Review and Experimental Long Range Forecast for SW England. May 2019.

Published 21 May 2019.

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1. Influences.

Changes in sea temperature.

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Despite sea temperatures around the UK being above average.,much of the northern Atlantic is somewhat below average as is parts of the Mediterranean.

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UKMO tropical N Atlantic forecast sea temperature

The forecast Atlantic tropical sea temperature (shown above) continues to suggest a warming trend although this has been reluctant to occur.

In the Pacific weak El-Nino conditions continue, see CFS2 and NMME multi model ensemble forecast below. Current forecasts are mostly slightly less warm, a more marginal El-Nino.

ninosst

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) continued a negative phase which in summer would suggest colder / wetter condition for the UK. There is roughly a balance of models suggesting wetter verses drier but the majority suggest warmer.

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NAO background information see the Met Office NAO information.

Recent Climatology

In the SW of England, main station SYNOP data to May 1800 20th shows the average temperature anomaly to be around -1 C with below average rain and near average sunshine hours. tempanomn

Winter and Spring temperature until recently though have been above normal and rainfall for the last 12 months mostly normal or below.

climat rain 05

River flows are again showing below normal values in the east of the SW Region although groundwater seems near normal in the April report.

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Full details can be found in the  April 2019 Hydrological summary PDF 

The reservoir levels in the SW of England (12th May) show 86% storage which is slightly below the normal May level.

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The experimental product from the Global Flood Awareness System (Version 2) based on early May ECMWF seasonal forecast for May to August 2019. The system does not suggest an enhanced flood risk for SW England.

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Atmosphere predictions.

A:  1: Stratosphere 

The stratospheric has reverted to “summer” mode as shown by the ECMWF 30 and 10hPa images below for 1200UTC on the 18th..

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A 2: Upper Troposphere

CFSv2 200hPa monthly mean forecast height (contours) and anomaly (lower row).

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Generally above normal heights are forecast by CFS2 system for the summer months.

 

B: Lower Troposphere:

June to August 2019 solutions in low resolution from WMO . JJA and individual months. Top row temperature anomaly and lower row precipitation anomaly. Above normal orange/reds below normal in blue shades.

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WMO multi model ensemble probabilities JJA

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UKMO ensemble probabilities JJA

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ECMWF JJA probabilities

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NMME JJA 

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CFS2 JJA (E3 data)

For the summer; Data from Australia BoM, Brazil, South Africa and Russia suggest below normal summer temperatures but the vast majority of solutions suggest above normal values for the season and each month. Rainfall solutions are more evenly balanced between the wetter and drier solutions.

For the Autumn.

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Autumn NMME data

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Autumn CFS2 E3 data

C: Recent results for the period February March and April:

Continuing on from recent reviews temperature forecast were fairly good but rainfall forecast were generally poor. Verification page for February to April 2019

2. Forecast. SW England.

Summer 2019  (June July August) 

A better than average summer is more likely than a less good summer.

A warmer than average summer for the season is likely and probably each month is likely to be above normal too.

Models are mixed at which month could be drier or wetter. Howver some longer dry spells seem likely. Overall near normal rainfall seems likely but uncertain as to whether it is the number of dry days or the rain total that will be lower/higher. Higher temperatures may lead to shorter periods of heavier rain but fewer wet days which has been a pattern in recent years.

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 2019  (September October November) limited data. 

A warmer September then milder than normal, monthly and for the season.  Still looks likely that the Autumn could start drier in September then end with a wetter November, hence overall average rain for the season is possible.

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 2019  (December January February) minimal data. 

Milder than average throughout. Wetter start to winter then ending below average in February. Below average snowfall.

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 Free University Berlin

Climate data from The Met Office UK and NCEP USA