Experimental Long Range Forecast for SW England July 2016 issue.

Published 19 July 2016.

Potential influences that might affect the forecast:

SEA TEMPERATURE.

anomnight.7.18.2016

NOAA Night time sea surface temperature anomaly 18 July 2016

The cooler than normal North Atlantic sea temperature area has expanded towards the UK again due to more frequent W or NW winds. This may impact on day time temperatures in some western parts of Eire and the UK. Note though that sea temps to the SW of UK remain above normal so winds from SW or S probably warmer/milder than average. (Sea Temperature in the west Atlantic including the tropical areas are above normal and this may impact rainfall amounts through late summer and Autumn).

El Nino/La Nina conditions are neutral at present with La Nina conditions predicted to develop over the next few months.

figure1

IRI/CPC probability of occurance

ECMWFNina

ECMWF forecast sea temperature anomaly Nino 3.4 location

The duration and strength of the La Nina is uncertain consequently the potential impact on the rainfall as shown by the IRI statictical relationship may be unreliable at the start of Autumn. Consequently the drier than normal signal could be much weaker than shown in Graphic 1: below.

SONnina

Graphic 1: IRI probability for drier/wetter/normal during La Nina event September to November

For the winter months the relationship is not so clear as in Autumn, slightly favouring normal to below normal rainfall across much of SW England apart from N Devon and Somerset which have similar probabilities of each category..

DJFnina

Graphic 2: IRI probability for drier/wetter/normal during La Nina event December to January

MODEL OUTPUT.

A text summary of the seasonal model output from the UK, various USA, Canada, China, Japan, Russia, Korea, Brazil and South Africa can be seen in the UK and Eire summary at www.weatherservice.co.uk

200hPa heights predicted by CFS2 and NMME to be above normal but hints at enhanced zonal pattern in September and October but not in November. This may steer rain bearing weather systems further north and away from SW England. There is some support for this from other models.

200SON

September, October and November 2016 200 hPa. Top CFS2 anomaly. Middle NMME anomaly. Bottom CFS2 mean. Data 7 July 2016.

200nmmeDJF

December January and February 200hPa CFS2 and NMME anomaly and CFS2 ensemble mean

The anomalies suggest lower than normal heights during February, perhaps hinting at slow moving low pressure centres across or near the N of UK.

Looking at the NMME near ground level output for Autumn and Winter a trend to mild conditions is shown but with a hint of less mild temperatures in February. Rainfall anomalies favour less wet conditions in parts of the S and E of England compared to elsewhere.

NNMESON

NMME September October and November top Temp anomaly Bottom Precipitation indicator

DJFNNME

NMME December January February Top temperature anomaly Bottom PPN indication

 

Forecast:

Remainder of Summer (August and possibly into September) 2016:

In summer the  main risk of heavier rain can come from showers or thunderstorms. These can give locally high rain totals but may not be widespread across a region. Despite this risk it seems more likely than not that August and parts of September will see some long dry spells. The S and E of the Region in particular may have below normal rainfall compared to nearer normal elsewhere.

In most parts the temperature for August and for a time in September is probably going to be  above normal although some western parts may be  nearer normal.

(1981 to 2010 average mean temperature  15°C  to 17°C or more 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. Average 1981 to 2010 rain for August 100-150mm over the moors, typically 60-100mm but locally E of Dartmoor and inland in Somerset 40 to 60mm)

Autumn  (most of September, October and November) 2016:

The temperature for the season probably above normal. September probably a little warmer than normal, October nearer normal and November is expected to be especially mild.

Rainfall probably near normal for the season as a whole but parts of SE  Devon and Dorset below  normal. September and some of October possibly drier than normal but with a wetter than normal November for Cornwall, N and W Devon but perhaps not in SE and E parts of the Region.

La Nina conditions expected to become established in the Pacific in the Autumn which may start to influence global weather patterns. For the SW of England this implies drier than normal conditions may develop through the Autumn which is consistent with at least some of the model forecasts.

 (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, although February may see temperatures nearer normal. Near normal rainfall across the SW is more likely, especially in the N and W but with a chance that SE and E parts of Devon and Dorset might be a little drier than normal for the season as a whole. There is a chance that February may see above normal precipitation totals across the region. 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)

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

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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Experimental Long Range Forecast for SW England

Published 19 March 2016 (layout change – Forecast then influences) .

The remainder of Spring (April and May) 2016:

April is likely to have unsettled/changeable weather with spells of rain or showers and shorter drier intervals, hence above normal rainfall accompanied by near or a little above normal temperature. For May a less clear signal, though rainfall is again more likely to be above normal than below normal. This does not rule out some drier spells and parts of Dorset and SE Devon probably near normal rainfall rather than above normal. The temperature in May probably a little above normal.

(1981-2010 average mean temperature roughly Apr 8 or 9°C  May 11 or 12°C but a few degrees cooler over the moors. Average 1981 to 2010 monthly rain April and May 60 to 80mm locally up to 150mm over the moors but 40-60mm around Exeter and widely across Somerset levels.)

Summer  (June July August) 2016:

Probably a typical summer with rather mixed weather, with no strong indication for a long dry summer despite some signs of above average pressure. Rainfall near normal or a little below normal in June and July  (a little more likely to be below normal in July than June) but above normal in August. There is chance that July may see some longer drier spells.

