Published 18 August 2023 delayed due technical issues until 24 August 2023
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A: Overview
1. Influences.
Changes in sea temperature
Sea temperature anomalies in most of the North Atlantic remain warmer than climatology for the middle of August. Areas around the UK, western Europe, western Mediterranean and the Baltic show 1 to 3 Celsius anomalies.
Ocean anomalies along the equator show an established El Nino state. The extent of the cold anomaly to the west of North America has markedly decreased in size.
Atlantic Hurricane season 2023 has seen nine tropical storm strength systems but only one hurricane and only one known fatality. The East Pacific has had six hurricanes and five known fatalities. The west Pacific has had 6 typhoons (2 super typhoons) 163 known fatalities and damage estimated at 15.6 Billion USD.
Time line to late August shown below (Wikipedia).
Potential new developments as stated by NOAA on the 18th August.
The Tropical Atlantic sea temperature anomalies are warmer than average having exceeded the forecast issued in June. Sea temperature in the tropical region are forecast to maintain positive anomalies through Winter 23/24. (see Met Office graphic below).
Tropical Pacific Sea Surface Temperatures (Nino3.4) are in an El Nino state and forecast to remain so into 2024. This may result in a higher global average temperature for 2023 and 2024 and there are suggestions that over the next few years the global temperature average will set further new records.
The effects of the El-Nino may be difficult to predict due to the excessive heat in water around coastal western Europe. A statistical link between El-Nino and rainfall suggests a slightly increased probability for a wetter than average autumn.
The North Atlantic Oscillation index (500hPa index shown below) became negative in early July through to mid August and is forecast to remain neutral or negative for the next few weeks. If westerlies are suppressed, as in the case of a negative NAO the temperature across Europe is more extreme in summer leading to heat waves which in 2023 has led to record high temperatures across many parts of Europe.
See https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere/north-atlantic-oscillation
2. Recent Climatology
Following the global hottest ever June, Nasa announced that July 2023 had the highest global temperature recorded for that month since 1880. It was also the warmest month on record. In the UK however July was wet with nearer normal temperatures.
There have been two named storms in August so far.
SW England
August to the 18th has been rather mixed with near average temperature (17 C) slightly above average rainfall and near or a little below average sunshine.
UK:
Met Office provisional Temperature and Rainfall anomalies for 2023 May June July are shown below. You can see the effect of an increase in average temperatures in the 1991 to 2020 period when comparing the image for July where anomalies are shown as negative when compared to the newer thirty year period. Temperatures in most areas were above average. Due to the wet July rainfall ended up near or above average in many places but below average for the three months in SE England, parts of Wales and Scotland. Changes in rainfall are less clear cut but suggest slightly higher rainfall in the most recent 30 year average.
Copernicus, European sourced temperature and rainfall anomalies for July 2023 and the 12 months to the end of July compared with the 30 year period 1991-2020, are shown below:
K River flows and ground water levels in July 2023 can be found in the Hydrological Summary
River flows across SW England were average or above in July 2023. River flow data for the 13th August from the UK Water Resources Portal shows mostly above normal flows.
Reservoir levels, according to SW Water are above the long term average across SW England as of 13th August 2023. Despite this hosepipe bans remain in force across Cornwall and much of Devon.
The Global Flood Awareness System – GloFAS – forecast from August 1st to end of November 2023 shows a “normal” flood risk across SW England – does not cover flash flooding from for example thunderstorms.
B: Forecast data:
1. Stratosphere:
The polar vortex remains in summer mode as shown by the GFS 17/1200UTC analysis at 10hPa
2. Upper troposphere:
CFSv2 200hPa contours (top row) and CFSv2 anomaly (middle row) and NMME anomaly (lower row) for September October November 2023.
3. Lower Troposphere:
A selection of model solutions for Autumn 2023 using August data are shown below. The full set, including available graphics for Winter 2023 can be found at https://www.weather-info.co.uk/wxsvc/seaslatest.html
Top row temperature anomaly. Lower row precipitation anomaly
September to November 2023
Data from WMO:
Three month average anomaly followed by September October November 2023. Top row temperature anomaly and lower row precipitation anomaly.
