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

Published 20 June 2017.

1. Influences.

Changes in sea temperature.

Sea temperature anomaly 19 June 2017

The sea temperature has warmed near and to the southwest of the UK as well as in the Mediterranean. Much of the North Atlantic has cooled, probably due to mixing of the surface layers due to large deep low pressure areas during June as shown in the composite of analysis for June to date. The colder North Atlantic sea temperatures increases the risk of cooler temperatures this summer with winds from the NW but winds from other directions flow over warmer than normal seas.

Analysis 1 to 20 June Midnight UTC

The risk of a developing El Nino looks to have reduced as shown by the CFS SST forecast for Nino 3.4 location in the Pacific. El Nino typically acts to reduce Atlantic Hurricanes hence this effect is not likely for the coming season. Tropical Atlantic temperatures are above normal as shown by the UKMO graphic below.

The North Atlantic Oscillation (shown by a 500hPa index in the image below) went negative during May but has since return to a positive phase. See Met Office NAO information.

 

Analysis and Numerical (model) output.

A: Rainfall during May was again below normal across much of SW England but not everywhere with parts of south Devon not far from normal. River flows and groundwater in the region are below normal – for more details, see the Hydrological Summary –  May 2017 summary PDF

June so far has seen rather mixed rainfall with period of heavy rain in the first third of the month followed by a dry period around the middle of the month and what looks like a wetter end to June. Following a “dry” winter and Spring reservoir levels remain below normal for the time of year and also below the 1995 drought levels. Water storage is much greater than during the 1980’s when significant water saving measures came into play and there has been a slight reduction in the deficit early in June.  For reservoir details see SW Water web site.

B: Upper Troposphere:

200hPa heights are forecast to be above normal although in August the NCAR anomaly suggest at least some risk of more troughing.

200hpa  CFS2 July and August mean and anomaly with lower images being the NCAR anomaly.

For the Autumn above normal heights continue to be forecast at 200hPa

September to November 200hPa mean and anomaly. Lower images NCAR anomaly.

C: Lower Troposphere:

NMME data continues to indicate strong positive temperature anomalies through the remainder of summer and into Autumn

NMME June data. Top Temperature mean anomaly July and August. Bottom row indicator for below (blue) or above (orange) normal rain rate anomaly.

NMME June data. Top Temperature mean anomaly September to November. Bottom row indicator for below (blue) or above (orange) normal rain rate anomaly.

Although all models agree on normal or above normal temperatures (mostly forecast above), for the summer and autumn, rainfall predictions are not so reliable.

NMME data suggest wetter periods for SW England in September and perhaps in November but overall indicates a continuation of the drier than normal types. A note of caution is required because model resolutions prevent the forecasting of smaller scale rainfall for example showers and thunderstorms. Consequently the drier types may be indicative of less wet days in a month rather than total rainfall. The sample CFS2 shown below indicates a wetter October in the SW for example.

Verification. Recent results Spring 2017.

Models overall had a good idea that temperature would be above normal but rainfall forecasts where not good with little signal for a very dry April and May which was wetter in the SE third of UK but drier elsewhere. An example of  forecast reliability is shown below with the March April and May temperature and rainfall anomalies compared to the Copernicus multi model ensemble forecast based on data in February 2017 data.

Spring temperature (top) and rainfall (lower) anomaly forecast from Feb 2017 data compared with observed anomalies.

 

2. Forecast.

Remainder of Summer 2017  (July and August):

Above normal temperatures are most likely and possibly well above normal values. There is a risk that Easterly winds may be more frequent than normal, due to higher than normal pressure over and to the north of the UK.

After and unsettled start to July probably becoming drier than average in terms of number of dry days and possibly also rain total. August may be also drier than normal but perhaps modified by shower/thunderstorm risk which can bring locally high rain totals but not for the whole region.

