Published 19 July 2020.
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1. Influences.
Changes in sea temperature
Equatorial sea temperatures look warmer than climatology as is part of the N Sea, Norwegian Sea and Baltic sea temperature. The North Atlantic west of Britain has become much cooler than average although the western side of the Atlantic is warmer than average. The colder than normal sea temperatures to the west of the UK produces below average temperatures in August in some models, assuming winds are from a westerly direction.
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 very slightly warmer than average through the remainder of 2020 (See Met Office graphic above).
No more Atlantic tropical storms are forecast for the remainder of July but there have been six tropical storms in the Atlantic/Gulf of Mexico area this season but no hurricanes. Names are Arthur, Bertha, Cristobal, Dolly, Edouard and Fay. – Wikipedia link to details.
In the Pacific sea temperatures continue to trend towards cooler / La Nina conditions which may now last until the end of Spring 2021 before returning to neutral conditions.
The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been negative since late June and is forecast to remain so over the next few weeks which suggests a colder and wetter period may develop/continue across the N and E of Europe.
NAO background information see the Met Office NAO information.
Recent Climatology – SW England
July to 19th Temperatures have been running slightly below average across the SW with other parts of the UK seeing cooler values. Sunshine also looks a little below average but rainfall has been well below average. Further north in the UK rainfall has been above average in places.
NCEP EU area temperature anomaly and rain total 5 to 11 July
Previously April to June had been warmer than average and drier in most places although June 2020 was very wet across the SW.
River flows in June 2020 were normal and reflected the wetter month. Groundwater levels were above normal. Details can be found in the June 2020 Hydrological summary PDF
The reservoir levels in the SW of England (12th July) showed 75% storage which is close to average. Burrator was down to 47% in May but recovered during the wet June.
Global Flood Awareness System. July forecast has near normal flood risk for southern parts of UK (Orange below normal Blue above).
Atmosphere: Predictions.
A: 1: Stratosphere
After a long period (during winter and spring) with a strong polar vortex the Stratosphere settled into summer mode around the end of April and remains with an easterly circulation as shown by the 30hPa chart for 18th July.
A: 2: Upper Troposphere
CFSv2 200hPa contours for August to October 2020 are shown (top row) along with the anomalies from CFS (middle row) and the NMME anomalies (lower row). Forecast of above normal heights through the period although in September lower anomalies may occur near the UK.
B: Lower Troposphere:
A selection of August to October solutions using July 2020 data are shown below. Three month season and individual months, top row temperature anomaly and lower row precipitation anomaly.
NMME
CFS2 E3 data
NASA
ECMWF
In the WMO data white areas are where the probabilities of below/normal/above are similar hence there is no overall signal. Precipitation forecasts over the UK end up with no signal because the wet models cancel out the dry forecasts showing there is no agreement between the models.
WMO super ensemble 11 models.
UKMO
CMA
BOM
JMA
The available selection, including plots for Autumn and Winter, can be seen at the weather-info site.
C: Comparing the output of seasonal models for April May June 2020, based on March 2020 data.
For graphics and details see Verification review 2020 Apr May Jun
Temperature: the main thrust for above normal temperatures was correct though if anything understated. Areas of normal or below normal were limited.
Rainfall: Below average rainfall was correct for the season. Monthly variation hard to find in the models but NMME and ECMWF did quite well.
2. Forecast. SW England.
Remainder of summer 2020 (August)
Temperatures in August probably near normal, perhaps cooler near northern and western coasts and a little warmer than average in southern and eastern parts of the region.
August is likely to be drier than average although there is a hint of nearer normal rainfall in N Cornwall and NW Devon. There is a typical summer risk that locally thundery outbreaks may upset totals in a few locations.
August climate: 1981 to 2010 average daily mean temperature 16 or 17°C in many areas, a little cooler over the N coastal areas of Devon and Cornwall to 16 or 17°C and a few degrees cooler over the moor. Locally over 17C in parts of Somerset. Average rain in August typically 60 to 80mm, but 40 to 60mm in places especially East of the moors and in Somerset. Locally over 100mm across the moors.
Autumn 2020 (September October November)
Overall near or a little above average temperature seems likely, especially later in the season. Rainfall very mixed indication, most models have some longer dry periods but suggest September could be wetter in many areas but near normal in the SW . Overall signal is for near normal rain although eastern parts of the region could be drier than average.
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 (2020 December 2021 January February) limited data.
A milder than average winter in indicated although February could see temperatures nearer average. Very mixed rainfall indications but Winter could end up near average although the first half of winter might be wetter than the second half. Below average snowfall is again likely.
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. 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
Stratospheric Diagnostics from Japan Met Agency and ECMWF via Free University Berlin
Climate data from The Met Office UK and NCEP USA
