Experimental Long Range Forecast for SW England. Update to January 2018 issue.

Published 4 February 2018.

Update.

Expected changes in the stratosphere illustrated by the ECMWF 30hPa sequence shown below may lead to an extension of “Winter” into early March due to increased chance of blocking and colder surface winds from an Easterly point.

030218SSW30hpa

This change may have been predicted by UKMO contingency forecast for February which suggested a colder and drier than normal February. Cansips (issued 31st Jan) hints at normal and locally colder temperatures for February. CFSv2 suggests colder and drier weather types for February and these may extend into the first half of March.

temprainseasonal

Forecast update.

The chance of a colder and drier February and start March for SW England is more likely than previously.

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

References.

CFS2  info

Stratospheric Diagnostics from Free University Berlin

 

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Experimental Long Range Forecast for SW England. January 2018 issue.

Published 17 January 2018.

1. Influences.

Changes in sea temperature.

SSTanom150118

Sea Temperatures are slightly cooler than average around the UK and coasts of western Europe but elsewhere the anomalies are similar to last month with Atlantic anomalies still above normal. Sea temperatures in the tropical Atlantic area are forecast to remain slightly above normal, as shown in the UK Met Office plot below.

160118tropnatl

The La Nina in the Pacific is well established, with a large area of below normal sea temperatures in the eastern Pacific, south of the equator. The CFS2 forecast for the La Nina suggests it may continue as a week feature through summer and autumn, as shown by the graphic below (left). This is supported by UKMO forecast ENSO plume but ECMWF is similar to the NMME consensus forecast shown below (right), which warms out the La Nina condithions.

nino160118

IRI published a statistical relationship between La Nina and UK rainfall and this is shown below for March to May. The Summer comparison is not shown due to likely weakening of the La Nina.

laNinaMAMppn

Probability for Dry/Normal/Wet based on a La Nina being active

The plots above suggest that SW England has slightly higher probabilities of Normal to Dry conditions for Spring.

Another statistical relationship which may play a part in the remainder of the Winter and for the Spring season is the Quasi-Biennial Oscillation (QBO). The QBO is linked to conditions over Western Europe during late autumn and early winter through an influence on the phase of the North Atlantic Oscillation (NAO) at the surface. An easterly phase of the QBO tends to moderately increase the chances of a negative phase of the NAO, which in turn increases the chances of below-average temperatures.”

QBO

QBO monitoring data (shown above) indicates that the easterly (negative) phase of the QBO remains above 50hPa and has had little impact this winter.

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) has been in the positive phase for some time and ensemble predictions suggest that a weak positive phase will continue, maintaining the signal for a  milder/wetter remainder of winter.

160118NAO

For more information see Met Office NAO information.

The next relationship to consider is the potential for the development of a sudden stratospheric warming. Should this occur there can be an impact on the Jet stream causing it to weaken and allowing High pressure to develop over Europe. This could bring colder temperatures further west and possibly into the UK. To date there has been periods of warming over the N Pacific and into northern Canada. The stratospheric polar vortex was displaced eastwards but then redeveloped further west resulting in a drop in North Pole 30hPa temperatures (shown below).

30hpa

Over recent weeks there have been a series of minor warming event across the N Pacific and into Canada with some model runs splitting the polar vortex, which might have been a prelude to a change away from the mobile westerly types currently affecting western Europe/Uk area. More recent ECMWF model runs have moved away from this idea and maintained a strong vortex although again being displaced further east away from the pole.

ecmwf30a12

ECMWF 30hPa T+00 +120 and +240 hours from 16th Jan 2018

Stratospheric temperatures in the North Polar vortex have been low enough for the formation of Polar Stratospheric clouds which are implicated in Ozone destruction. Should the vortex be maintained this could become an item of interest this Spring.

50hpa

Left Total Ozone. Right 50hPa minimum temperature plot.

 

Analysis and Numerical (model) output.

A: Recent Climatology.

temp

Temperatures across the SW of England have been slightly above normal and for many areas the last four months have been slightly drier, despite a wetter December.  January in SW England so far is running slightly milder than average, wetter in the west but drier in the east (based on limited data).

Recent reservoir levels remain above normal for the time of year and river flows in the region have returned to near average values.

