Review and Experimental Long Range Forecast for SW England. August 2018.

Published 15 August 2018

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1. Influences.

Changes in sea temperature.

SST00818

The sea temperature around the UK remains well above normal. The colder area in the northern North Atlantic shows signs of becoming relatively warmer around the Newfoundland area with the main colder than average area having transferred East and South.

The tropical Atlantic is continuing to become a little warmer than average and it perhaps interesting that the two recent tropical storms Debby and Ernesto have formed near 35 to 45 degrees North where there is a strong positive anomaly.

SST10818

Left: Tropical North Atlantic UKMO. Right: CFS2 Pacific nino 3.4 area

SST20818

Left: ECMWF nino 3.4 area forecast Right: UKMO nino3.4 area forecast 

In the East Pacific temperatures continue to move towards El Nino conditions, from the current neutral state.

The IRI statistical predictor for UK rainfall based on the Pacific sea temperature anomalies is unlikely to be of use until the El Nino has established. The graphic below shows the probs for normal precipitation this winter during an El Nino.

elninonormal

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) remains mostly in a positive phase and is forecast to remain so in the next few weeks suggesting only weak Atlantic mobility.

NAO0818

For background information see the Met Office NAO information.

Recent Climatology

Temperatures during August (to the 15th) in SW England have been above normal by a degree or so. Rainfall has been more variable with heavy rain on just a few days bringing totals near to the average (but in parts of Cornwall possibly a bit above normal) for half way through the month.

The image below shows the rainfall and temperature anomalies for June and July.

clim0818

Following the dry weather water content in the top of the soil remains low is likely to further reduce in most areas. See GFS analysis and forecast below.

soilm0818

Rover flows in July mostly reflect the drier period. Ground water in the east of the region is near normal. The full Hydrological Summary for  June is available from this link  – July 2018 summary PDF 

rivground10818

The reservoir levels in the SW of England (12th August) show 65% storage with a slight uptick in storage following recent rainfall.

The experimental product from the Global Flood Awareness System based on ECMWF seasonal forecast was not available at the time of writing.

Atmosphere predictions.

A: Upper Troposphere

CFS2 data (as supplied to NNME data set August 2018) for September to November at 200hPa continues to suggest above normal heights across the UK and possibly reduced jet strength but with a hint of stronger jet flow in November.

z200SON0818

CFS2 200hPa month mean and anomaly September to November 2018

z200DJF0818

CFS2 200hPa month mean and anomaly December 2018 to February 2019

B: Lower Troposphere:

September to November temperature anomaly and precipitation indication for below/above model normal. Caution ECMWF uses opposite colour scheme for above normal rain.

NNMESON0818

NMME Sept to Nov TOP: temperature anomaly LOWER: rain rate indication (blue is below normal)

E3SON0818

CFS2 E 3 data 13th August. Sept to Nov TOP: temperature anomaly LOWER: rain rate indication (blue is below normal) 

ECSON082018

ECMWF Sept to November Sept to Nov TOP: temperature anomaly LOWER: rain rate indication (rain rate ORANGE is below normal)

December 2018 to February 2019 temperature anomaly and precipitation indication for below/above model normal. Caution ECMWF uses opposite colour scheme for above normal rain.

NNMEDJF0818

NMME Dec to Feb (data 8th August 2018) (Rain rate ORANGE is above normal)

E3DJF0818

CFS2 E3 10 day mean data 13th August 2018 (Rain rate ORANGE is above normal)

ECDJF082018

ECMWF Dec to Feb (Rain rate BLUE is above normal)

Full size graphics and a text review of other seasonal models can be viewed at www.weather-info.co.uk/wxsvc/seaslatest.html

C: Recent results/comments:

UK Summary 17 April 2018 for May June July

Temperature:The main indication is for near normal temperature, perhaps starting below normal especially across the S/SW of Eire and SW of UK due to cooler than normal sea temperatures. Some models also indicate parts of NE England/SE Scotland at risk from colder that normal May due to colder N Sea temperatures. Hints at June being above normal. Strongest signal for above normal is in NW Scotland and Midlands England. Rainfall: A mixed signal. Mainly a signal for near normal rainfall for the season but also for the NW to be above normal. For models that provided monthly output the South is perhaps more likely to be drier in June than in July but it is not clear cut. Several models suggest the N and perhaps the far S could see a wetter period especially in May and July.

Result: some idea that temperature would rise from April anomalies but not generally very good. Rainfall some idea of drier June but not for the overall drier than normal period although indications that the N/NW could be wetter were OK. Score temperature poor rain fair.

Overall there was a poor indication of the very dry and very warm weather to come. More details see Verification link

2. Forecast.

Autumn 2018  (September October November) early indications. 

Milder or much milder than normal for the season as a whole and probably each month individually.

Rainfall starting below normal in September but trending wetter than average with the season probably wetter than average especially in western parts of the region. Less clear cut in the east where near normal rainfall 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 2018/9  (December January and February) limited data from 4 models. 

No indication for a colder than normal winter. Main theme is above or well above average temperatures, with just a hint of less mild in February.

Rainfall likely to be above normal for the season. There are some indications for drier periods but not much agreement between models in location or timing. May lead to some eastern parts being near average rainfall rather than above.

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  www.weatherservice.co.uk

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

ECMWF seasonal monthly data from weather.us

Stratospheric Diagnostics from Free University Berlin

Climate data from The Met Office UK and NCEP USA.

 

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