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

Published 17 September 2018

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1. Influences.

Changes in sea temperature.

130918sstAnom

The sea temperature around the UK remains well above normal. The colder area in the northern North Atlantic has reduced in extent slightly but recent tropical storms probably resulted in the cooler than normal SST extending from off the coast of Africa towards the USA. Despite this recent cooling the forecast, shown below, remains for a slightly warmer than normal tropical Atlantic but any future storms may need to track a little further south than Florence to benefit from warmer than normal sea temperatures.

tropsst

Cooling can also be seen in the Pacific following recent storms affecting Japan and the Philippines. Progress towards an El Nino state in the Pacific seems to have stalled a little, much like last months ECMWF plume suggested. For the ENSO area 3.4 the ECMWF and UKMO forecast (shown below) is just a little cooler than the CFS2 forecast (above).

tropsst1

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 for the winter. 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) had been mostly in a positive phase but is forecast to turn neutral or negative in the next few weeks suggesting some increased in Atlantic mobility.

naosep2018

For background information see the Met Office NAO information.

Recent Climatology

Temperatures during August in SW England were near or above average with the summer generally above normal making 2018 one of the warmest summers on record. Rainfall in August was more variable and closer to average but for the summer as a whole it was much drier than average as can be seen in the graphics below.

clim09

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.

09smchange

Rover flows in August reflect the longer term drier period. Ground water in the east of the region is near normal, see the full Hydrological Summary for  August for the full details – August 2018 summary PDF 

08riverflows

The reservoir levels in the SW of England (9th Septemer) show 55% storage which is a further decrease since last month.

resevoir

The experimental product from the Global Flood Awareness System based on ECMWF seasonal forecast for September and August data is shown below. It should be noted that recent severe flooding in North Carolina and also in China due to tropical storms was not captured by this system. It is likely that the system is better suited to persistent broad scale rain types rather than one off events as the data is typically means over a period.

SEP2018glofas

Atmosphere predictions.

A:  Stratosphere and Upper Troposphere

The N Pole stratosphere is cooling more or less along the mean although there have been some colder than normal occasions and the polar vortex is forming as shown by the 30hPa analysis on the 16th.

ecmwf30a12

CFS2 200hPa data (as supplied to NNME data set September 2018) for the December to February 2019 period continues to suggest above normal heights across the UK. By January enhanced jet strength across the Atlantic is indicated suggesting greater storminess for the UK. By February this is perhaps confined to the far NW.

z200DJF0918

CFS2 monthly mean 200hPa contour height (upper row) and anomaly (lower row) 

B: Lower Troposphere:

Graphics for October and November available via web link seasonal latest.

Comparing CFS2 NMME and ECMWF model output for the winter months (December January and February).

E3DJF0918

CFS2 E3 10 day mean of models. Top row temperature anomaly, lower row rain rate anomaly.

NMMEDJF0918

NMME data. Top row temperature anomaly, lower row rain rate indication.

ECDJF092018

ECMWF Top row temperature anomaly. Lower row rain anomaly (blue is below normal)

 

Most models forecast above normal temperatures for the remainder of Autumn and also for winter. ECMWF, however,  suggests below normal temperatures in January 2019 supported only by JAMSTEC although a few also have near normal values rather than milder temperatures.

Rainfall is a more varied picture in the models but some agreement that parts of the UK, probably the south may have above normal rain at least for December and January.

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

2. Forecast.

Remainder of Autumn 2018  (October November). 

Milder than average but perhaps for November much milder than normal.

Rainfall starting below normal in October but trending wetter than average for November but uncertainty as to when the change to wetter types will start.

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).

Almost no indication for a colder than normal winter. Main theme is above or well above average temperatures, with just a hint of the temperature only slightly milder than average in wither January or February, the latter being the most likely month.

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 and February may be drier than average.

Below average snowfall with the highest risk of snow possibly in January and chiefly over 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.

Spring 2019  (March April May) based on limited data.

Continuing with milder than average temperatures though some months may be near average. Rainfall below normal at least for one month and probably near or below normal for the season.

Spring climate: 1981-2010 average mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly Mar 7 or 8°C Apr 8 or 9°C  May 11 or 12°C. Average 1981 to 2010 rain 200 to 300mm lowest values in E Devon and over parts of Somerset.

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

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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