Experimental Long Range Forecast for SW England. October 2017 issue.

Published 17 October 2017.

1. Influences.

Changes in sea temperature.

anomnight.10.12.2017

The colder sea temperature anomalies to the SW of the UK have gone although the recent Extra Tropical Ophelia may have caused some cooling/up welling since the image date although this was not evident on an image for the 16th (not shown). Some relative cooling can be seen in the eastern Caribbean but in general the tropical North Atlantic is above normal and expected to remain warmer than normal (see image below). La Nina is evident in the Pacific and forecast to remain as a weak La Nina through to Spring 2018.

 

nino102017

NMME Nino3.4 forecast left. UKMO tropical North Atlantic forecast right.

ninoNAO

CFS Nino 3.4 forecast left. Observed NAO index right.

The North Atlantic Oscillation index (shown by a 500hPa index in the image above) turned positive fairly early in October after a negative phase in September. Weak indication at present for a milder/wetter winter season. For more information see Met Office NAO information.

La Nino conditions counter this to some extent with the IRI statistical relationship between La Nina and UK rainfall for the winter shown below. La Nina is weak so influence may not be strong.

La Nina

Winter season probabilities for Dry/Normal/We based on La Nina occurring 

Analysis and Numerical (model) output.

A: Rainfall during September was above normal across SW England, especially parts of S and SW Devon and S and E Cornwall, in terms of both measured rain and the number of wet days. Parts of Somerset and Dorset though were near normal or even a bit below.

Recent reservoir water levels remain above normal for the time of year. River flows and groundwater in the region are available from the Hydrological Summary  –  September 2017 summary PDF

Sunshine was again below normal during September and the temperature was near or even a little cooler than the 1981 to 2010 averaging periods but near normal when compared to the 1961 to 1990 period. The last generally warmer than average month being June 2017.

B: Upper Troposphere:

Average monthly 200hPa heights are again forecast to be above normal right through until February 2018 in the CFS2 data although increased zonal flow is also indicated January and February. Possibly suggesting low pressure tracks over the central or south of the UK during February rather than over or to the north as in January. Hint of more northerly flows in December?

z200

CFS2 200hPa heights (top) and anomalies (lower) November 2017 to February 2018

C: Lower Troposphere:

Temperature anomalies have been consistent in suggesting above normal temperatures in most model output, see NMME and CFS2 images below as example. Recently anomalies have been slightly warm so perhaps for November and December in SW England temperature may be nearer normal.

NMME102017

NMME Temperature anomaly (top). Rain rate anomaly (lower). November 2017 to February 2018

Rainfall forecasts are more variable in model output as can be seen by comparing NMME above and CFS2 below. There is also little if any consistency run to run. The main signal seems to be for above normal rain during the winter season but with some months possibly near or below normal made up for by wetter months.

E3DJF

CFS2  Temperature anomaly (top). Rain rate anomaly (lower). November 2017 to February 2018

D: Comment:  Seasonal temperature and rainfall forecast data have been poor recently, which is consistent with the data from 2016 which showed poor results for output issued during August and September. Rainfall forecasts were especially poor. Verification summary.

2. Forecast.

Remainder of Autumn  2017  (November) :

Temperature and rainfall are likely to average out to near or perhaps slightly above the long term average for SW England, although there is a chance that some eastern areas may be drier than normal.

November Climate: 1981 to 2010 November average mean temperature 8 or 9°C but nearer 6°C in upland areas. Maximum temperatures average 11 or 12°C but lower in upland areas. Average 1981 to 2010 rain 80 to 100mm to lee of Exmoor and Dartmoor (as low as 60mm over parts of Somerset) but  in western areas 100 to 200mm is fairly typical with over 250mm over the tops of the moors. 

Winter (2017 December, 2018  January and February) :

There is a strong indication for a mild winter in the SW of England which continues to be signalled by almost all model output. This does not rule out some colder spells but suggests that they will be short lived.  There is a suggestion that temperature anomalies could be lower early in the winter and higher later which hints at the temperature in December being nearer normal but with relatively milder months to follow.

