Review and Experimental Long Range Forecast for SW England. May 2020.

Published 19 May 2020.

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1. Influences.

Changes in sea temperature

170520ssta

Equatorial sea temperatures look warmer than climatology as are the N Sea and Baltic sea temperature. The North Atlantic west of Britain remains near or cooler than average towards North America but have warmed relative to average closer to the UK.

tna_anom_20200501

The tropical north Atlantic is forecast to remain very slightly warmer than average through Spring and Summer 2020 (See Met Office graphic above). The first Atlantic Tropical Storm “Arthur” developed off Florida but will weaken soon.

arthur

In the Pacific neutral El-Nino conditions are likely to trend towards cooler / La Nina conditions during Summer and into early Autumn 2020. The CFSv2 model looks quicker than most of the other ensemble systems of the NMME in completing this transition. The timing of the change from El Nino to La Nina might impact on the Atlantic hurricane season, El Nino tending to depress the formation of Atlantic Tropical Storms.

170520nino

Met Office Atlantic tropical storm forecast for June to November 2020 issued 19 May 2020: “The most likely number of named tropical storms (winds of at least 39 mph) predicted to occur in the North Atlantic during the June to November period is 13, with a 70% chance that the number will be in the range 9 to 17. The 1981-2010 long-term average is 12.

The most likely number of hurricanes (winds of at least 74 mph) predicted to occur in the North Atlantic during the June to November period is 7, with a 70% chance that the number will be in the range 5 to 9. The 1981-2010 long-term average is 6.

The most likely number of major hurricanes (winds of at least 111 mph) predicted to occur in the North Atlantic during the June to November period is 3, with a 70% chance that the number will be in the range 2 to 4. The 1981-2010 long-term average is 3. 

Note: Tropical Storm Arthur occurred in May 2020 and is therefore outside the period covered by this prediction (June-November).”

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been mostly negative since then end of March and is forecast to be roughly neutral over the next few weeks.

180520nao

NAO background information see the Met Office NAO information.

Recent Climatology

May to 18th has seen temperatures near average on the whole but with well below normal rainfall (5 to 15mm) and above average sunshine.

Because of the very wet February running average for the three months is above average.

190520clim

River flows in April 2020 reflected the drier January and February conditions and groundwater has largely recovered to above normal values in all areas. Details can be found in the April 2020 Hydrological summary PDF 

190520ground

The reservoir levels in the SW of England (10th  May) showed 89% storage.  Three of the main reservoirs are at over 90% capacity but Burrator is down to 68%.

res

Global Flood Awareness System. May forecast has a surprisingly high risk for southern parts of UK and may be because of high ground water following the very wet February. (Orange below normal Blue above)

190520glow

Atmosphere: Predictions.

A:  1: Stratosphere

After a long period with a strong polar vortex the Stratosphere settled into “summer” mode around the end of April.

strat

A: 2: Upper Troposphere

FSv2 200hPa contours for June to August 2020 are shown (top row) along with the anomalies from CFS (lower row). Forecast of above normal heights through the summer period.

z200JJA0520

B: Lower Troposphere:

A selection of  June to August solutions using May 2020 data are shown below. Three month season and individual months, top row temperature anomaly and lower row precipitation anomaly.

White areas are where the probabilities of below/normal/above are similar hence there is no overall signal for this period from the WMO data.

WMOJJA052020

WMO 

UKMOJJA052020

UKMO

TOKJJA052020

JAPAN

MOSCJJA052020

MOSCOW

ECLWMOJJA052020

ECMWF

NASAjja0520

NASA

0520nmmejja

NMME

0520E3JJA11may

CFS2 E3 data

The full selection can be seen at the weather-info site.

C: Comparing the output of seasonal models for  February, March and April 2020, based on January 2020 data.

There were some good indications for above average temperatures and the drier trend. NASA was one of the few monthly anomalies that picked out the near normal March and only UKMO and perhaps to lesser extent ECMWF picked the very much milder April.
CFS2 and NMME indicated drier for April as did Brazil and to a lesser extent Toulouse – some others did but were mostly drier throughout so rejected if did not get wet start to season.

For graphics and details see Verification review 2020 Feb Mar Apr from January

2. Forecast. SW England.

Summer 2020  (June July August)  limited data

 The main theme is for above normal temperatures although some indication that they may only be slightly above. August and possibly June could have the higher anomalies in the south. Rainfall very variable monthly detail from the models but probably below average for the season except perhaps in the North. Hints at August being above average in places, mainly in the south.

Slightly above average temperatures are expected for the summer season with higher anomalies possibly in the second half of the summer

Very mixed rainfall patterns but an indication for a drier than average June and July but possibly a wetter August. Summer rain can be showery or thundery in nature with wide variations in rain days and rain totals across a region.

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 2020 (September October November) limited data.

Overall above average temperature, especially later in the season. Rainfall  may be below average early in the season followed by a wetter October although indications from models are quite mixed.

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) very limited data.

A milder than average winter in indicated although February could see temperatures nearer average. Rainfall could end up below average but January looks to be wetter and February drier. Below average snowfall (snow chiefly over the moors) is most 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