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 in July typically 60 to 80mm, but 40 to 60mm in places especially East of the moors and in Somerset. Locally over 100mm across the moors. August slightly small areas with the lower rain totals due to July often being drier than August.

Autumn  2017  (September, October and November) :

Consistent indication for a warmer/milder than normal season. Possibly well above normal for September and October but perhaps less so in November.

Rainfall probably near normal overall but above normal in September and perhaps November. October probably near or below normal rainfall.

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)  – very limited data:

December probably near normal although hints at a colder spell late in the month. January and February milder than normal, hence overall a milder than normal winter.

Probably windier than typical and rainfall is likely to be above normal for the season. Snow risk mainly 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. May 2017 issue.

Published 17 May 2017.

1. Influences.

Changes in sea temperature.

The Sea Temperature Anomaly (above) shows further cooling in western parts of the North Atlantic and little change in the warmer than average area in the East Pacific. The North Sea, Biscay and northern Mediterranean areas look to be a little cooler than average for mid May.

El Nino 3.4 predictions suggest a slight strengthening as shown by the NMME and CFS2 forecast plots below, although CFS2 has been trending nearer normal but with a large spread in solutions

The influence of an 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.

The North Atlantic Oscillation (shown by a 500hPa index in the image below) has gone negative during May. Should this be maintained then this might imply a drier and warmer summer for the UK. See Met Office NAO information.

Analysis and Numerical (model) output.

A: Rainfall during April was unusually low across much of SW England – for details see the Hydrological Summary –  April 2017 summary PDF

There was some very heavy rain for two days spanning the end of April and start of May across parts of S Devon and W Dorset. Depending on whether rainfall measurement period 00-24 or the standard UK climate period of 0900-0900 is used half the rain total was either included in the May or in the latter case the April rainfall figures. Since then, following a dry week or so, May has become wetter. Following a “dry” winter reservoir levels remain below normal for the time of year and also below the 1995 drought levels. Water storage is much greater than during the 1980’s when significant water saving measures came into play.  For reservoir details see SW Water web site.

B: Upper Troposphere:

NMME 200hPa ensemble mean was not available. Looking at four of the available models there was agreement in that heights would be mostly above normal in July and possibly other summer months and that an enhanced SW jet was likely in November near UK and between 40 and 50 deg North across the Atlantic.

CFS2 200hPa mean height (top row) and anomaly (lower row) for June to August

Taken together 3 out of 5 of the models in the NMME grouping suggest above normal heights for the June to October period.

CFS2 200hPa mean height and anomaly September to November

Looking at data from the Copernicus ensemble system for 500hPa height anomalies there is a strong signal for above normal heights this summer although Meteo France hints a lower heights near to the South of the Uk and more especially further south over France and the Mediterranean.

C: Lower Troposphere:

There is a very strong signal from most models for above normal temperatures during both Summer and Autumn, although Meteo France suggest coastal areas near E or SE facing coasts might well be cooler in June and given the cooler N Sea temperatures at present this may be correct for the early part of June. BCC China suggests that the next colder than average month might be December. The stronger anomaly in November probably linked to increased Atlantic mobility which shows up in the rainfall rate anomaly.

June July and August 2017. NMME whole month temperature anomaly (top row) and  indication for above or below model normal rainfall (lower row).

In general rainfall anomalies are less well forecast and show bigger variations in detail between the models.  That said there is some agreement that July could be the drier month and November the wetter.

September October and November 2017. NMME whole month temperature anomaly (top row) and  indication for above or below model normal rainfall (lower row).

Verification. Recent results summary for 2017.

January to April 2017 models overall had a good idea that temperature would be above normal but rainfall forecasts where not especially good with little signal for a very dry April. UK area Seasonal Forecast Verification

 

2. Forecast.

 

Summer 2017  (June July August):

Above normal temperatures are most likely and possibly well above normal values. There is a risk that Easterly winds may be more frequent than normal, due to higher than normal pressure over and to the north of the UK. This may make some coastal areas exposed to the east wind cooler at times especially during early June until sea temperatures increase.