Decriver

The  full summary is available from the Hydrological Summary  –  December 2017 summary PDF 

An experimental product from the Global Flood Awareness System suggests the flood risk areas for the next 4 months based on January ECMWF seasonal forecast.Glofass

For the SW of England the river flood risk does not appear to be elevated compared to normal with the main risk areas across Eire/N Ireland and N England/S Scotland.

B: Upper Troposphere:

z200

CFS2 forecast for 200hPa suggest above normal heights across the UK but still with a signal for enhanced jet across the Atlantic for a time in Spring.

C: Lower Troposphere:

Comparing the CFS2 mean of 10 days forecasts and the NMME output for Spring 2018. Top row of each pair, temperature anomaly. Lower row indication for above or below normal rainfall (reds are above).

mamcomp

NMME forecast for Summer 2018.jjacomp

The forecasts tend to be a little warm and the rainfall output is often poor with month to month detail. The below average rainfall in the S of UK does fit to some extent with the IRI probabilities when experiencing a La Nina in the Pacific.

D: Subjective model scores for temperature and rainfall for the seasonal forecasts issued December 2016 to September 2017 valid for the following three months. (e.g. September forecast valid for Oct Nov Dec). Left hand column is total of all months where there is a potential max score of 30. Scores for rainfall show only a few good forecasts.

verif

2. Forecast.

Remainder of Winter ( 2018 February) :

There are strong indications for a milder than average February in the SW of England.  This does not rule out some colder spells, probably with North or NW winds rather than easterly, but suggests that the cold spells will be short lived.

Mobile westerly patterns look like continuing to dominate the weather leading to above normal  rainfall especially in Cornwall and in areas to the west of the Moors. Elsewhere near normal or slightly above normal rain totals are most likely. The snow risk remains reduced compared to average with little if any snow for lower ground but some snow for the moors.

February Climate: 1981-2010 Temperature; average temperature values for lowland areas 5 or 6°C but over 6°C in West Cornwall. Rainfall;  February. 1981-2010 average 80-100mm mm lowlands but 60-80mm in areas to E of Dartmoor and 40-80mm East of Exmoor. Over the Moors 100-250mm. 

Spring 2018 (March April and May):

Temperature for the season is likely to be near or a little above the long term average. There are, however, hints of a colder spell which is more likely in April than the other months. Risk of air frost continues into April though not very frequent.

Rainfall probably near normal but in the south and east of the region rain totals may well be below normal. Although a mostly unsettled theme is likely there is a chance of a longer drier period sometime later in March or in April. Month to month detail lacks consistency between models and is often unreliable.

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 2018  (June July August). Limited model data:

The average temperature is most likely to be near or only slightly above normal for the season. During August there is the best chance of above average temperatures.

Rainfall for the season probably below the long term average (in terms of number of wet days, if not actual rain totals) but with a chance that August could see above normal rainfall. Summer rainfall is often difficult due to showery nature leading to large variation in totals over the 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 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

GLOFAS Acknowledgement: Data were provided by the Global Flood Awareness System – GloFAS (http://www.globalfloods.eu/) of the European Commission Joint Research Centre and the European Centre for Medium-Range Weather Forecasts.
Reference: Alfieri, L., Burek, P., Dutra, E., Krzeminski, B., Muraro, D., Thielen, J., and Pappenberger, F.: GloFAS – global ensemble streamflow forecasting and flood early warning, Hydrol. Earth Syst. Sci., 17, 1161-1175, doi:10.5194/hess-17-1161-2013, 2013.

‘Copernicus Products’  as listed in the C3S or CAMS Service Product Specification or any other items available through an ECMWF Copernicus  http://climate.copernicus.eu

Stratospheric Diagnostics from Free University Berlin

 

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

Published 18 December 2017.

1. Influences.

Changes in sea temperature.

sstanom141217

Apart from a little cooling, relative to “normal”, near the North of Scotland Atlantic anomalies continue to be above normal. The tropical area is forecast to remain slightly above normal as shown in the UK Met Office plot below (right). The La Nina in the Pacific is well established, with a large area of below normal sea temperatures in the eastern Pacific south of the equator. The La Nina is forecast to slowly decline during the Spring and early Summer as shown by the NMME plot below (left).

tropsst1

IRI published a statistical relationship between La Nina and UK rainfall and this is shown below for January to May, however, as the La Nina becomes weak so the statistical relationship may decline.

