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. June 2017 issue.

Published 20 June 2017.

1. Influences.

Changes in sea temperature.

anomnight.6.19.2017

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.

anal

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.

TropicalSST

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.

NAO200617

 

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.

z200JULAUG

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

CFS2z200SPN0617

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

JA0617

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

SON0617

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.

CFSPPN

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.

mamcap

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.

anomnight.5.15.2017

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

nino34.rescaling.ENSMEANnino34Mon

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.

nao.mrf.obs

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.

Z200CFS2JJA0517

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.

Z200CFS2SON0517

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.

jja0517

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.

son0517

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. 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

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

Published 18 December 2016.

1. Potential influences.

Changes in sea temperature.

anomnight-12-15-2016

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.

decnino

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

jfmnina

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.

nmmejfm1216

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.

z200dec2016comps

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


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.


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. November 2016 issue.

Published 18 November 2016.

1. Potential influences.

Changes in sea temperature.

anomnight-11-17-2016

The sea temperature around the UK remains above normal and the colder anomaly in the North Atlantic is similar to last month. In the Pacific a large colder than normal area has developed and the La Nina remains fairly week. The La Nina is expected to weaken further during the course of the winter months as shown by the consensus probability forecast below.

nino

ENSO 3.4 Comparison Forecast and Probability plot

The probability plot above suggests a move to Neutral ENSO conditions later in the winter. Until then the statistical relationship between La Nina and UK precipitation may be worth a look, especially as there is support in some climate model output for a drier December (eg: NMME CFS  BCC China Cansips).

DJFnina

Probability for Dry Wet Normal precipitation during La Nina

Numerical (model) output.

CFS ten day mean ensemble precipitation output for December has moved towards a drier forecast, as shown below. Despite this change there remains a weak signal for a wetter than normal winter across parts of SW England in the CFS output.

cfs

There is also a corresponding change in the temperatures with a less mild forecast for December and January made up for by a milder February. At the time of writing almost all the available seasonal models predict a normal to mild winter for SW England.

NMME data also shows variation in the precipitation but is fairly consistent with an above normal temperature forecast.

nmmedjf

NMME Winter (DJF) forecast temperature anomaly (top) and precipitation anomaly indicator for below (blue) and above (orange) ensemble model normal

nmmemam

NMME Spring (MAM)

CFS (200hPa) upper wind forecast mean and anomaly suggests enhanced jet over Canada, extending at times across the Atlantic towards UK. This implies stronger Atlantic low pressure systems during the January to March period. Later model runs appear to have maintained a stronger than normal jet.

z200com

CFS2 December to March 200hPa monthly mean height (top) and anomaly

2. Forecast.

Winter (2016 December, 2017  January and February) :

Temperatures this winter are most likely to be near or a little milder than the long term average (which does not rule out some frost). The second half of the winter may well be relatively milder than the first half when compared to the average for the individual months.

The early part of winter may be drier than average but this is likely to be offset by wetter weather later. (This would be different from the long term average in which December and January are wetter than February). Overall near normal or above normal precipitation is most likely for the  N and W of the Region with parts of the SE and E of Devon, E Somerset and Dorset closer to normal and in places a little below normal. Below average snowfall is more likely than above average, which means some snow accumulation over the moors but not much elsewhere.

Chance of increased number of storms in January and possibly February although the tracks may be to the north of the Region.

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 (2017 March April and May) :

Slightly above average temperatures are most likely for the season as a whole. Rainfall probably above normal overall  with near or below normal precipitation early in Spring offset by wetter than normal April and/or May.

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.

Limted data early look at Summer 2017  (June July August) 2016:

Conflicting temperature forecast perhaps suggesting near normal values. Rainfall, hint at drier June and/or July and wetter August which is not unusual.

(1981 to 2010 average mean temperature 14 or 15°C in many areas to 16 or 17°C in main urban areas also locally east of the moors and more widely in Somerset. Maximum temperatures average 19 to over 21°C in similar areas. July often warmer than August.  Average 1981 to 2010 rain 300mm over the moors, typically 200 to 250mm in many coastal and eastern areas. June often drier than July and July drier than August)

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