Experimental Long Range Forecast for SW England. September 2016 issue.

Published 17 September 2016.

Potential influences that might affect the forecast:

SEA TEMPERATURE.

anomnight-9-15-2016

NOAA/NESDIS night time sea temperature anomaly 15 September 2016

The area of below normal sea temperature in the north Atlantic has expanded somewhat during September possibly due to overturning/up-welling caused by fairly frequent areas of low pressure/strong winds in this area. This contrasts with the warm anomalies in the North Sea, Baltic and to the SW of the UK. The cold anomaly may impact on temperatures later in the winter if this expands and winds are as typical westerly but in the Autumn the residual warmer temperature around the UK may help keep temperatures above normal.

It was anticipated that, after the Pacific El Nino of 2015/16, a La Nina event would follow as is mostly the case. The La Nina however has so far been very week and barely cooler than cool Neutral Conditions. The forecasts produced by various models since late 2013 has been assembled by by IRI and is shown below:

ensofcst_dynam_sep16_l

The most recent multi model forecast for the Nino3.4 location shows that most models suggest only a weak La Nino through the winter then a rise in sea temperature.

figure4

The latest probability forecast (below) suggests that Neutral conditions are most likely to prevail.

figure1

A consequence of the weaker than expected La Nina is that the statistical relationship between rainfall in the UK area and La Nino is unlikely to be helpful.

MODEL OUTPUT.

A text summary of several seasonal model outputs can be seen in the UK and Eire summary at www.weatherservice.co.uk. The models show reasonable agreement with regards to temperature but much less consensus in the precipitation output (which is often the case).

CFS2 200hPa (15 Sept 2016) ten day ensemble monthly mean forecast and anomaly suggests an enhanced westerly jet strength across the Atlantic, initially across the north and mostly in central and the West of the Atlantic during November and February. In December some upper level ridging is suggested but in January a more generally cyclonic pattern is suggested over the UK area.

z200

The NMME temperature and rainfall anomaly forecast for the period October 2016 to February 2017 is shown below:

ondjf

The lowest temperature anomaly is show for December with above normal rainfall for SW England except in November and to a lesser extent in October and December

 

Forecast:

Remainder of Autumn  2016 (October and November):

Good agreement with respect to the temperature which is likely to be above normal through October and November. There is a chance of some very mild spells.

Rainfall probably near or below normal taking the two months together but with some longer dry spells quite likely especially in the SE and E of the Region. Uncertain as to which month has the best chance of the drier spells with recent forecasts suggesting November but earlier ones October.

 (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  (2016 December, 2017  January and February) :

A mild winter is most likely (does not rule out some frost), although December may see temperatures closer to normal. In January and February temperatures probably above normal, possibly very mild at times.

Near normal or above normal rainfall across the SW is most likely for the season as a whole. The N and W of the region is most likely to have above normal rainfall. There is a chance that SE and E parts of Devon and Dorset might be a little drier than normal for periods in December and possibly February too but make up for this in January. Below normal snowfall is more likely than above normal which means just a little snow accumulation over the moors but not much elsewhere.

(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 (2017 March April and May) :

The limited data available for this period suggests milder and wetter than normal.

(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.)

Caution:

Experimental Long Range Forecasts do not have a good success rate. The data used for the above forecast summary and brief verification of previous Long Range Forecast summaries can be seen at   http://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

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/

Stratospheric images prepared by Freie Universität Berlin using European Centre for Medium Range Weather Forecasting data supplied by Deutscher Wetterdienst

NMME information:   http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-12-00050.1

CFS2  info

 

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