Rainfall is uncertain month to month but the main theme of a wetter than normal winter seems to be consistent although December may be less wet. The snow risk is reduced compared to average with little if any snow for lower ground but some snow for 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 (2018 March April and May)  limted data:

Temperature probably near or slightly above normal for the season. Rainfall near or a little below normal at least in parts of the region.

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 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

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Experimental Long Range Forecast for SW England. September 2017 issue.

Published 19 September 2017.

1. Influences.

Changes in sea temperature.

anomnight.9.18.2017

The Atlantic to the W of the UK changed to a positive anomaly for a time but the colder area to the SW has expanded probably due to recent strong winds in this area. Overall, as can be seen in the graphic above, much of the Atlantic remains above normal for the time of year. In the Pacific conditions are moving towards La Nina as illustrated by the NMME forecast graphic below.

Statistically (according to IRI) La Nina conditions might imply near normal rainfall over much of UK in the winter period (Dec to Feb) but with the NW being wetter and the S possibly drier. The La Nina may not be strong enough for this effect to be noticeable.

SSTNINO

Left: NMME forecast for sea temperature in the Nino 3.4 region. Right: Tropical north Atlantic forecast sea temperature UKMO

AO

The North Atlantic Oscillation index (shown by a 500hPa index in the image above) turned positive in June and more or less remained in a positive phase until the third week in July before reverting to a negative phase which remains the dominant phase in the latest analysis. See Met Office NAO information.

Analysis and Numerical (model) output.

A: Rainfall during August was near or perhaps slightly above normal across much of SW England, in terms of measured rain, and together with the higher totals in June and July this has made up for the deficit from previous months.  The number of “dry” days was near or a little above normal for August but for the summer period as a whole was close to the average despite well above normal rain totals. This adds to the evidence for rain events to be “heavier” when they occur rather than there being more “wet” days.

Recent reservoir water levels have recovered and are now well above normal for the summer months. River flows and groundwater in the region should be available from the Hydrological Summary (Web says it has been published but pdf file was missing as of 19 Sept2017) –  August 2017 summary PDF

Sunshine was below normal for the summer despite temperatures being near or slightly above normal – largely due to warmer June offsetting near normal or cooler values in July and August.

B: Upper Troposphere:

Average monthly 200hPa heights are forecast to be above normal right through until February in the CFS 2 data although increased zonal flow is also indicated later in the Winter period which is somewhat later than earlier forecast output.

z2001

CFS2 200hPa height and anomaly Oct Nov Dec 2017

z200DJF

CFS2 200 hPa mean and anomaly Dec 2017 Jan and Feb 2018

C: Lower Troposphere:

NMME data continues to indicate strong positive temperature anomalies into Autumn and through Winter, although October and November values are nearer normal in SW England possibly due to the cooler than average sea temperatures just west of the UK. There remains strong agreement between most available models that temperatures will be above average through winter. Note rainfall anomaly is indicated in the form of an index for above (oranges) or below (blues) based on daily rain rates average over a month.

NMME1

NMME Temperature anomaly and rainfall index October and November 2017

NMMEDJF

NMME Temperature anomaly forecast and rainfall index. Dec 2017 to Feb 2018

D: Comment: 2016 Seasonal forecast rainfall data available in August and September were generally poor and the 2017 output for Summer 2017 were also quite poor.  Temperatures were generally better forecast although for the summer months there seems to be a warm bias in many models. Verification summary October to December last year.

2. Forecast.

Remainder of Autumn  2017  (October and November) :

Temperatures are likely to be near or perhaps slightly above normal perhaps with November seeing slightly higher positive anomalies than October.

Rainfall probably near normal, perhaps a little below normal in October and hence slightly above normal in November although month to month detail lacks consistency  between model output .