JFMMAMlaNinprobs

IRI probabilities of Wet /Normal /Dry seasons based on possible links to La Nina

The plots above suggest that SW England has slightly higher probabilities of Normal to Dry conditions as we move into Spring.

Another statistical relationship which may play a part in the remainder of the Winter and Spring seasons is the Quasi-Biennial Oscillation (QBO). The QBO is linked to conditions over Western Europe during late autumn and early winter through an influence on the phase of the North Atlantic Oscillation (NAO) at the surface. An easterly phase of the QBO tends to moderately increase the chances of a negative phase of the NAO, which in turn increases the chances of below-average temperatures.”

qbo

QBO monitoring data (shown above) indicates that the easterly (negative) phase of the QBO remains above 50hPa.

The North Atlantic Oscillation index (shown by a 500hPa index in the image below) continues to oscillate between positive and negative phases with ensemble predictions of remaining in a positive phase through to the end of December. This maintains a signal for a  milder/wetter winter season to develop.

nao181217

The North Atlantic Oscillation can also be affected by the Madden-Julian Oscillation. In an early issue of the contingency forecast (15 December 2017), the Met Office states that “the Madden-Julian Oscillation is an area of enhanced thundery activity that moves eastwards through the tropics over a period of several weeks. It has recently been active and is currently in a location that frequently leads to a negative phase of the North Atlantic Oscillation about 10 days later. The negative phase of the NAO brings colder-than average conditions to the UK, implying an increased chance of colder than-average temperatures in early January.” For more information see Met Office NAO information.

The next statistical relationship to consider is the development of a sudden stratospheric warming. Should this occur there can be an impact on the Jet stream causing it to weaken and allowing High pressure to develop over Europe. This could bring colder temperatures further west and possibly into the UK.

The stratospheric polar vortex has been displaced eastwards over recent weeks by a series of warming events across the N Pacific and into Canada, as can be seen in the temperature plot for the North Pole.

pole30_nh

It is not clear whether there will be a full disruption of the polar vortex, though this remains a risk, possibly leading to an upset in the model forecasts. Several of the seasonal models are able to handle changes in the stratosphere.

30hpa

ECMWF 30hPa chart 17 December and forecast for 27 December 2017

Analysis and Numerical (model) output.

A: Recent Climatology.

tandr181217Nov

Temperatures across the SW of England have been near normal and for many areas it has been a fairly dry period. Recent reservoir levels remain above normal for the time of year, but further dry weather may impact the storage levels for the winter as a whole.  River flows in the region for the Autumn season show western areas above normal and eastern areas below, although November data shows reduced river flow in more areas.

riverflowsNov2017

The  full summary is available from the Hydrological Summary  –  November 2017 summary PDF 

An experimental product from the Global Flood Awareness System suggests the flood risk areas for the next 4 months based on ECMWF seasonal forecast.

181217riverflow

Hint at below normal river flows in Somerset and high flows in the Exe estuary, the latter looking odd. Compared to the previous forecast there is a lower probability of flooding in some Wales and N England.

B: Upper Troposphere:

z200JFM1117

CFS2  200hPa monthly mean Jan Feb March 2017 and anomalies (lower)

z200MAM1217

CFS2 200hPa monthly mean contour March April May 2017 and (lower) anomalies

CFS2 forecast for 200hPa suggest above normal heights across the UK but still with a signal for enhanced jet across the Atlantic in February.

C: Lower Troposphere:

Comparing the CFS2 mean of 10 days forecasts (top row of each pair temperature anomaly, lower row indication for above or below normal rainfall (reds are above).

cfandnmmjfm2017

Jan Feb March 2017 temperature and precipitation anomalies

cfandnmmmam2017

March April May Temperature and rainfall anomalies

The main theme from the above is for above normal temperatures and a wet period early in 2018 but a drier Spring with temperatures nearer normal, for at least some of the months.

