November 28, 2015


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Reigate Grammar School, UK. Local weather station and forecasts for education. Reporting on local and global weather and climate. RMetS education committee. Town VP2 updates website every 10mins, wind every 5secs. CoCoRaHS manual rain gauge. Data to Met Office + Weather Underground.  Status: Reigate data uploads all good! :-)

badge_2015Delighted to announce that Reigate Grammar School has won a new MetOffice and Royal Meteorology Society MetMark Award 2015 for excellence in weather teaching and promotion of weather understanding and climate awareness in schools. Read some of our best weather club events that helped win the award: HAB launch; BBC school report; River Mole and Gatwick flood reports and St Jude storm post as reported on Radio4 and published student authors in Weather magazine.


Quick October weather summary for Reigate 2015.

Summary stats for October in Reigate, Surrey, England

  • T average 11.6C
  • Tmax 19.33 C
  • Tmin 3.2C
  • Rainfall total 61mm (CoCoRaHs)
  • Sunshine 95.3 hours
  • Max gust 24mph (24 Oct)

October CET temperature came out just above average at +0.4C. Significantly, globally, October 2015 was the warmest October ever on record for this month by a significant margin, probably due in part to a strong El Nino.  In fact every month of 2015 so far has been record-breakingly warm and each month has been the warmest recorded anomaly… since 1880.


October global temperature anomaly 2015

Due to anomalously high pressure for much of the month, South East England rainfall was lower than average at some 76% of the long term average.  In Reigate this amounted to 61mm of rain.

Despite the high pressure sunshine amounts were average or slightly below in the south east with Reigate totting up 95.3 hours of sunshine in the month.

October is sometimes correlated with winter outcomes. Anomalously high pressure in October has sometimes been correlated with cold winter patterns, though this is highly experimental.  In addition, October Siberian snow cover expanded rapidly and beyond the 2014 cover for the same period.  This got winter forecasters further interested in the possibility of high pressure, northern blocking and the increased potential for a cold late winter in the UK.  However, seasonal forecasts have a habit of going awry, as was illustrated by the ill-fated “October Pattern Index” much heralded in the run-up to winter 2014 when it predicted a cold one. The OPI, as far as I can tell, has now disappeared off the seasonal forecasting scene completely.



MetOffice Fax chart Sat 21 Nov: cold UK weather

A cold Arctic surge will spill south over Friday night bringing a shock to the record-breaking warm November so far.  The airmass trajectory shows the extraordinary journey the air has taken to get to London.


airmass trajectory (GFS)

It’s a brief Arctic attack but rather potent and Saturday morning will feel especially cold in strong northerly winds.

Snow is also forecast for Scotland and parts of the North of England as the Arctic air is brought south with a North Sea low and attendant fronts.  This system will reach Reigate and the SE early Saturday morning and likely bring a wintry miserable start to the weekend with sleety rain, maybe even some wet snow for the Downs. (in the event: snowfall in Reigate for an hour or so as front passed through, settled momentarily. front cleared off quickly)

In the South East, whilst the airmass temperature is just cold enough for snow and sleet, falling to a cool -5C at 850hPa by early Saturday morning, the dew point is shown to rise quickly above freezing as precipitation arrives in the morning and this will reduce the chance of any proper snow fall. This is fairly typical of the SE!

The main feature of Saturday morning in Reigate will be a truly miserable wet, cold windy start with an impressive wind chill well below freezing lasting all day, something we haven’t experienced in Reigate and Surrey for a long time!


wind chill November 21 2015

Overall, this cold snap is not due to last long, warming slowly into next week but turning unsettled as the jetstream blasts back from the Atlantic. It will not, however, become as warm as it has been.


unsettled due to return as jet bites back

Winter forecasters look high into the atmosphere to get an idea of what the winter will bring.  One feature they look at is the Polar Vortex.  The PV is currently getting stronger, which it usually does this time of year.  The vortex is the winter circulation of high altitude westerly winds in the stratosphere that keep the cold polar air locked up in the Arctic.  To get sustained cold outbreaks the vortex needs to be broken down: something that can occur when the polar stratosphere warms, sometimes suddenly.

However, with North Pole stratospheric temperatures taking a steep dive, lower than average, the temperature gradient between the Pole and mid-latitudes is increasing and so the subsequent pressure gradient is also steep: with generally low pressure over the Pole and high pressure further south.  This all really means cold air is less likely to leak south.