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 (2017 December, 2018  January and February) :

A mild or very mild winter continues to be indicated by almost all model output. This does not rule out some colder spells but suggest they will be short lived. Individual station plots indicate brief cold snaps in late December and in the first half of January although this detail is unlikely to be reliable.

Probably windier than a typical winter with near normal rainfall in the S and E of the Region but probably above normal in the N and W of the Region. Snow risk reduced compared to average, mainly in late December or first hale of January then little if any snow, except perhaps 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 (2018 March April and May)  limted data:

Temperature probably above normal for the season with rainfall near or a little below normal although some agreement that April could be wetter than average.

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 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. August 2017 issue.

Published 20 August 2017.

1. Influences.

Changes in sea temperature.

anomnight.8.17.2017

East and Central parts of the North Atlantic have cooled or remain cooler than normal but Northern and Western parts are now significantly above normal. In the short term winds from the NW may result in cooler than normal temperatures but longer term the potential rise in temperatures hints at a mild Autumn and perhaps Winter.

El Nino is expected to remain in a neutral phase as shown in the CFS2 and NMME ensemble forecast below. A developing  El Nino typically acts to reduce Atlantic Hurricanes hence this effect is not likely for the coming season although the Atlantic season has been a bit slow to develop so far.

nino0817

Tropical Atlantic temperatures are above normal as shown by the UKMO graphic below and the North Atlantic Oscillation (shown by a 500hPa index in the image below) turned positive in June and more or less remained in a positive phase until the third week in July before reverting to a negative phase. See Met Office NAO information.

trop

Analysis and Numerical (model) output.

A: Rainfall during June and July was above normal across much of SW England, in terms of total rain, and this has mostly made up for the deficit from previous months except perhaps in some eastern parts of the region. Interestingly the number of “dry” days was close to the average adding to the evidence for rain events to be “heavier” when they occur which may be linked to global warming as temperatures despite the rain have been normal or above.

Recent reservoir water levels although slightly below average, have recovered. River flows and groundwater in the region show mixed results with moorland rivers showing increased flow as can be seen in the Hydrological Summary –  July 2017 summary PDF

Riverflows

B: Upper Troposphere:

Average monthly 200hPa heights are forecast to be above normal right through until February in the CFS 2 data although increased zonal flow is also indicated later in the Autumn and into to Winter period.

z200SON0817

CFS2 200hPa height month mean and anomaly September to November 2017

z200DJF0817

CFS2 200hPa month means height and anomaly December 2017 to February 2018

C: Lower Troposphere:

NMME data continues to indicate strong positive temperature anomalies into Autumn and through Winter, although September values are near normal in SW England in the latest NMME forecast possibly due to the cooler than average sea temperatures just west of the UK. There remains strong agreement between most available models that temperatures will be above average through winter.

SONT

NMME Sep to Nov 2017 temperature anomaly

DJFT

NMME temperature anomaly December 2017 to February 2018

Forecasts of precipitation issued based on August data have a low success rate compared to the temperature anomalies. That said there is a signal for a wetter than normal winter period with the main uncertainty in the Autumn.

sonP

NMME precipitation anomaly for September to November 2017

djfP

NMME precipitation anomaly December 2017 to February 2018

cap

Combined ECMWF/UKMO/Meteo France. Temperature and Rainfall Tercile summary September to November 2017. 

 

2. Forecast.

 

Autumn  2017  (September, October and November) :

There remains a consistent indication for a warmer/milder than normal season. Possibly well above normal in October but perhaps close to normal in September.

Rainfall probably near normal or a little below normal for the season but the month to month detail is inconsistent and at least one month may be see above normal rain. The trend is for increasing rainfall rates later in the Autumn.

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 (2017 December, 2018  January and February) :

A mild or very mild winter is likely which does not rule out some colder spells. Individual station plots indicate brief cold snaps around the turn of the year and perhaps in February.

Probably windier than typical and rainfall is likely to be above normal for the season. Snow risk reduced compared to average, mainly in late December then little if any snow, except perhaps 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 (2018 March April and May)  very limted data:

Average temperature probably above normal for the season and rainfall near or a little below normal.