Temperature near or a little above normal in parts of E Devon, Somerset and parts of Dorset but cooler than normal Atlantic sea temperatures may result in near or below normal temperatures elsewhere. This could be due to reduced day time maximum temperature rather than overnight minimum temperatures. July may be a slightly warmer month than June or August compared to the average, which is not unusual.

(1981 to 2010 average 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  (September October November) 2016 with limited data:

Overall temperature for the season near normal, perhaps a little below at first then becoming above normal. Rainfall near normal for the season but in September a higher risk of above normal rain than below normal.

La Nina may conditions may become established in the Pacific by Autumn which may start to influence global weather patterns. For the SW of England this implies drier than normal conditions may develop. The onset of this potential impact is very uncertain.

 (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)

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

Potential influences that could affect the forecast.

Sea temperature.

SST170316

NOAA/NESDIS night time sea surface temperature anomaly 17 March 2016

Typically the coldest time of the year for sea temperatures around the UK but note the further cooling of North Atlantic temperatures relative to normal. Long range forecast systems tend to retain a colder than North Atlantic right through the summer months, much like last year. The impact of this is a likely reduction in temperatures, especially maximum temperatures, in at least the western half of the UK – assuming winds from a westerly direction predominate.

The strong El Nino in the Pacific continues but forecast systems tend to weakening this by summer with potential La Nino conditions for the Autumn and Winter 2016/17 as shown by the graphic below.

ENSO0316

Nino 3.4 Sea Surface Temperature Anomaly prediction comparison

Rainfall probabilities linked to El Nino.

IRIelninoAMJ

IRI rainfall probabilities during El Nino for April May and June (Dry Wet Normal left to right)

The El Nino linked probabilities for April May and June suggest above normal rainfall most likely but nearer normal in parts of Devon, roughly Torbay and eastwards, also in Dorset.A graphic for the summer months is not included due to the weakening El Nino.

There is a chance that La Nina conditions develop in the Pacific later this year. The graphic below shows the Autumn rainfall probabilities based on IRI research for a well established La Nina.

SONnina

IRI rainfall probabilities if strong La Nina develops in time for Autumn – not very likely.

Changes in the Stratosphere.

The anticipated Sudden Stratospheric Warming arrived too late to influence the winter pattern with a major warming event starting during the first week of March, roughly at the same time as High pressure developed across the UK. The 50hPa analysis for the 17th March is shown below for comparison with earlier publications.

50hpa170316

ECMWF 50hPa analysis by FU Berlin

200hPa predicted anomalies.

z200com0316

200hPa CFS2 mean and anomaly

NMME mean not available. CFS2 suggests above normal heights over UK area except perhaps in April. A slightly enhanced westerly jet is predicted in the NW Atlantic in May but with ridging over the UK area.

Conflicting model output.

nmme0316

NMME March 2016 1 deg GRIB data.

The North American Multi Model Ensemble (NMME) which includes CFS2 data, suggests slightly above normal temperatures for SW England and a drier than (model) normal June and July. CFS2 is not so clear cut, some output did suggest drier in SW England in July but the set shown below implies near normal rain although better agreement for April. The data for CFS2 used in the NNME had June drier in the SW. Temperature anomalies show below normal across SW England in the summer month otherwise near average +/-0.5C which is not far from the NNME anomalies. Other models to indicate a risk of cooler summer temperatures include UK Met Office and Beijing Climate Centre and these centres also suggest that slightly wetter than normal is more likely than below normal.

Many long range forecast models seem fairly poor at picking drier than normal months.

 

CFS20316

CFS2 averaged 6 to 16th March. Left temperature anomaly. Right precipitation anomaly.

 

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

 

UK Autumn 2015 and Winter 2015/16 weather outlook

Can the weather for SW England be forecast for Autumn 2015 and Winter 2015/16?

Many International National Weather organisations issue experimental long range forecasts for up to a year ahead using some of the most powerful computers available.

Are the forecasts any good? The answer is not very good but improving. Combining output from many sources led to the conclusion that summer 2015 would be drier in June and then turn wetter and that temperatures would be near normal. This forecast turned out to be a good guide.

For the coming Autumn and Winter there are two complicating factors.

1: The El Nino, in the Pacific Ocean, is likely to last into the middle of 2016 and can impact on Global weather. The International Research Institute for Climate and Society (IRI), a joint USA project between Columbia University and National Oceanic and Atmospheric Administration (NOAA), has published research which implies that SW England may have normal or above normal rainfall this winter (see map).

Featured image

Mason, S.J. and L. Goddard, 2001: Probabilistic precipitation anomalies associated with ENSO.Bull. Amer. Meteor. Soc.82, 619-638. doi

2: Unusually cold North Atlantic Sea Temperature – shown by the sea temperature anomaly map below. The forecast models tend to hold on to these cold ocean temperatures well into winter. This implies that winds from the west or northwest may be cooler than in typical years.

Featured image

NOAA USA National Environmental Satellite Data and Information Service (NESDIS).

Experimental Forecast based on available August 2015 data.

The forecasts, which should take account of the above factors, suggest that the weather for SW England may be quite unsettled in Autumn and for the start of Winter.

Temperatures on average are likely to be near normal or above normal through to January 2016. Consequently, snow risk is reduced until nearer the end of winter, which is not unusual. This does not mean there will be no frost but suggests less frequent frost.

Rainfall is likely to be above normal for Autumn and Winter, although September and February may have less precipitation. The forecast for above normal rain agrees with the statistical El Nino probabilities as published by IRI.

Prepared August 2015 weatherservice.co.uk