D: Comment:  Seasonal temperature and rainfall forecast data for the Autumn have tended to over forecast above normal temperature and rainfall. There has been some indications of below normal rain in southern areas but these have not been consistent. Forecasts of below normal temperature seem to be rare in the model output. So far in December temperatures have been near or slightly below average and the rainfall about half of the total for a normal month but this can easily change. Verification summary.

2. Forecast.

Remainder of Winter ( 2018  January and February) :

There remains a strong indication for a milder than average January and February in the SW of England  This does not rule out some colder spells, probably with North or NW winds rather than easterly, but suggests that the cold spells will be short lived.

Rainfall is uncertain month to month but the suggestion that mobile westerly patterns will come to dominate the weather patterns suggests above normal  rainfall especially in Cornwall and to the west of the moors. Elsewhere near normal or even slightly below normal rainfall seems possible, perhaps with January being the wetter month. The snow risk is reduced compared to average with little if any snow for lower ground but some snow for the moors.

Due to the statistical relationships, outlined in part 1, there is a moderate risk that colder and drier weather may develop making this forecast summary uncertain.

Winter Climate: 1981-2010 average temperature values for lowland areas 7°C in the West and 5°C in the East. Rainfall;  January 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):

Temperature probably slightly above normal for the season but possible colder periods in March and again in May. Rainfall near normal in the west, elsewhere drier than average.

Spring climate: 1981-2010 average mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly Mar 7 or 8°C Apr 8 or 9°C  May 11 or 12°C. Average 1981 to 2010 rain 200 to 300mm lowest values in E Devon and over parts of Somerset.

Summer 2018  (June July August). Limited model data:

Temperatures near normal, perhaps a cooler start and a warmer end to summer compared to average. Uncertain rainfall but perhaps above normal in June and near normal later.

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

GLOFAS Acknowledgement: Data were provided by the Global Flood Awareness System – GloFAS (http://www.globalfloods.eu/) of the European Commission Joint Research Centre and the European Centre for Medium-Range Weather Forecasts.
Reference: Alfieri, L., Burek, P., Dutra, E., Krzeminski, B., Muraro, D., Thielen, J., and Pappenberger, F.: GloFAS – global ensemble streamflow forecasting and flood early warning, Hydrol. Earth Syst. Sci., 17, 1161-1175, doi:10.5194/hess-17-1161-2013, 2013.

‘Copernicus Products’  as listed in the C3S or CAMS Service Product Specification or any other items available through an ECMWF Copernicus  http://climate.copernicus.eu

Stratospheric Diagnostics from Free University Berlin

 

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

Published 18 November 2017.

1. Influences.

Changes in sea temperature.

161117ssta

Positive temperature anomalies (shown in the image above for the 16th November) are most marked in the North Atlantic south of about 50 degrees North but are less marked towards the equator. Consequently winds from a generally SW direction are likely to be milder than average, at least for the next month or so.

La Nina is evident in the Pacific and forecast to remain as a fairly weak La Nina through to Spring or Summer 2018.

Nino3.41117

NMME ENSO area 3.4 ensemble and (right) UKMO tropical N Atlantic ensemble

IRI statistical relationship between La Nina and UK rainfall for the winter is shown below. La Nina is weak so influence may not be strong but many of the seasonal model forecast hint at wetter in north and drier in south –  see models section below.

La Nina

Winter season precipitation probabilities during La Nina events.

Other statistical relationships may play a part in the Winter and Spring seasons, for example according to the Met Office late October Seasonal Forecast; “the Quasi-Biennial Oscillation (QBO), an oscillation of the equatorial winds in the stratosphere, is in an easterly phase. The QBO is linked to conditions over Western Europe during late autumn and early winter through an influence on the phase of the North Atlantic Oscillation (NAO) at the surface. An easterly phase of the QBO tends to moderately increase the chances of a negative phase of the NAO, which in turn increases the chances of below-average temperatures.”

QBO monitoring data is shown below along with the 50hPa ECMWF analysis on the 17th. Minimal warming is shown during the subsequent 10 days in the forecast at 50 and 30hPa.

QBO

Left QBO monitoring (left )shows negative phase descending in the stratosphere. ECMWF analysis at 50hPa shows the well established winter polar vortex and minimal warming.