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 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 2017 issue.

Published 17 July 2017.

1. Influences.

Changes in sea temperature.

anomnight.7.13.2017

The sea temperature remains warmer than average near and to the southwest of the UK as well as in the Mediterranean. Much of the North Atlantic remains below average and this colder area has extended towards Ireland.  The colder North Atlantic sea temperatures increases the risk of cooler temperatures this summer with winds from the West or NW but winds from other directions flow over warmer than normal seas.

El Nino is expected to remain in a neutral phase as shown in the CFS2 and NMME ensemble forecast below. A developing  El Nino typically acts to reduce Atlantic Hurricanes hence this effect is not likely for the coming season.

nino3.4

Tropical Atlantic temperatures are above normal as shown by the UKMO graphic below and the North Atlantic Oscillation (shown by a 500hPa index in the image below) turned positive in June and has more or less remained in a positive phase. See Met Office NAO information.

NATL

Analysis and Numerical (model) output.

A: Rainfall during June was above normal across much of SW England but this has not made up for the deficit from previous months.

rainfall

In addition most of the rain in June fell on just two days and reservoir water levels remain below average. River flows and groundwater in the region show mixed results with moorland rivers showing increased flow as can be seen in the Hydrological Summary –  June 2017 summary PDF

res

SW Water Resevoir Levels

B: Upper Troposphere:

There is some agreement between models that pressure in the south of England may be above average through the Autumn. Average monthly 200hPa heights are forecast to be above normal right through until February in the CFS 2 data.

z200SAON0717

CFS2 mean monthly 200hPa heights and anomaly August to November

z200DJF0717

CFS2 mean monthly 200hPa heights and anomaly December to February

C: Lower Troposphere:

NMME data continues to indicate strong positive temperature anomalies through the remainder of summer, into Autumn and through winter. There is good agreement between most available models that temperatures will be above average through winter.

Autumn

August to November NMME monthly temperature anomaly (upper row) and (lower row) rainfall rate indication for below in blue or above in orange 

Copernicus and CFS2 three month means differ in the temperature anomaly and rainfall but this may be due to smaller anomalies during this period.

soncomp

Rainfall forecasts are the least reliable (as shown in the verification section below) but the strongest anomalies for the Autumn are in September and October. For the winter there is a strong signal for an unusually mild and wet season across much of the UK.

winter

NMME temperature and rainfall anomaly forecast for Dec 2017 to Feb 2018

Verification. Recent results June 2017.

NMMEPPN

The image above shows June rainfall anomalies based on forecast data available in March April and May and the associated observed anomalies (right) in CPC and Met Office data. The most recent 3 month verification is available at this link.

2. Forecast.

Remainder of Summer 2017  (August):

Above normal temperatures are most likely though perhaps not averaging out with a large anomaly. There is a chance that Easterly winds may be more frequent than normal, due to higher than normal pressure over the UK. Probably drier than normal, reduced number of wet days if not rain total. Rain total may be modified by shower/thunderstorm risk which can bring locally high rain totals but not for the whole region.

August 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. Average 1981 to 2010 rain typically 60 to 80mm, but 40 to 60mm in places especially East of the moors and in Somerset. Locally over 100mm across the moors. 

Autumn  2017  (September, October and November) :

There remains a consistent indication for a warmer/milder than normal season. Possibly well above normal in October but perhaps only slightly above normal in other months.

Rainfall probably near normal or a little below normal for the season but the month to month detail is inconsistent and at least one month may be see above normal rain. The trend is for increasing rainfall later in the Autumn.

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 (2017 December, 2018  January and February)  –  limited data:

A mild or very mild winter is likely which does not rule out some colder spells. Individual station plots indicate brief cold snaps in late December and again in January and February. February may be slightly less mild than the other months, compared to average.