The Arctic Oscillation is an index showing how atmospheric pressure varies between the North Pole and mid-latitudes.  In a positive phase the westerly winds are stronger and the jetstream more powerfully moves depressions across the Atlantic to the UK.  A negative AO is required for wintry outbreaks… El Nino can induce stratospheric warming episodes and many winter forecasters are expecting the Polar Vortex to come under attack later this winter, after Christmas, and possibly produce a colder second half.  meanwhile, expect more unsettled, relatively mild weather to return after this cold snap.


The brief cold snap is shown in the ensemble forecast from the GFS and ECM models below: cold snap followed by a return to mild.


Originally posted on xmetman:

I thought that I would just place the infographics for both of these “storms” gales in a single blog, and allow you to compare wind speeds and gusts, which to me look rather similar, but whereas Abigail mainly affected the north and west of Scotland, Barney ran right through central parts of Ireland, NW Wales and across central parts of England and into the North Sea.

Infographic for the British Isles on Fri, 13 Nov 2015 Infographic for the British Isles on Fri, 13 Nov 2015

Infographic for the British Isles on Tue, 17 Nov 2015 Infographic for the British Isles on Tue, 17 Nov 2015

View original


El Nino: massive Pacific heat engine

El Niño has no obvious or strong effect on UK winter weather.  Historically, El Niño years have coincided with both mild/wet and cold/dry winters in the UK.  By itself, El Niño does not directly drive our winters in any single, simple direction. For example, El Niño winter 2009/10 was the coldest winter for 30 years with a notable “Big Freeze”, while the El Niño winter 2006/2007 was the second warmest winter on record.  The strongest recent “Mega” El Niño in 1997-98 turned out to be a stormy and mild December in the UK, with just two minor snow events and then a notably mild January and February, with Tmax even reaching 17C on occasions.  However, despite weak and ambiguous El Niño signals for UK winters, when other weather drivers and teleconnections are combined with El Niño there is some research to suggest that stronger impacts such as stormy early winters and cold dry late winters are possible. The current El Niño could turn out to be one of the most powerful in 50 years (though recent measurements still show it 3rd in the league table of Mega-El Niños).  Of course, wintry weather can also occur completely independently of any El Niño event, such as the snow of January 2013.

“In Britain, the impact of El Niño is nowhere near as marked as in other parts of the world. But it does tip the balance a little bit more in favour of wet and windy weather. It makes it more probable,”

Jeff Knight, a climate modeller at the Met Office’s Hadley Centre, Exeter.

Read on for more details on how this fascinating and topical weather story might or might not impact our winter weather!


Some drivers, teleconnections and indicators of UK winter weather


Winter 2015-16 most powerful El Nino in 50 years?

El Niño: What is it?

El Niño is a natural change in the atmospheric pressure and wind patterns and flow of ocean currents in the Equatorial Pacific.  In normal “non-El Niño” conditions, a 200 metre deep pool of the world’s warmest sea surface water builds up in the West Pacific.


Average Pacific Ocean sea surface temperatures non-El Nino conditions

This is known as the West Pacific Warm Pool and it is formed by intense insolation, a piling up of warm water driven by the easterly trade winds and low evaporation in light winds found round Indonesia.  Moist warm air over the West Pacific Warm Pool creates an area of instability and convergence in low pressure systems where air rises forming deep tropical clouds, heavy rain and thunderstorms and sometimes typhoons.  A contrasting cold pool exists in the East Pacific where upwelling of deep ocean water reaches the surface off the coast of Peru courtesy of the cold Humboldt current bringing Antarctic water up from the ocean depths . Cool dry air subsiding over the cold pool in the East Pacific condenses moist air and forms low cloud and fog that acts as a feedback loop by reducing insolation and creating cooler conditions. Normally, brisk Easterly Trade Winds drive this cool tongue of ocean water west and, on it’s journey along the Equator, the sea surface warms up.  This normal Pacific pattern is known as the “Walker Circulation”.

Normal Pacific Walker Circulation

Normal Pacific Walker Circulation (RGSweather diagram)

Strong El Niño episodes result in a reversal of the normal pattern of Pacific Ocean wind and ocean currents and dramatically changes the sea surface temperatures across the Pacific.