 

Rainfall, probably drier than 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 but some  hints that June could be wetter at least for a time followed by a drier July and more mixed picture in August. Indication of lower pressure than normal over France and the South of the UK may imply increased risk of thunder showers during the summer leading to highly variable rainfall totals.

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) :

Consistent indication for a warmer than normal September with temperatures above normal throughout the season. Possibly well above normal in November, although this may be offset by stronger winds?

Rainfall in September and October probably near or below normal but followed by a wet November, so on balance ending up near normal for the Autumn season.

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)  – very limited data:

December may have colder than normal temperatures in places although the SW of England probably near normal. Returning to milder than normal for the remainder of winter.

Rainfall is likely to be above normal with 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. April 2017 issue.

Published 18 April 2017.

1. Potential influences.

Changes in sea temperature.

The Sea Temperature Anomaly (above) shows a little cooling in the western parts of the North Atlantic and an expansion of the warm area in the East Pacific. The Mediterranean and areas around Spain look to have increased warm anomalies.

El Nino predictions suggest a slight strengthening as shown by the NMME and CFS2 forecast plots below, although only a minority suggest a strong El Nino this summer.

Nino 3.4 Forecast. Left NMME 08Apr17 and Right CFS2 18Apr17.

The influence of an 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 has been completely warmed out as shown by the ECMWF 30hPa plot.

B: Hydrological Summary – see March 2017 summary PDF

Due to higher rainfall in March river flows across SW England were near normal and in the north somewhat above normal. October 2016 to March 2017 rainfall was below normal and the groundwater levels in the SE of Devon and into Dorset were below normal. (Parts of this area relies on water extraction from underground sources). Reservoir levels in the SW Water area stand at about 87% of capacity which is about 10% below the level prior to  the summer 1995 drought levels. April to date has been a drier than normal month.

C: Troposphere:

Forecast data from CFS2 and NMME show above normal 200 hPa heights through the Summer and into Autumn suggesting reduced westerly jet winds compared to a typical year. Pressure may be higher than average during the May to October period.

May to August CFS2 200hPa monthly mean (top) NMME anomaly (middle) CFS2 anomaly (lower)

Sept to November 200hPa

 

D: Surface outlook.

There is good support from international seasonal model output (April 2017 data) for temperatures to be above normal as illustrated by the NMME temperature anomaly plots (shown below). BCC China hints at the next colder than normal month may be December although this is not supported by other models. There is no significant indication of a colder than normal summer but note that the models have not been good at picking out a colder month in a sequence of mild months.

May to August NMME temperature mean anomaly (top) and rainfall rate indicator (below)

September to November NMME temperature anomaly and rainfall indicator.

Rainfall has been less reliably forecast and forecasts are not consistent between models or between new runs of the same model. The developing El Nino statistically leans towards to near normal rainfall over at least parts of the SW through summer although the El Nino may not become strong enough to be sure of this relationship holding true.

NMME signals are quite weak for SW England suggesting a wetter June and possibly August although May could well start drier and cooler than normal. There is some support for November being the next “wet” month,

Verification Recent results summary for 2016.

Rough and ready assessment of temperature and rainfall from various models  three month seasonal forecasts.  Note how forecasts issued in November to April had better results than other months. The left hand column is the total out of a potential max score of 36.

Temperature. Most models gave favourably results over 50% of the time.

Rainfall. Fewer models got favourable results over 50% of the time. 

There are hints that multi model ensembles may detract from a single models better performance.

2. Forecast.

Remainder of Spring (2017 May) :

Temperature probably near normal in May after a colder than normal start, becoming milder than average later. Chilly nights may be compensated for by warm daytime sunshine.

Mixed rainfall signal but probably near normal although parts of South and East Devon, Somerset and Dorset may be drier than normal.