Probably windier than typical and rainfall is likely to be above normal for the season. Snow risk reduced compared to average, mainly in late December then little if any snow, except perhaps 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.

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. 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. February 2017.

Published 17 February 2017.

1. Potential influences.

Changes in sea temperature.

anomnight-2-16-2017

Apart from some relative cooling in Biscay and around Spain and Portugal, the sea temperature anomaly in the North Atlantic is similar to those in mid January. The La Nina in the mid Pacific has decayed with the current neutral conditions likely to slowly return to a weak El Nino state during the 2017 summer. This transition can been seen in the CFS2 and NNME output shown below which is perhaps a little quicker and stronger indication than last months forecast.

Should an El Nino develop more quickly the influence on rainfall in the SW of the UK (according the statistical output IRI statistical output) 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:

A major stratospheric warming event took place in early February with a splitting of the Polar Vortex and a temporary reversal of the stratospheric wind in the UK area. Temperature at the North Pole rose sharply at 30hPa but has since returned to normal, as shown in the graphic below.

pole30_nh

Tokyo issued at major stratospheric warming start message on the 3rd and an end message on the 10th.

startstopstrat

Subsequently the western part of the vortex has declined with a new strong vortex developing to the north of Russia during the second half of February.

contstrat

It is not clear whether this will aid the development of high pressure to the west of the UK and more cyclonic patterns further east hence an increased risk of chilly NW types for the UK?

B: Troposphere:

200hPa CFS2 and NMME monthly mean anomalies hint at slightly stronger jet flow towards the S of UK in March but with above normal heights forecast to develop later in Spring.

z200mam0217

March April May 2017 CFS2 200hPa mean heights (top) anomaly (mid) and NMME anomaly (bottom row) Data 6FEB2017.

z200jja0217

June July August CFS2 mean heights anomaly and NMME anomaly

For the summer months positive anomalies continue but with differences between CFS2 and he other models making up the NMME group of ensembles.

C: Surface NMME output:

mam0217

NMME Mar Apr May temperature anomaly (top) Rainfall indication for above/below normal.

There is good support from the various seasonal models for above average temperature or at worst normal values. Rainfall patterns are less clear but other output does suggest a wetter March in the west.

jja0217

NMME Jun Jul Aug temperature and rainfall indication.

There is a consistent signal for above average temperature (up to plus 1 C anomaly) across the UK  the rainfall is less consistent but at least a hint of drier overall in places.

Recent results.

UK: Verification for the November 2016 to January period showed that the temperature for the season was a reasonably guide for the UK but month to month detail was unreliable. Precipitation detail is often poor mainly due to dry spells being under forecast.

SW England:

2. Forecast.

 

Spring (2017 March April and May) :

Average temperature probably above normal for the season. March less clear could be nearer normal, with chilly NW or N winds for periods although if rainfall correct then milder nights may tip the balance to above normal. April slightly above normal with May possibly the mildest relative to average values.

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. March could be the wetter month of the three.

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 2017  (June July August):

Temperatures probably warmer than 1981 to 2010 average although some western coastal areas may be nearer normal. Rainfall, probably drier then average in terms of number of dry days and possibly also rain total. Drier types more especially for July and August perhaps after a wetter June. This is a change from last months forecast and may be unreliable.

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.

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

 

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

(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. January 2017.

Published 18 January 2017.

1. Potential influences.

Changes in sea temperature.

anomnight-1-16-2017

Sea temperature anomalies are very similar to a month ago with areas of above normal near to the UK and a similar area of below normal in the North Atlantic. In the Pacific the weak La Nina continues to weaken with essentially neutral conditions over the next several months, possibly trending to weak El Nino conditions late in 2017. This is illustrated by the CFS2 and NMME ENSO 3.4 area forecast sea temperature shown below.

nino34mon

Numerical (model) output.

A: Stratosphere:

The North Polar Vortex was slow to establish during the early winter but is now well established with temperatures low enough for Ozone reduction. Zonal 50hPa and Polar 30hPa temperature plots are shown below.