North Atlantic Oscillation monitoring using 500 hPa is shown below and includes a forecast for the next season. NCEP ensemble forecast suggest NAO may move into negative phase later in November and for much of December. Consequently increased chance of blocking patterns which could be cyclonic or anticylonic over the Uk area.

For more information see Met Office NAO information.

nao.sprd2

Analysis and Numerical (model) output.

A: Recent Climatology.

Following on from a couple of unsettled wetter months, October / early November was drier and milder than average across the SW of England. Recent reservoir water levels remain above normal for the time of year, although Wimbleball was only at 55% of capacity at the time of writing. River flows in the region show western areas above normal and eastern areas below with groundwater in eastern parts of Devon and Dorset at notably low levels.. The  summary is available from the Hydrological Summary  –  October 2017 summary PDF 

A new experimental product from the Global Flood Awareness System suggests the flood risk areas for the next 4 months based on ECMWF seasonal forecast. Hint at below normal river flows in Somerset and high flows in the Exe estuary which looks odd.

FloodriskNDJF

B: Upper Troposphere:

CFS2 forecast for 200hPa suggest above normal heights across the UK but with below normal heights to the north suggesting increased zonal types (unsettled systems from the west) with February heights below normal across the north of the UK.

z200DJF1117

200hPa CFS2 forecast mean for 2017 December, 2018 January and February

C: Lower Troposphere:

Only BCC China seasonal forecast suggest below normal temperatures this winter and it is often a little cooler than other centres. The main signal from almost all seasonal forecasting centres is for above normal temperatures this winter. Highest anomalies appear to be in the S and E of the UK.  NMME and CFS2 winter season graphics are shown below.

nmmewinter

NMME temperature anomaly (top) and rain rate indicator (lower row)

DJF131117CFS2

CFS2 mean of 10 days output. Monthly mean temperature and rain rate anomaly (winter)

Probably worth noting that the multi-model ensemble tends to be a little warm and that rainfall forecasts in general are not good with month to month detail. Despite a warm signal colder spells are still possible – see for example temperature plot from a single CFS2 model run.

Exeter1117

Temperature forecast for Exeter area to August 2018

NMME plots for spring 2018 continue with a mild theme but note the much drier signal.

nmmespring

D: Comment:  Seasonal temperature and rainfall forecast data could have been better recently but 2016 data suggets data in November might fare better Verification summary.

2. Forecast.

Winter (2017 December, 2018  January and February) :

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

Rainfall is uncertain month to month but the suggestion that mobile westerly patterns may bring cloud and rain suggest above normal  rainfall especially in Cornwall and to the west of the moors. Elsewhere near normal or even slightly below normal rainfall seems likely, perhaps with January being the wetter month. The snow risk is reduced compared to average with little if any snow for lower ground but some snow for the moors.

Winter Climate: 1981-2010 average temperature values for lowland areas 7°C in the West and 5°C in the East. Rainfall; Dec and Jan typically wetter than February. 1981-2010 Autumn average 300-400 mm lowlands but 200-300 mm areas to E of Dartmoor. Snow climatology less than 5 days lying snow over lowland areas 5 to 10 hills, say hills around 200 metre elevation – one in three years have no lying snow.

Spring (2018 March April and May):

Temperature probably slightly above normal for the season but close to normal in April. Rainfall near normal in west elsewhere drier than average.

Spring climate: 1981-2010 average mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly Mar 7 or 8°C Apr 8 or 9°C  May 11 or 12°C. Average 1981 to 2010 rain 200 to 300mm lowest values in E Devon and over parts of Somerset.

3. Caution.

Experimental Long Range Forecasts do not have a good success rate. The data used for the above forecast summary and brief verification of previous Long Range Forecast summaries can be seen at   http://www.weatherservice.co.uk

The attempt at a Regional Forecast for SW England aims to test whether such a forecast of temperature and rainfall variation from average can be made using numerical model data available on the internet. The forecast should not be used for any other purpose. A brief verification summary for the UK and Eire is routinely published at http://www.weather-info.co.uk/wxsvc/Verification.html or Click here for the Teignmouth and Dawlish summary

