9°C  May 11 or 12°C. Average 1981 to 2010 rain similar totals in April and May roughly 70mm but 150mm over moors and 40mm in drier east each month.

Summer 2021  (June July August)  

Temperature above average for the season with June possibly seeing the warmest anomalies and August the lowest, with values closer to the average.

Rainfall for the season uncertain but probably above average rain totals. Higher rain rates are typical of a warming atmosphere but may not imply an increase in the number of days with rain. Suggestion of one month being drier than average but no agreement as to whether it could be June or August. Slightly reduced indication for a wetter August.

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.

Autumn 2021  (September October November).

Milder than average overall but perhaps near average, or even cooler than average, in November. Wetter than average Autumn with little agreement which month if any might be drier.

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 which is colder than September. 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. September often drier than October or November.

Winter 2021 /22 (December January February) very limited data.


Milder overall and also wetter than average especially early in the winter but February drier and possibly colder than average.

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.

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. 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: 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

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

Contains public sector information licensed under the Open Government Licence v3.0.

By Master0Garfield – Created using WikiProject Tropical cyclones/Tracks. The background image is from NASA [1]. The tracking data is from the National Hurricane Center’s Atlantic hurricane database, Public Domain, https://commons.wikimedia.org/w/index.php?curid=90697173

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