Climate review and Experimental Long Range Forecast for SW England. August 2021.

Published 17 August 2021.

1. Influences.

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Changes in sea temperature

Atlantic sea temperatures are generally warmer than climatology, especially so in the North and Northwest, although cooler than average values persist in an area to the SW of the UK and SW of Portugal. The North Sea, Baltic and Med are well above average for the time of year.

Tropical Atlantic is near or slightly above average and is forecast to remain warmer than average through the remainder of 2021 (See Met Office graphic below).

Colder sea temperatures around Japan follow several storms in the area after being well above normal previously. The East Pacific has similar anomalies to last month. Forecasts suggest that tropical Pacific Sea Surface temperatures may cool to a La Nina state for the Autumn and Winter before returning to a neutral state in Spring 2022.

The North Atlantic Oscillation index (500hPa index shown below) had been slightly positive for early August before turning negative. The negative NAO is consistent with a blocked or non mobile pattern which can be anticyclonic or cyclonic over the UK depending on the location of the “blocked” high and low pressure centres. Indications are not strong in either direction and the blocks so far have been fairly week allowing some mobility to occur.

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

August to the 17th has been quite mixed with about 9 dry days and between 25 and 50mm of rain, which is about right for half the month. Temperatures have been a fraction below normal averaging around 16 or 16.5C. Sunshine seems to be about one third of the average, suggesting a cloudier than average month so far. (Sunshine data is limited).

May June July 2021 UK Met provisional Temperature and Rainfall anomalies are shown below: Temperature has shown marked swings from a cold May to a warmer June and July. Rainfall deviated markedly from the long term average noting the dry W Highlands of Scotland and the wetter than average S/SE of England.

UKMO Top row: Temperature anomalies. Lower row: Rainfall anomalies

UK River flows in July 2021 were above average across parts of Cornwall and South and West Devon. Details can be found in the July 2021 Hydrological Summary

Data as of 17th August shows near normal river values across most of the SW but with some locations (in orange) showing below average values.

Reservoir levels, according to SW Water, were fairly high across SW England as of 8th August 2021.

Global Flood Awareness (updated to version 3) August forecast for the period August to November indicates slightly above normal flood risk for SW England. Blue above normal flood risk, White (river in grey) normal and Orange below normal.

2021 Atlantic hurricane season Started 22 May 2021. Data from Wikipedia.

Storm timeline – active storms as of 17th August Henri, Grace and Fred.

Death toll 9 or 10, estimated cost more than $1.2 billion.

Track map to 15th August.

A named storm “Evert”, occurred over SW England during the 29 to 30 July 2021
Southern and south-west England experienced some unusually strong winds for the time of year from storm Evert. Winds gusted widely at over 40Kt (46mph) and was much strong in exposed locations across SW England reaching 60mph in the Isles of Scilly.

Atmosphere: Predictions.

A:  1: Stratosphere

North polar stratospheric circulation is in “summer” mode as seen by the 50hPa and 10hPa analysis charts for the 16th August 1200UTC.

2: Upper Troposphere

CFSv2 200hPa contours (top row) and CFSv2 anomaly (middle) and NMME (lower row) for September to November 2021. There is a hint from CFSv2 data for a less mobile patterns across the Atlantic for October but more cyclonic patterns near the N of the UK for September and possibly November although NMME data differs from raw CFSv2 data showing a lack of agreement between models.

Similar plot but for December 2021 to February 2022 showing potential for increased zonal flow in February.

B: Lower Troposphere:

Selection of model solutions for September to November using August 2021 data are shown below. 

Top row temperature anomaly and lower row precipitation anomaly. 

CFSv2 using mean of 10 days model data to 7th August
NMME multi model
NASA
ECMWF orange/red colours are above model hindcast average.

Data (as supplied to WMO) shown below.

Top row temperature anomaly and lower row precipitation anomaly. Left image three month mean then individual months.

WMO multi model ensemble mean
Beijing CMA

Brazil

DWD Germany
CMC
Russia
Korea
Japan
France
UK Met Office

Graphics for the Autumn and Winter can be viewed at http://www.weather-info.co.uk/wxsvc/seaslatest.html

Winter December 2021 to February 2022 – limited graphics.

WMO multi model Winter

NMME multi model Winter.

C: Comparing the output of seasonal models for 2021 May June July based on April 2021 data for the UK and Ireland.

For graphics and details see Verification review May June July

Original – Summary – 170421 –
Temperature: The three month average probable above normal, a few models suggest the north rather than the south but the majority favour the south being above normal. Monthly data suggest June nearer normal with May and July more likely above. Rainfall: Fairly good indication for below average rainfall in the first half of the season then trending above average. Overall for the three months most likely below average in S and far N, elsewhere normal. Pressure: near normal perhaps above in the south.

Comment:
Temperature: Three month average good monthly detail not correct.
Rainfall: Generally poor/misleading forecast for monthly although a few models had a drier April. For the three months overall poor.

Scoring will attempt to state good, fair, poor or no signal.
Comment: Moscow got the sequence quite well.
1. Russia (WMO): Temp good . PPN fair (but good for sequence) .
2. USA – CFS2 : Temp good . PPN poor .
3. UKMO Contingency: Temp good . PPN fair .
4. UKMO : Temp good. PPN poor
5. USA – IRI : Temp no signal . PPN poor .
6. KMA APCC : Temp fair . PPN poor.
7. JMA : Temp good . PPN poor . PMSL fair
8. NMME : Temp good . PPN fair .
9. WMO multi : Temp good . PPN poor .
10. BCC : Temp poor . PPN poor .
11. NASA : Temp fair. PPN poor .
12. Brazil: Temp poor . PPN poor .
13. CanSips : Temp fair. PPN poor .
14. IMME : Temp good . PPN fair .
15. Copernicus Temp good . PPN fair . PMSL fair
16. CMCC Temp good . PPN fair . PMSL fair
17: DWD Temp good . PPN fair . PMSL poor
18. EC Temp good . PPN fair . PMSL poor
19 JMA Temp poor. PPN poor . PMSL poor
20 UKMO Temp poor. PPN poor . PMSL poor
21. MF Temp poor . PPN poor. PMSL poor`
22 NCEP Temp good . PPN fair . PMSL poor
23 ECMWF (monthly) Temp: poor . PPN poor .

2: Forecast.

Autumn 2021  (September October November).

Milder than average but chance that temperatures could be closer to average earlier in the Autumn then milder. Near normal or wetter than average overall but rainfall may be below normal in September and/or November little agreement on this detail.

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) limited data.

Milder than average Winter although there might be near average values early in the winter. Rainfall may be below average early in the winter but overall above average rainfall is more likely for the season with hints that February could be mild and wet. Below average snowfall is most likely, which mostly limits snow to 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.

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.

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