Climate: 1981-2010 average period. May mean temperature 11 or 12°C but a few degrees cooler over the moors.  Spring average rainfall 200 to 300mm lowest values in E Devon and over parts of Somerset,  May rainfall 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. Increased chance of well above normal temperatures (anomaly more than 1 deg C) during the period. 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 but some  hints that June and August could be a little wetter than July but not by much.  Indication of lower pressure than normal over France and the South of the UK may imply increased risk of thunder showers in August leading to highly variable rainfall totals.

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) :

Warmer than normal especially in September with temperatures mostly above normal throughout the season.

September and October probably near or below normal rainfall but followed by a wet November.

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. March 2017.

Published 18 March 2017.

1. Potential influences.

Changes in sea temperature.

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.

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.

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.

 

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.

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.

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.

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

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.

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.

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

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:

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.

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.

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.

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.

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

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.

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.

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

As above but for the period June to August 2017

C: Surface NMME output:

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

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

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

Published 18 December 2016.

1. Potential influences.

Changes in sea temperature.

The temperature of the sea around the UK remains slightly above the seasonal norm with the pattern of slightly cooler temperatures SE of Greenland and SW of Iceland being very similar to mid November.

ENSO 3.4 Early December probability plot and Mid December model comparison. 

The weak El Nina is expected to become neutral and then in late summer trend towards another El Nino. Consequently any statistical relationship between ENSO conditions and rainfall becomes even more uncertain by Spring.

The IRI produced graphic for January to March rainfall probability during La Nina (shown below) suggests near normal totals but hints at wetter in the NW of the UK and drier in East of England

Rainfall probability for dry/normal/wet during La Nina January to March

Numerical (model) output.

Most of the model output for the Jan to March period also implies near normal rainfall but with a chance of above normal in more northwestern parts and less than average in the South or East of the UK. (Text based summary of model output for the UK and Eire). Much like the CFS2 output in November the data from mid month suggests a drier than normal January, suggesting that February or March might well be wetter in order to get a near or above normal season average.

NMME data illustrates this quite well with mean rain rates (shown on the lower part of the graphic) showing below model norms in Jan and Feb then above normal March to May. February has the least positive temperature anomaly being not far from normal.

NMME: Top row – temperature anomaly (+/-1C). Lower row – rainfall rate indicative anomaly blue below and orange above average rain rates

Rain rate anomalies might reflect the intensity or the number of rain days but implies a wetter Spring for many. Skill in the output for Autumn was not good – see verification Autumn 2016.

NMME 200hPa mean values not available but the CFS2 mean and anomalies (below) imply developing enhanced jet/zonal flow for part of February and more especially for March.This replaces higher heights shown in January. Above normal heights are also evident in the forecast for April and May.

CFS2 data from 8th December 2016. 200hPa  January to May. On top row month mean heights and on lower row the month mean anomaly.

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2. Forecast.

Remainder of Winter ( 2017  January and February) :

 

Temperatures in early January may be colder than average followed by a recovery to milder weather and then near normal values in February. This means at least some frost, especially early in the period, although the overall temperature for Winter (Dec to Feb) may well be near or milder than the long term 1981 to 2010 average.

Rainfall may also be lower than average in January, especially the early part of the month, before returning to near average rainfall figures. The snow risk remains lower than average which means some snow accumulation over the moors but not much elsewhere.

 

Winter climate: 1981-2010 average temperature values for lowland areas 7°C in the West and 5°C in the East. (Jan and Feb similar to season values) 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. (Jan 60 to 100mm to lee of moors, 100 to 200mm elsewhere but higher totals over the moors. February, slightly lower values than in January). 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) :

 

Above average temperatures are most likely for the season, as well as for each individual month. Rainfall probably near normal but perhaps above in the more exposed western parts of the Southwest but less likely in places to the East of the Moors, e.g: in parts of South and East Devon due to pressure being higher than normal in the south of England

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  limited data (June July August):

Warmer than average summer temperatures with below average rainfall  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 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.

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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