4. References.

SST anomaly NOAA Remote Sens. 2014, 6(11), 11579-11606; doi:10.3390/rs61111579

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

UK climate details see: http://www.metoffice.gov.uk/public/weather/climate/

NMME information:   http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-12-00050.1

CFS2  info

GLOFAS Acknowledgement: Data were provided by the Global Flood Awareness System – GloFAS (http://www.globalfloods.eu/) of the European Commission Joint Research Centre and the European Centre for Medium-Range Weather Forecasts.
Reference: Alfieri, L., Burek, P., Dutra, E., Krzeminski, B., Muraro, D., Thielen, J., and Pappenberger, F.: GloFAS – global ensemble streamflow forecasting and flood early warning, Hydrol. Earth Syst. Sci., 17, 1161-1175, doi:10.5194/hess-17-1161-2013, 2013.

‘Copernicus Products’  as listed in the C3S or CAMS Service Product Specification or any other items available through an ECMWF Copernicus  http://climate.copernicus.eu

Stratospheric Diagnostics from http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/index.html

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

Published 17 October 2017.

1. Influences.

Changes in sea temperature.

anomnight.10.12.2017

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

 

nino102017

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

ninoNAO

CFS Nino 3.4 forecast left. Observed NAO index right.

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

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

La Nina

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

Analysis and Numerical (model) output.

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

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

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

B: Upper Troposphere:

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

z200

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

C: Lower Troposphere:

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

NMME102017

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

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

E3DJF

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

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

2. Forecast.

Remainder of Autumn  2017  (November) :

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

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

Winter (2017 December, 2018  January and February) :

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

Rainfall is uncertain month to month but the main theme of a wetter than normal winter seems to be consistent although December may be less wet. The snow risk is reduced compared to average with little if any snow for lower ground but some snow for the moors.

Winter Climate: 1981-2010 average temperature values for lowland areas 7°C in the West and 5°C in the East. Rainfall; Dec and Jan typically wetter than February. 1981-2010 Autumn average 300-400 mm lowlands but 200-300 mm areas to E of Dartmoor. Snow climatology less than 5 days lying snow over lowland areas 5 to 10 hills, say hills around 200 metre elevation – one in three years have no lying snow.

Spring (2018 March April and May)  limted data:

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

Spring climate: 1981-2010 average mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly Mar 7 or 8°C Apr 8 or 9°C  May 11 or 12°C. Average 1981 to 2010 rain 200 to 300mm lowest values in E Devon and over parts of Somerset.

3. Caution.

Experimental Long Range Forecasts do not have a good success rate. The data used for the above forecast summary and brief verification of previous Long Range Forecast summaries can be seen at   http://www.weatherservice.co.uk

The attempt at a Regional Forecast for SW England aims to test whether such a forecast of temperature and rainfall variation from average can be made using numerical model data available on the internet. The forecast should not be used for any other purpose. A brief verification summary for the UK and Eire is routinely published at http://www.weather-info.co.uk/wxsvc/Verification.html or Click here for the Teignmouth and Dawlish summary

4. References.

SST anomaly NOAA Remote Sens. 2014, 6(11), 11579-11606; doi:10.3390/rs61111579

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

UK climate details see: http://www.metoffice.gov.uk/public/weather/climate/

NMME information:   http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-12-00050.1

CFS2  info

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

Published 19 September 2017.

1. Influences.

Changes in sea temperature.

anomnight.9.18.2017

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

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

SSTNINO

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

AO

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

Analysis and Numerical (model) output.

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

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

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

B: Upper Troposphere:

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

z2001

CFS2 200hPa height and anomaly Oct Nov Dec 2017

z200DJF

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

C: Lower Troposphere:

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

NMME1

NMME Temperature anomaly and rainfall index October and November 2017

NMMEDJF

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

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

2. Forecast.

Remainder of Autumn  2017  (October and November) :

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

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

Autumn climate: 1981 to 2010 average mean temperature 11 or 12°C but nearer 10°C in rural areas. Maximum temperatures average 14 to 16°C. November normally colder than October which is colder than September. Average 1981 to 2010 rain 300 to 400mm, 600 to 800mm over the moors but east of the moors and in lowland Somerset more like 200 to 300mm . September often drier than October or November.

