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

Published 20 October 2018

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1. Influences.

Changes in sea temperature.

181018sst

The sea temperature around the UK remains well above normal in the south but has shown some cooling in the north. The northern North Atlantic remains below normal as is the Great Lakes area which is a little surprising given the milder than average temperatures in the area over the later month or so. Recent tropical storms do not seem to have material depressed sea temperatures to the S and E of the USA and transition towards El Nino can be seen in the Pacific.

tna_anom_20181001

Met Office plot of tropical North Atlantic sea temperature (above right) seems to have picked out a colder area but in the Pacific there is broad inter-model agreement for an El Nino (Enso 3.4 area) and possibly a longer lasting event than previously forecast.

The dominant IRI statistical predictor for UK rainfall based on the Pacific sea temperature anomalies for the winter season is shown below. Probabilities are for “near normal” precipitation,

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 sharply negative in a week or so reflecting the cold northerly plunge into UK/Central Europe after the 26th of October rather than increased Atlantic mobility although this may occur in the far North of the Atlantic.

NOA

For background information see the Met Office NAO information.

Recent Climatology

Temperatures during September in SW England were near average following an unusually warm summer. Rainfall in September was near or a little below average and followed a drier than average summer.

HSRA0918

Although there has been some significant rainfall recently, after the preceding drier months soil moisture content remains below normal in the south of UK. The present dry spell looks like reducing the water content further; as shown by the GFS analysis and forecast below.

sm1018

Rover flows in September reflect the longer term drier period being near or below normal for September and more especially the June to September period. Ground water in the east of the region is was near normal. The full Hydrological Summary for September can be viewed here – September 2018 summary PDF 

river

The reservoir levels in the SW of England (14th October) show 51% storage which is a further decrease since last month although recent heavier rain has caused a slight uptick in levels.

res

The experimental product from the Global Flood Awareness System based on early October ECMWF seasonal forecast for October to January. It should be noted that severe flooding due to tropical storms has not been captured this year and it is likely that the system is better suited to persistent broad scale rain types. This month the output does not indicate a preference for below or above normal area rainfall but has local river flow shown as above (blue) or below (orange) normal.

glofas

Atmosphere predictions.

A:  Stratosphere and Upper Troposphere

The N Pole stratosphere is cooling in line with normal variations and the polar vortex has formed as shown by the 30hPa analysis on the 19th October 2018.

30hpa

CFS2 200hPa data (as supplied to NNME data set October 2018) for the December to February 2019 period continues to suggest above normal heights across the UK. for both December and January there are signs of enhanced jets suggesting stormy spells at least for the north of the UK.

z200DJF1018

Winter monthly mean 200hPa height forecast and anomaly

For Spring above normal heights predominate but with a rather more cyclonic pattern for parts of April hinted at in the anomalies.

z200MAM1018

Spring monthly mean 200hPa height forecast and anomaly

B: Lower Troposphere:

Graphics for November available via web link seasonal latest.

Comparing CFS2 NMME and ECMWF model output for the winter months (December January and February). Oranges/red are above “model normal” blues are below.

CFS2e3djf102018

CFS2 E3 10 day mean of output. Top temp anomaly Lower precipitation rate anomaly.

nmmedjf102018

NMME.  Top temperature anomaly Lower precipitation rate indication.

ECDJF102018

There is some agreement for temperature anomalies to become more positive during the winter season but rain rate anomalies are unclear and by February EC has the opposite signal to that of  NMME.

NMME data for Spring (March to May 2019) perhaps indicate a colder start to spring in the SW and a milder end with increased precipitation in April compared to average. There is perhaps an increase snow risk in the southwest in March than average following a decrease in snow in Winter.

nmmemam102018

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.

November 2018

Probably milder and wetter than average. See month ahead forecast  here

Winter 2018/9  (December January and February).

Main consensus is for above average temperatures for the season but perhaps starting off with near normal values in December then trending milder than average later in the winter.

Rainfall likely to be above normal for the season especially in the west with some parts of the east of SW England having near normal precipitation.

Below average snowfall is likely with the highest risk of snow possibly late in the winter 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 less data.

Possibly a colder than average March then recovering to milder than average temperatures for the season.

March may be drier than average but with increased snow risk. April and May probably wetter than average especially April.

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.

Summer 2019  (June July August) based on very limted data.

Another warmer than average summer according to 2 out of 3 available models with above normal rain although early summer may be drier than average.

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.

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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