Pacific Ocean sea surface temperatures during warm El Nino phase

The reversal of winds and currents causes the West Pacific Warm Pool to move to the Central and East Pacific where there is normally cold ocean water, hence El Niño are known as “warm phases”.  Strong El Niño phases produce a tongue of above average sea surface temperatures extending 13,000 km long and 1000 km wide across the Equatorial Pacific and this has a major impact on weather patterns across parts of the world, especially during the Northern Hemisphere winter when El Niño usually reaches a peak of intensity.

El Nino phase Pacific Ocean

El Nino phase Pacific Ocean (RGSweather diagram)

During El Niño episodes the Pacific trade winds weaken, the subtropical jetstream can reverse and strengthen and wind driven upwelling slackens.  As a result the Equatorial ocean current reverses as warm water starts moving to the east. Whilst it is not known what causes an El Niño, a key change is in the pressure pattern across the Pacific basin.


Tahiti sea level pressure correlates negatively with Darwin i.e. when one is high, the other is low. ENSO warm phase yields HIGH pressure over Darwin.

ENSO: El Niño Southern Oscillation or Pacific Pressure See-Saw

In El Niño phases the normally LOW pressure measured over Darwin, Australia changes to higher pressure and the reverse goes for pressure over the east Pacific, measured in Tahiti, where pressure falls.  The changing fortunes of these pressure cells is known as the Southern Oscillation.  The reversal of pressure gradient weakens or reverses the trade winds and allows the West Pacific warm pool to “slosh” east across the Pacific towards South America.  The resulting thermal expansion and the reversal of ocean currents, actually raises the sea level in the East Pacific.  This all takes months and the coupling of the ocean currents and atmospheric winds is critical in creating a complete El Niño Southern Oscillation (ENSO). This video explains the phenomenon well:

So during an El Niño event, the easterly trade winds converging across the equatorial Pacific weaken.  This in turn slows the ocean current that draws surface water away from the western coast of South America and reduces the upwelling of cold, nutrient–rich water from the deeper ocean, flattening out the thermocline (boundary between deep cold water and surface warm water) and allowing warm surface water to build in the eastern part of the Pacific.  Once the ocean currents and atmospheric winds “couple-up” then a positive feedback loop is established which causes further sea surface warming in the East Pacific.  Here the air is warmed above, becomes more buoyant and rises, lowering pressure so further drawing in more westerly winds.  These changes transport enormous amounts of heat and energy to the East Pacific which alters the subtropical jetstream which transfers changes in the atmosphere further “downstream” to other parts of the world.

El Nino

El Nino sea surface temperature anomalies October 2015


El Nino sea surface temperature anomalies November 5th 2015 (courtesy xmetman wordpress)

Significant global Impacts

Due to the release of immense amounts of heat from the Pacific Ocean, El Niño years often become record-breakers for global average temperature.  The energy and moisture released flows “downstream” into the global circulation and has significant impacts on weather elsewhere.  El Niño reaches a peak around Christmas, hence the name “Christ Child” bestowed on the phenomenon by Peruvian fishermen who suffer from the collapse of their fisheries during warm episodes as the upwelling of nutrient rich bottom waters are capped by the invasion of the nutrient poor warm pool.  This causes a temporary collapse in sea life in the East Pacific.  El Niño occur periodically but irregularly over a cycle of 3 to 7 years, they differ in strength and are sometimes followed by a corresponding reversal to a strengthened “normal” flow called La Nina. The last mega-El Niño was 1997-1998 and our 2015-2016 El Niño looks like matching that strength or possibly exceeding it (update November: not so likely now* see Xmetman blog post in refs at foot of page)

The effects of El Niño around the Pacific and neighbouring continents are the most obvious and well correlated with the event, for example wetter and stormier conditions in South America, drier drought conditions with more wildfires in Indonesia and Australia and NE Brazil and a weaker SE Asian monsoon and wet winters in SE USA.  El Niño years also correlate with 44% fewer Atlantic hurricanes due to the enhanced subtropical jetstream shearing the heads off developing thunderstorms and enhanced Pacific hurricanes due to warmer SSTs e.g. Patricia October 2015.  Some of these effects have already occurred in the 2015 El Niño with hurricane activity correlating well with expected changes and an Indian heatwave with reduced monsoon.

The chart below shows a composite of analogue surface temperature anomalies for October in El Niño years (source JMA) compared to the actual conditions measured for mid-October 2015.  The patterns match surprisingly well, especially for the more significant and more strongly correlated locations.  This hints at how patterns for this El Niño might expect to map out as expected. Note the lack of any significant impact in NW Europe.