Winter (2017 December, 2018  January and February) :

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

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

Winter Climate: 1981-2010 average temperature values for lowland areas 7°C in the West and 5°C in the East. Rainfall; Dec and Jan typically wetter than February. 1981-2010 Autumn average 300-400 mm lowlands but 200-300 mm areas to E of Dartmoor. Snow climatology less than 5 days lying snow over lowland areas 5 to 10 hills, say hills around 200 metre elevation – one in three years have no lying snow.

Spring (2018 March April and May)  limted data:

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

Spring climate: 1981-2010 average mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly Mar 7 or 8°C Apr 8 or 9°C  May 11 or 12°C. Average 1981 to 2010 rain 200 to 300mm lowest values in E Devon and over parts of Somerset.

3. Caution.

Experimental Long Range Forecasts do not have a good success rate. The data used for the above forecast summary and brief verification of previous Long Range Forecast summaries can be seen at   http://www.weatherservice.co.uk

The attempt at a Regional Forecast for SW England aims to test whether such a forecast of temperature and rainfall variation from average can be made using numerical model data available on the internet. The forecast should not be used for any other purpose. A brief verification summary for the UK and Eire is routinely published at http://www.weather-info.co.uk/wxsvc/Verification.html or Click here for the Teignmouth and Dawlish summary

4. References.

SST anomaly NOAA Remote Sens. 2014, 6(11), 11579-11606; doi:10.3390/rs61111579

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

UK climate details see: http://www.metoffice.gov.uk/public/weather/climate/

NMME information:   http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-12-00050.1

CFS2  info

 

 

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

Published 20 August 2017.

1. Influences.

Changes in sea temperature.

anomnight.8.17.2017

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

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

nino0817

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

trop

Analysis and Numerical (model) output.

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

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

Riverflows

B: Upper Troposphere:

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

z200SON0817

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

z200DJF0817

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

C: Lower Troposphere:

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

SONT

NMME Sep to Nov 2017 temperature anomaly

DJFT

NMME temperature anomaly December 2017 to February 2018

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

sonP

NMME precipitation anomaly for September to November 2017

djfP

NMME precipitation anomaly December 2017 to February 2018

cap

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

 

2. Forecast.

 

Autumn  2017  (September, October and November) :

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

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

Autumn climate: 1981 to 2010 average mean temperature 11 or 12°C but nearer 10°C in rural areas. Maximum temperatures average 14 to 16°C. November normally colder than October which is colder than September. Average 1981 to 2010 rain 300 to 400mm, 600 to 800mm over the moors but east of the moors and in lowland Somerset more like 200 to 300mm . September often drier than October or November.

Winter (2017 December, 2018  January and February) :

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

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

Winter Climate: 1981-2010 average temperature values for lowland areas 7°C in the West and 5°C in the East. Rainfall; Dec and Jan typically wetter than February. 1981-2010 Autumn average 300-400 mm lowlands but 200-300 mm areas to E of Dartmoor. Snow climatology less than 5 days lying snow over lowland areas 5 to 10 hills, say hills around 200 metre elevation – one in three years have no lying snow.

Spring (2018 March April and May)  very limted data:

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

Spring climate: 1981-2010 average mean temperature 9 or 10°C but a few degrees cooler over the moors. Roughly Mar 7 or 8°C Apr 8 or 9°C  May 11 or 12°C. Average 1981 to 2010 rain 200 to 300mm lowest values in E Devon and over parts of Somerset.

 

 

3. Caution.

Experimental Long Range Forecasts do not have a good success rate. The data used for the above forecast summary and brief verification of previous Long Range Forecast summaries can be seen at   http://www.weatherservice.co.uk

The attempt at a Regional Forecast for SW England aims to test whether such a forecast of temperature and rainfall variation from average can be made using numerical model data available on the internet. The forecast should not be used for any other purpose. A brief verification summary for the UK and Eire is routinely published at http://www.weather-info.co.uk/wxsvc/Verification.html or Click here for the Teignmouth and Dawlish summary

4. References.

SST anomaly NOAA Remote Sens. 2014, 6(11), 11579-11606; doi:10.3390/rs61111579

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

UK climate details see: http://www.metoffice.gov.uk/public/weather/climate/

NMME information:   http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-12-00050.1

CFS2  info