WMO super ensemble (below) is a mean of data supplied by National Meteorological organisations across the globe. Because of the amount of data the output is a low resolution. Note rainfall anomalies show cancellation between wetter and drier solutions in some of the multi model ensembles.
Individual members of the WMO super ensemble are shown below (Beijing China missing)
WMO super ensemble for Winter 2023/24
C: Comparing previous output:
May to July 2023 – based on April 2023 data.
NOTE: 30 year period 1981-2010 no longer available from UKMO web site now using newer 30 year period 1991 to 2020 which has higher temperature averages. 1961-1990 shown for comparison.
Review of details looked for in seasonal forecast.
Observed data
Temperature. Despite a cooler/near average July values were above average for season.
Rainfall. Difficult to fully quantify but looks like the wet July more than offset the previous drier months in some places. More clear cut in the 1961-1990 averages. Scotland and Wales below average but England and N Ireland above. Did models suggest wet July?
Sunshine. Above average sunshine for the three months despite cloudier July.
Pressure. Three month average above normal but July was well below normal
Forecast data
Original Summary – 18April2023
Consistent forecast for above normal temperature but hints at nearer normal in May and strong signal for above normal in June and / or July. Rainfall forecast rather messy with wetter models cancelling drier output. Hint at wetter than average or normal but with slightly stronger signal for drier than average in the south in June.
Notes: 1: Large scale blocking with easterly types seem to only be forecastable about two weeks ahead.
Comment on summary: Signal for warmer June good not so clear for July, overall above average good signal. Rainfall overall above average good as was drier June in south.
Scoring will attempt to state good, fair, poor or no signal.
SCORE based on THREE MONTH AVERAGE
1. Russia (WMO): Temp fair . PPN fair .
2. USA – CFS2 : Temp good . PPN fair .
3. UKMO Contingency: Temp fair . PPN fair .
4. UKMO : Temp good . PPN good . PMSL fair
5. USA – IRI : Temp fair . PPN poor .
6. KMA APCC : Temp good . PPN poor .
7. JMA : Temp good . PPN poor . PMSL poor
8. NMME : Temp good . PPN poor .
9. WMO multi : Temp good . PPN fair .
10. BCC : Temp good . PPN poor.
11. NASA : Temp good . PPN fair.
12. CanSips : Temp good . PPN poor.
13. Copernicus Temp good . PPN fair. PMSL fair
14. CMCC Temp good . PPN poor . PMSL good
15: DWD Temp good . PPN poor . PMSL good
16. EC Temp good . PPN poor . PMSL good
17 JMA Temp good . PPN poor . PMSL good
18 UKMO Temp good . PPN good . PMSL fair
19. MF Temp good . PPN poor . PMSL fair
20 NCEP Temp good . PPN poor . PMSL good
21 ECCC Temp: fair . PPN fair. PMSL fair
Only Canada got the monthly sequence best for temperature but perhaps a little cool.
WMO low res data:
BoM Temp GOOD . PPN poor.
Can Temp GOOD . PPN fair.
DWD Temp GOOD . PPN poor.
KMA Temp GOOD . PPN fair.
MOSC Temp poor . PPN fair .
WASH Temp GOOD . PPN fair .
CMA Temp GOOD . PPN good .
ECMWF Temp GOOD . PPN poor .
UK Temp GOOD . PPN good .
JMA Temp GOOD . PPN poor
MF Temp GOOD . PPN fair .
CMCC Temp GOOD . PPN poor .
CPTEC Temp poor . PPN fair.
D: Text Forecast for SW England
Autumn 2023 (September October November)
Temperature: A milder than average Autumn is most likely, consistent across models.
Rainfall: Probably wetter than average for the season. Month to month detail unreliable but hints at a drier start and wetter end to the season can still be found.
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 (December 2023 January and February 2024)
Milder than normal winter overall although February may be colder than average. Rainfall average or above for the season but with a chance that December and / or February could see longer drier spells which is consistent with July data. Snow probably again below average so any accumulations more likely to be across 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.
E: Caution.
Experimental Long Range Forecasts do not have a good success rate. especially with regards to precipitation. The models are good at predicting above average values but not for picking out a one off colder month. 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
F: 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
International seasonal monthly data from WMO
Climate data from The Met Office UK and NCEP USA
Contains public sector information licensed under the Open Government Licence v3.0.
t2mike 