Around the Pacific, very roughly, places that are normally wet and stormy become drier and more settled but can also suffer drought and fires e.g. Indonesia and Australia, while those places which are normally dry become stormy and wet and suffer from flash floods and landslides e.g. Peru and California. The most extreme weather impacts occur during the cold winter season of each El Niño in the Pacific but the knock-on effects can last into the following summer and link with places over great distances. The charts below show some of the recognised El Niño impacts. Note the complete absence of any reliable or linear teleconnections in Europe recognised by NOAA.


Impacts of El Nino

Weak UK and European Impacts…

Impacts on the weather further away from the Pacific mostly consist of weaker signals that are often reversible due to other stronger weather drivers.  The impact of El Niños on European weather, especially the UK, fits into this category because there are no strong, reliable impacts based solely on El Niño episodes on UK weather.

“There is really no effect in the U.K. that we can say is definitely caused by El Niño” AccuWeather Meteorologist Tyler Ros states.

Other drivers of weather become more significant because the UK is located further downstream and along way from action in the Pacific.  Research by Judah Cohen, Atmospheric and Environmental Research (AER), suggests El Niño warm years overall bring warmer winters to the Northern Hemisphere. However, other research has picked up on some weak “teleconnections” between El Niño events and colder European winters. Some of the connections are illustrated below:


El Nino connections to European winter weather (diagram RGSweather)

…But some possible El Niño signals for UK and Europe?

In Europe, research shows that any El Niño signals are “strongest” in middle and late winter and they approximate to a negative North Atlantic Oscillation.  A negative NAO corresponds with higher pressure over Iceland and a weaker meridional (wiggly) jetstream.  This situation can lead to cold outbreaks for the UK as a sinuous jet can provide chances for Arctic air to leak out of the Poles. In addition, El Niño is associated with low temperatures and decreased precipitation over NE Europe, connected with higher than normal pressure here. This provides the UK with the risk of cold North Easterly winds coming from Russia… so called “Beast from the East”. Some modest El Niño signals emerging from research for European winter weather are listed below but it is important to point out that these are weak signals and other research finds no reliable El Niño winter signals at all!

  • Atlantic storm tracks shifted south taking storms over Mediterranean
  • More cyclonic weather patterns over Central Europe
  • Pressure over Scandinavia HIGH; or western Russia anticyclone expanded over Europe: this would increase the chances of cold Easterly winds
  • High sea level pressure over Iceland across to Scandinavia and NE Europe
  • NE Baltic cold impact: but not in very strong El Nino events when warm impact may occur (UK MetOffice)
  • LOW sea level pressure across central Europe and Western Europe: higher precipitation
  • Some El Ninos have cold winters in NE Europe and enhanced precipitation in Central Western Europe
  • Positive NAO in Nov-Dec : this would mean a stronger jetstream with milder conditions for much of Europe, especially NW
  • Negative NAO late winter into Spring: this would mean a weaker more meridional jetstream with the possibility of blocked patterns and potential Arctic outbreaks or easterlies (other things coming into play)
  • High rainfall in the Mediterranean and decreased precipitation over NW Europe and Scandinavia
  • Frequency of upper troughs over central Europe was very high
  • Temperatures and precipitation over Turkey are high.
  • Israel high rainfall

There are mixed messages regarding the El Niño signal for European weather. Overall, the signal is most consistent in late winter and resembles the negative phase of the North Atlantic Oscillation which itself links to higher chances of cold winter episodes with northern blocking.  The prolonged 1940–1942 El Niño was accompanied in northeastern Europe by three of the coldest winters of the 20th century. In early winter the signal is almost the opposite with a positive NAO, stronger jetstream which brings milder stormier conditions to Europe.  Variability between El Niño impacts on Europe is also large and range much larger than the impacts themselves. Some research shows such variability might be due to volcanic eruptions in the tropics prior to El Niño events.  There is some evidence that pronounced El Niño impacts on European weather follows major volcanic eruptions e.g. El Chichon 1982, Pinatubo 1991.


Calbuco Volcano erupted April 2015 but is this eruption big enough to trigger stronger El Nino teleconnections this winter?

