Experimental Long Range Forecast for SW England. March 2017.

Published 18 March 2017.

1. Potential influences.

Changes in sea temperature.

anomnight.3.16.2017

The Sea Temperature Anomaly (above) shows fairly small differences from previous anomaly charts. There looks to be some cooling, relative to average, in the Atlantic along 30 deg North and a little warming in the eastern Pacific near the equator which is consistent with the slow transition to El Nino conditions as shown in the graphic below.

nino34

Ensemble forecast for the Nino 3.4 area of the Pacific. Left NMME+IMME and Right CFSv2.

The influence El Nino has on rainfall in the SW of the UK according the statistical output produced by IRI is such that there is an increased probability of near normal summer rainfall for the N and W of the Region but less impact in the S and E.

Analysis and Numerical (model) output.

A: Stratosphere:

The polar vortex at 30hPa remains displaced with further weakening likely in the next couple of weeks, s shown by the ECMWF 30Hpa forcast below. Analysis by JMA shows N Pole Temperature above normal following the February Stratospheric Warming.

strat

B: Troposphere:

NMME 200hPa data was not available. CFS2 data continues to suggest above normal heights and hence reduced westerly Atlantic jet flow for the next several mouths. Note, however, that in August the anomaly is reduced over the UK area.

 

CFSz200

C: Hydrological Summary – see February 2017 summary PDF

River flows across SW England were only slightly below normal in February but for the Winter (Dec to Feb) flow were generally below normal and in the north of the region, exceptionally low. Rainfall for the winter was below normal and showed a return period of between 10 and 20 years. Looking at longer lead periods for example; Sept to Feb and March 2016 to Feb 2017, the return period for the recorded below average rainfall extends to as much as 25 years. According to South West Water reservoir storage is about 82 percent of normal  and is running below the drought level of 1995, although much depends on rainfall over the next few months as o whether this will have an impact on summer water availability. There is a lot more water storage available in the SW than during the 1980’s when there were significant water restrictions in places.

D: Surface outlook.

There is good support from international seasonal model output (March 2017 data) for temperatures to be above normal as illustrated by the NMME temperature anomaly plots below.  Rainfall is more problematic and there is much less agreement between the models with regards month to month and location specific detail. For example recent CFS2 data (output date 16th March) suggests SW of UK being drier in April and wetter in July. The drier weather in NMME output being a little further south over France in April, which is typical of the type of variability between models.

NMME0317

Some models indicate above normal sea level pressure over the UK during the next several months, although this becomes a weaker signal in the south of the UK for the summer with hints at lower pressure in the south and over France.

Recent results.

UK: Verification for the December 2016 to February 2017 period showed that the temperature for the season was a reasonably guide for the UK but precipitation was often over estimated. Looking back to Summer 2016 NMME forecast in March and May 2016 gave some good indication for drier types in the south of UK in July.

2. Forecast.

Remainder of Spring (2017 April and May) :

Average temperature probably above normal for both April and May although with the chance of periods of High Pressure could not rule out some chilly nights in April. In May perhaps only slightly above the long term average.

Rainfall probably near normal for the season, perhaps above normal in the more exposed western parts of the Southwest but below normal in parts of South and East Devon, parts of Somerset and into Dorset. There is a chance for above normal rainfall in all areas but rainfall tends to be over forecast in the models.

Climate: 1981-2010 average period. Spring mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly Apr 8 or 9°C  May 11 or 12°C. Spring average rainfall 200 to 300mm lowest values in E Devon and over parts of Somerset,  April  typically 60 to 80mm but in SW Devon and parts of Cornwall 80 to 100mm to as low as 40 mm East of the Moors also in Somerset and parts of Dorset to more than 100 mm over the moors. May typically 60 to 80mm but as low as 40mm E of Moors and in Somerset and parts of Dorset and more than 100mm over the moors.

Summer 2017  (June July August):

Consistent indication for temperatures to be warmer than 1981 to 2010 average although some western coastal areas may be nearer normal. July may be further above average then August but there are no indications for a cooler than normal summer.

Rainfall, probably drier then average in terms of number of dry days and possibly also rain total. Uncertain as to which month might have the highest number of wet days. Fairly strong signal for July to be drier. Indications for lower pressure than normal over France and the South of the UK may imply increased risk of thunder showers with highly variable rainfall in August.

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.

Autumn  2017  (September, October and November) 2017 – limited data:

Possibly a drier and warmer start to Autumn then near normal or wetter with milder than normal temperatures.

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.

3. 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

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

 

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