50mbnhlopole30_nh

Ozone shows at least a reduction of 20% in the polar vortex area although this area remains out of sunlight for now.

current

Areas of stratospheric warming developing across the Pacific are likely to lead the polar vortex being disturbed. Consequently the colder temperatures in the polar vortex may relocate and not persist resulting in a recovery in Ozone values.

ecmwf50a12

The 50hPa analysis 17th Jan 1200UTC shows the strong polar vortex and the forecast sequence suggests a relocation of the vortex and perhaps the start of conditions favourable to more blocked surface patterns for the start of February. The evolution of the polar vortex/stratospheric warming will need to be watched to see if moves to a pattern more conducive to blocked surface patterns for the UK area due and a reversal of the stratospheric flow.

B: Troposphere:

The NMME 200hPa output suggest above normal heights over the UK in the period February to August 2017 but there is also a suggestion of enhanced jet flows across parts of the N Atlantic steering low pressure towards or to the North of the UK. Consequently a winter blocked pattern may be in the wrong place to bring significantly colder temperatures to the UK for the last part of Winter/Early Spring.

z200fmam0117

NMME 200hPa anomalies with (lower row) CFS2 mean and anomaly Feb to May 2017

z200jja0117

As above but for the period June to August 2017

C: Surface NMME output:

nmmefmam0117

NMME month mean anomaly. February to May 2017. Top row temperature. Lower row indication of above (oranges) or below (blues) model normal rainfall rates.

nmmejja0117

NMME data as above but for period June to August 2017.

Model output (as available between 1st and 18th January) mostly agrees with the idea of above normal temperatures (as shown in the graphics above) although, in February, the outcome for the S of the UK is perhaps less clear cut with NASA data hinting at a colder and CFS2 suggesting near normal. Rainfall is always more problematic because rainfall is often about mean rain rates and short periods of heavy rain not likely to be captured, hence sometimes drier than normal might mean less rain days rather than lower rainfall totals.

Recent results.

Verification for the October to December 2016 period showed that seasonal model rainfall output was quite misleading, partly due to the poor prediction of blocking High Pressure and associated dry weather. Consequently given the risk of a stratospheric warming event leading to increased chance of blocking there may also be an increased chance of drier and colder weather for a time in February or March.

The first half of winter has been drier and milder than average with below average sunshine (cloudier skies) but above average pressure. This is probably unusual because dry weather and high pressure in winter is more likely to lead to colder than milder temperatures.

2. Forecast.

Remainder of Winter ( 2017 February) :

The mean temperature in February is most likely to be near or above the longer term average with less frequent frosts. The early part of the month is most likely to be mild, though with a risk of colder spells later. Despite higher than average pressure winds may often be from between West and South suggesting rather cloudy skies.

Rainfall probably near or below normal at least during the first half of the month. The snow risk remains lower than average which means some snow accumulation over the moors but not much elsewhere.

February climate: Temperature 1981-2010  typical mean 5 or 6°C but over 6°C in West Cornwall and only 2 or 3°C over the higher ground). Rainfall: 60 to 80mm to lee of moors, but 40 to 60mm over parts of Somerset. 80 to 100 elsewhere but higher totals over parts of Cornwall and NW Devon but over 200mm  over the higher moors). Snow climatology  many low lying areas less than 1 Day on average, hills 1 to 4 days but tops of moors 4 to 10 days. One in three years have no lying snow.

Spring (2017 March April and May) :

Mean temperatures probably above normal for the season, as well as for each individual month. March may well be relatively milder (stronger anomaly) than April or May which could be closer to normal. 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. March could be the “wetter” month of the three.

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 2017  (June July August):

Slightly warmer than average summer temperatures although some western areas may be nearer normal due to more frequent west winds. Rainfall, probably drier then average in terms of number of dry days if not the rain totals. Drier types more especially for June and July rather than August. Note a drier June and/or July and wetter August is not unusual.

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 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