Links between El Niño  and other atmospheric drivers

In the 20th Century all three of the strong El Niño events followed major volcanic eruptions. Even so, the signals were not consistent between these events.  Some studies also show a connection of El Niño events to stratospheric conditions. Warming of the stratosphere (sudden stratospheric warming) and subsequent weakening of the Polar Vortex have been linked to increased chances of cold winter weather in Europe (due to a weakening of upper westerly zonal winds propagating down into the Troposphere, allowing cold easterlies to break out into Europe).  Some research finds an increased frequency of such stratospheric events, especially in late winter, during El Niño years. Additionally, volcanic eruptions might also play a role in warming the lower stratosphere and encourage SSW (sudden stratospheric warming) events but further research is needed to establish any firm connection.

In addition to volcanic activity and stratospheric behaviour, other drivers and atmospheric behaviours can have significant influences on UK winters and these might enhance or reduce any El Niño signal or overwhelm it completely. Examples of some drivers / indicators and teleconnections that seasonal forecasters use include:

  • Solar activity: low sunspot numbers connect to northern blocking.  Currently low.
  • Atlantic hurricane activity: more hurricane activity injects heat to the Poles that increases the chance of northern blocking and cold winters. 2015 season very low hurricane activity.
  • October Siberian snow cover: high and rapid expansion of Eurasian snow cover in October links to increased chance of sudden stratospheric warming later in the winter which can cause cold late winters. Current Siberian snow cover is more than more recent recorded years.
  • October weather patterns: recent research shows that an anticyclonic October in the UK (dry) can link to cold winters with LOW pressure in Europe and northern blocking at high latitudes. This enhances a negative NAO.
  • Quasi-Biennial-Oscillation: westerly upper tropical wind pattern surpresses chances of cold outbreaks in mid latitudes. Currently westerly QBO.
  • Atlantic sea surface temperatures: tripole of warm/cold/warm pattern hints at potential for -ve north atlantic oscillation in winter.  Currently no tripole but cold pool anomaly in central North Atlantic could cool NW flow a little more than usual.

How the different teleconnections work together is complicated and many are at the cutting edge of climate long range forecasting and research.

The search for ENSO / winter correlation

Reanalysis of groups of strong El Niño years can be correlated with years exhibiting current atmospheric patterns from the list above and these show interesting results for UK winters illustrated below.  The following recent twitter chat is an example of such reanalysis widely undertaken by weather experts and enthusiasts:

//platform.twitter.com/widgets.jsThese can then be rolled forward to see how things pan out through the winter. Here is an interesting example from consultant meteorologist Anthony Masiello.  He has reanalysed El Niño winters with years with November positive Arctic Oscillations (as now).


December enso: westerly influence, positive NAO


January enso: with Siberian snow cover: Atlantic blocking with -Ve AO and potential for Arctic incursions


February enso: Siberian snow cover years yield a beast from the east

The results above seem to match the idea of warmer unsettled Atlantic driven conditions before Christmas and colder blocked patterns after Christmas i.e. January and February have a decidedly blocked patterns to the north with low pressure to the south… as predicted in El Niño years as a possibility. HOWEVER.. if you check the number of years represented there are only 10 for El Niño years with +NAO and only 5 with widespread October Siberian snow cover. This is therefore not a significant finding, as Anthony himself points out on twitter.


Too much inter-event variability for headline news

Globally, there is great inter-event variability between different El Niño years. The charts above, from JMA, show composite impacts of El Niño events and their significance over several decades. These appear to show some warmer and wetter than average winter conditions in Europe which agrees with Cohen et al but is in contrast to other findings.  To complicate matters further there are also different types of El Niño such as Central Pacific (Modoki) events that are correlated with different impacts on global weather e.g. colder winters in USA.  The latest 2015-16 El Niño appears to be turning out as a “standard” East Pacific mega-El Niño event with a long continuous tongue of warm SST anomalies stretching across the east Pacific.  Even so, no two El Niño events are the same.


annual nonsense headlines from Daily Express about winter weather

One thing is for sure, recent newspaper reports touting confident headlines suggesting certainty over severe winter weather impacts in the UK and Europe “caused” by El Niño are not based on the findings of climate research or historic precedent which show only tentative and conflicting connections with our winter weather. It might be more accurate to suggest that no one really knows how El Niño  mixing with all the other connections will play out this winter!  Nevertheless, this should not stop the efforts of scientists trying to find clues for long range forecasts.

The last word should go to the UK MetOffice who state the following for the UK winter outlook with regard to El Niño 2015:

What does El Niño imply for the UK this winter?

