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

Published 19 July 2021.

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1. Influences.

Changes in sea temperature

Atlantic sea temperatures are generally warmer than climatology except for the area west from Biscay. 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 in the East Pacific remain and show similar anomalies to last month. Forecasts suggest that Pacific Sea Surface temperatures may remain in a neutral or slightly negative state through summer and trend towards La Nina after October. The West Pacific, especially near Japan and also the Northern Indian Ocean are well above normal for mid July.

The North Atlantic Oscillation index (500hPa index shown below) has been neutral or slightly positive for much of July but is again forecast to turn negative over the next week or two, although perhaps only briefly. 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.

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

Until mid month July was wetter than average in terms of the number of wet days and total rainfall for the whole month with close to average temperatures. For about a week from mid July though there has been a longer dry and very warm spell lifting average temperatures above normal for the month as a whole. Sunshine up to mid month looks to have been below average.

April May June 2021 UK Met provisional Temperature and Rainfall anomalies are shown below and illustrate the potential for extremes. Temperature swings from cold April and May followed by warmer June and Rainfall that deviated markedly from the long term average indicates the unusual times for the UK climate.

UKMO top row Temperature anomalies April to June, lower row Rainfall anomalies April to June.

UK River flows in June 2021 were well below normal except in the wetter South and SouthEast of England . Details can be found in the June 2021 Hydrological summary PDF 

River levels on the 17th of July were more or less normal but starting to reduce in places

Reservoir levels, according to SW Water were fairly high across SW England as of 11th July 2021.

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

Rainfall during the severe flooding in Germany amounted to at least 100 to 200mm in 24 to 48 hours over a fairly widespread area following an already wet period. Forecast models suggest 100mm totals a few days before the event and the river flow forecast tool suggested river flows in excess of the 1 in 20 year return period but only shortly before the event.

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

Wind speeds reaching tropical depression category = 5, storm category = 5, hurricane strength 1. Total fatalities 19. Estimated damage more than 390 million USD.

No current Atlantic tropical storms as of 19th July 2021.

Atmosphere: Predictions.

A:  1: Stratosphere

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

2: Upper Troposphere

CFSv2 200hPa contours (top row) and CFSv2 anomaly (middle) and NMME (lower row) for August to November 2021 hint at a less mobile patterns across the Atlantic for August and October but more cyclonic patterns near the N of the UK for September and possibly November.

B: Lower Troposphere:

Selection of model solutions for August to October using July 2021 data are shown below. Top row temperature anomaly and lower row precipitation anomaly. 

CFSv2 mean of 10 days data to 7th July for August September October
NMME multi model ensemble data to 7th July for August September October
NASA July data for August September October

ECMWF July 2021 data

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 anomalies
Biejing China

BoM Australia

Brazil
DWD Germany
ECMWF
Canada
Russia
S Korea
Japan
France
UK
USA

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

Winter December 2021 to February 2022 – limited graphics.

NMME Winter 21/22
NASA winter 2021/22

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

For graphics and details see Verification review April May June

Details looked for in seasonal forecast:

Temperature;. for the season the warmer June did not offset the two colder months hence below average for the season but can we see any trend colder then warmer?

Rainfall. Parts of S and SE England were above normal for the season but elsewhere the wet May was mostly offset by the other dry months.

Sunshine above average for the season

Pressure above average for the season (Slack) (April well above, May well below, June above)

Original Summary – Summary – 180321 –
Temperture: There looks to be more models suggesting near normal than typical although the main signal is for above normal with hints that this could occur later in the three months than earlier in the period.
Rainfall: Very mixed indications, possibly near normal overall. Probably some longer drier than average periods, mostly in April and more likely in the south but not a clear signal. For the three months some areas above average perhaps most likely in the north. Pressure possibly a little above average in the south.

Comment: 030721 – Although the season turned out below average the indication for warmer later was correct and the increased number of near normal solutions may have been a pointer to colder values. Reasonable indication for drier April and for some areas above normal but suggestion that this would be the north was wrong. Pressure was slightly above average for the season but not just in the south.
200hPa not enough troughing in May but CFS2 anomaly hinted at lower heights over France than in April or June.

Scoring will attempt to state good, fair, poor or no signal.
THREE MONTH SCORES
1. Russia (WMO): Temp good (model tends to be colder than others). PPN fair .
2. USA – CFS2 : Temp poor . PPN poor .
3. UKMO Contingency: NIL
4. UKMO : Temp poor. PPN poor. PMSL poor
5. USA – IRI : Temp poor. PPN fair.
6. KMA APCC : Temp poor. PPN no signal .
7. JMA : Temp poor. PPN poor. PMSL good
8. NMME : Temp poor. PPN fair .
9. WMO multi : Temp poor. PPN poor.
10. BCC : Temp poor. PPN poor.
11. NASA : Temp poor. PPN poor.
12. Brazil: Temp fair. PPN poor.
13. CanSips : Temp fair trend poor overall. PPN fair overall poor trend.
14. IMME : Temp poor. PPN poor.
15. Copernicus Temp poor. PPN poor. PMSL fair
16. CMCC Temp poor. PPN poor. PMSL good
17: DWD Temp fair. PPN fair. PMSL fair
18. EC Temp fair. PPN fair. PMSL fair
19 JMA Temp poor PPN fair. PMSL fair
20 UKMO Temp poor. PPN poor. PMSL poor
21. MF Temp poor. PPN fair. PMSL good
22 NCEP Temp poor. PPN fair. PMSL good
23 ECMWF (monthly) Temp poor. PPN poor.

2: Forecast.

Remainder of Summer 2021  (August)  

Temperature above average most likely.

Rainfall uncertain but a good number of solutions suggest below average rainfall in south UK. This can easily be upset by a few thundery rain events. Hence possibly more dry days than average even if rain totals are higher.

August climate: 1981 to 2010 average daily mean temperature 16 or 17°C in main urban areas also locally east of the moors and more widely in Somerset, 13 to 15 Celsius elsewhere. Average 1981 to 2010 rain 40 to 80mm but over the moors, typically 125-150mm.

Autumn 2021  (September October November).

Milder than average but chance that October could be closer to average. Some western parts of the region could also be near average. Wetter than average overall. Rainfall may be below normal in one of the months but no agreement as to which month.

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 in January but overall above average rainfall is more likely for the season. 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.