Unlike some parts of the world, the effect of El Niño on Europe is relatively subtle. In El Niño years there is a tendency for early winter to be warmer and wetter than usual and late winter to be colder and drier. Despite this, it is just one of the factors that influence our winters, so other influences can overwhelm this signal – it is relatively straightforward, for example, to find years where these general trends were not followed.

El Niño moderately increases the probability of the positive phase of the North Atlantic Oscillation (NAO) in late autumn and early winter and the negative phase of the NAO in late winter. (In winter) the positive phase of the NAO is associated with milder- and wetter-than-average conditions, whilst the negative phase is associated with colder- and drier-than-average conditions.

useful references:











winter 2006 – 7 http://news.bbc.co.uk/1/hi/sci/tech/6401063.stm

winter 2009-10 http://www.metoffice.gov.uk/about-us/who/how/case-studies/winter09-10

winter 2013 http://www.metoffice.gov.uk/about-us/how/case-studies/january-2013-snow



Originally posted on xmetman:


I have a number of applications that parse and display climate data from SYNOP reports, but the one I have just finished is a little different. It counts the number of days that anyone location is the top warmest, coldest, wettest, sunniest or windiest place from across the British Isles in the last year. A SYNOP report holds enough climate data to generate the following categories:

  1. The warmest place by day (highest day maximum 06-18)
  2. The coldest place by day (lowest day maximum 06-18)
  3. The wettest place by day (highest rain 06-18)
  4. The warmest place by night (highest night minimum 18-06)
  5. The coldest place by night (lowest night minimum 18-06)
  6. The lowest Grass Minimum
  7. The wettest place by night (highest rain 18-06)
  8. The wettest place in 24 hours (highest rain 06-06)
  9. The sunniest place
  10. The highest mean wind speed (18 & 06)
  11. The highest gust (18 & 06)
  12. The highest…

View original 715 more words


Forecasting confidence decreases over time

If we are not certain about the weather next week, how can we be confident about climate change predictions 50 or 100 years into the future? The charts below show two predictions about the temperature in Europe.. one was forecast for a day in June 2015, some 198 hours ahead, the other is a prediction for European July temperature by 2100, some 876,000 hours ahead.  If computer model forecasts beyond 120 hours are known to become significantly more unreliable pushed further into the near future, how confident can we be in similar computer models predicting atmospheric conditions some 876,000 hours into the distant future?


Two temperature anomaly predictions for 198 hours and 876,000 hours ahead.

Of course, weather and climate are different things: weather is short term atmospheric conditions over days and weeks and is usually forecast at high resolution over small areas, while climate describes long term weather over decades or more for larger regions or the whole planet.  Nevertheless, much like the computer models that broadcast meteorologists use to issue short range weather forecasts, climate models use equations of fluid motion and thermodynamics to determine the behaviour of the atmosphere and ocean and to project predictions of Earth’s climate into the future. Both short range and long range predictions require powerful super-computers to run similar complex models that ingest millions of real time observations and perform trillions of calculations to produce predictions of atmospheric conditions on a huge three dimensional grid across different surfaces and altitudes, including the oceans.

The atmospheric system is essentially unpredictable.   Even the most powerful super-computer short range weather predictions can become quite unreliable beyond about 120 hours.  This is perhaps why the MetOffice still only widely publish short term forecasts out to 5 days.  It’s also why charts beyond 300hours are sometimes called “Fantasy Island”… they are so unreliable and not to be taken seriously as a forecasting tool on their own.

In the medium range, accurate ten day forecasts are still something of a holy grail and longer range seasonal forecasts arguably remain largely experimental as they are based on the unreliable time-frame of most numerical model output and the application of complex teleconnections and /or the extrapolation of observations from historic analogue patterns. The MetOffice have even put their seasonal long range predictions into obscure parts of their website after notable public failures in past seasonal forecasts, where they still languish today despite huge investment in computer power.  The charts below show examples of MetOffice contingency planner long range forecast information.

Even with a “perfect model”, weather and climate prediction will always suffer from uncertainties due to the immense complexity of the climate system, the chaotic nature of the atmosphere and the simplifications and approximations necessarily built into models themselves.  The ensemble charts below illustrate this growing uncertainty over just a short time scale of a few weeks.  Note the increasingly wide range of possible outcomes from the individual members showing the growing uncertainty as time progresses, and this is in a relatively settled period of weather.

So, if we really cannot be not certain about the weather next week, how can we be confident about related computer model predictions of climate for 50 or 100 years ahead?  To answer this we need to outline the three different types of uncertainty over climate change prediction:

There are three main types of uncertainty over climate change predictions:

  1. Model uncertainty: climate models have to approximate and estimate feedbacks and processes, they do this slightly differently.
  2. Internal variability: weather is chaotic partly due to uncertain internal forcings, like volcanic eruptions.
  3. Scenario uncertainty: future estimates of human behaviour and emissions of greenhouse gases in particular are uncertain

So, do these uncertainties increase over longer time scales thus rendering computer model predictions of climate in 100 years completely unreliable?  The answer is “No”!  Or more precisely, most of these uncertainties actually decrease over time making model predictions of climate in 100 years possibly more reliable than a seasonal forecast for next summer! Time scale and geographic scale are two reasons that can explain why this happens:

  1. Time scale: climate models deal with longer timescales better than short. The chart below shows lines indicating different model output for temperature change between 1850 and 2010. Note the various models (shown as different lines) all reaching a similar overall temperature increase of +0.8C by 2010.

temperature change more certain over longer time scale

Over the long timescale shown above, the model runs all agree on an overall temperature rise of +0.8C due to initial changes in radiative forcing linked to combinations of, for example, increases in emissions of greenhouse gases, changes in solar radiation and frequency of volcanic eruptions etc.  Models confidently handle these changes over long time scales because climate system and model uncertainty both decrease over time..

Now… if we zoom into one part of the chart above the predictive capability of the model is shown to be much more questionable as the lines wiggle about much more over shorter timescales and sometimes go in completely opposing directions.  This shows uncertainty increasing over shorter timescales.


less accurate predictions over a short time scale

On the shorter timescale of decades or less, model uncertainty increases as climate variability over short time scales is naturally large from one year to the next i.e. one year can be colder or warmer than the previous due to internal variability of the climate system (i.e.chaos).  Models don’t handle this small scale short term climate chaos very well!  One model run predicts cooling in a particular decade, while another predicts warming for the same decade.  It turns out that predicting climate change over smaller timescales is more unpredictable than predicting changes over broad sweeps of time! This is because of the internal chaotic nature of the climate system and model uncertainty being greater at this higher resolution. For example, models will handle these short term variables differently: How will ocean heat uptake respond?  What will happen to ocean currents and regional climates? How will snow cover respond?  Will volcanoes erupt? How will cloud cover and type change? … and many more besides.


climate model variables

Despite these transient climatic uncertainties over short time scales the overall direction of change towards a new climatic equilibrium in the long term is confidently predicted by all the models. So… longer timescales are better handled by models than short.


Scenario uncertainties increase over time in long range climate prediction models

The exception to this reduction in uncertainty over time is scenario uncertainty.  Most long range climate model charts include a wide range of predictions.  This wide range is not due to model uncertainty or uncertainties over internal climate variability. Scenario uncertainty is the uncertainty over predicting future emissions of greenhouse gases.  In other words, uncertainty over our own human behaviour in the future, which is historically difficult to predict!  This is the reason why the IPCC charts show several “RCP trends”.  Representative concentration pathways (or emissions scenarios) show a range of human response to the climate crisis… ranging from drastic curbs to emissions and lower growth, through business as usual, to increased emission pathways presumably due to high growth with no curbs to greenhouse gas emissions. These all yield obviously different outcomes.  The uncertainty over human response, known as the scenario uncertainty, increases over time.  Despite scenario uncertainty, each RCP is an accurately modelled temperature change based on changes in radiative forcing, the wide range of results is due to our own unpredictable behaviour more than climate chaos or uncertainties in the models.

2. Geographic coverage

On a long term global scale, climate system uncertainties are reduced while uncertainties increase over how climate change will occur over small geographic regions.  Predicted global mean annual temperature change is therefore more certain than, for example, regional European or UK temperature change in 100 years.  Future climate change on regional scales is more uncertain than on a global scale due to small scale variabilities.


So… prediction of climate change over short time scales and regional scales is more uncertain than predicting longer range changes on a global scale.  Models struggle to resolve climate change at small time scales and small geographic scales. Overall however, longer term climate outcomes are more certain than short term small scale weather action.  Even with the immense complexity of the climate system, computer models can provide confident predictions of future climate within a range of scenarios.  The IPCC quote levels of confidence and certainty for their climate predictions and, even for the end of the century, they claim high confidence in their predictions ranging from “likely” to “more likely than not” for various scenarios.


IPCC AR5 report on confidence in computer model temperature predictions


the range of climate predictions will hit the probable climatological outcome

In conclusion, maybe this analogy might help: Perhaps climate prediction is a bit like a football match: complex uncertainties about precisely where the ball will go during the game are perhaps completely impossible to forecast beyond the first few seconds of the game.  Any detailed minute-by-minute action on the field thereafter becomes increasingly uncertain over time as countless variables come into play, including some chaos.  For example, individual player performance on the day, how the players and teams interact, the chaotic nature of the ball, the nature of the pitch… these are internal variables that make it almost impossible to model accurate step by step action of a game perhaps beyond the first few kicks after the whistle blows.  This is why small scale high resolution weather forecasts are still limited to less than a week ahead.  Nevertheless, despite such short term uncertainty the overall score and outcome of the game is still possible to predict with confidence, particularly within a certain range. The same can be said for long range climate models.

This post has only scratched the surface of climate and weather uncertainty. Please let me know of any errors you spot in this post. Further reading available here:

further reading







September 2015 Reigate weather summary

Reigate September summary weather statistics

Tmax 22.4C

Tmin 3.9C

Average temperature 13.3C

Total rain 79.8mm (CoCoRaHs)

Max gust 25mph (14 Sept)

Mean SLP 1016mb

Sunshine 132.9 hours

September in Reigate turned out to be cooler than average with rainfall amounting to around average totals mostly falling in showers so that there were relatively long dry sunny spells, especially towards the start of the month, with HIGH pressure in charge or nearby for much of the time.

It was sunnier than usual for September at over 130 hours. This was good news for Run Reigate which turned out to be a beautiful day for the runners.

Much of the September rainfall fell in heavy showers, especially in some storms mid-month.  This meant 2015 September rainfall at nearly 79.8mm turned out wetter than 2014, at a mere 22mm.  Despite this it was still just about equal to the long-term average rainfall according to MetOffice anomaly charts.

At a mean temperature of 13.4C September was slightly cooler than average and the CET came out at 1C below the long term average. Globally, September was 0.31C warmer than the long term average.

Originally posted on xmetman:

Killerton - 0822 on the 18th of December 2010 Killerton – 0822 on the 18th of December 2010

I was looking at the way this month was faring up as far as circulation patterns were concerned using the objective Lamb Weather Type data. September had ended strongly anticyclonic and that’s how October [2015] started. It wasn’t long though before the whole thing had unravelled and a low pressure system zipped across the country, but looking at the latest NWP data I notice that , according to the GFS model at any rate, high pressure would build and the next week looks like being anticyclonic easterly [AE] and very similar to the 27 – 30th of last month. So I decided to look for the most anticyclonic October from the 1871 data set and to my surprise found that 1879 turned up at the top of the list.

Lamb Weather Type Analysis for the October [01 Oct - 31 Oct] Lamb Weather Type Analysis for the October [01 Oct – 31 Oct]…

View original 348 more words


Hurricane Joaquin tracks NE then E across Atlantic

Hurricane Joaquin, previously a Category 4 hurricane, tracked across Bermuda today and dropped to Cat 1.  Nevertheless, maximum sustained winds were near 85 mph with higher gusts. Joaquin is expected to transition to a large extra-tropical low pressure system on Wednesday. The chart below shows how many extra-tropical storms (those named previously as tropical cyclones) reach or have got near to the UK, many of these would have passed unnoticed as regular autumnal stormy weather.


extra-tropical storms that have reached or nearly reached the UK

The tracks below show the remnants of Joaquin apparently taking direct aim at the UK but it’s not unusual to have extra-tropical storms arriving in UK waters during the autumn.  Of course, extra-tropical storms lose much of their tropical characteristics and potency as they recurve and track across the cooler mid-latitude Atlantic.  Even if extra-tropical storms track directly across the UK they are usually not much more than a nuisance storm, like Bertha in August 2014.

Beyond Friday, model tracks are more uncertain on whether the remnants of Joaquin will actually impact the UK at all, or linger offshore.  A ridge of high pressure builds later this week and acts as an effective buffer to block off Atlantic LOWS, including Joaquin.  He is therefore reduced to sitting offshore.


HIGH pressure ridge blocks entry to Joaquin

Of the main models the ECM sees ex-Joaquin taking a more direct track across the country later in the weekend.  The GFS is keen to disrupt the trough and send a secondary LOW across the south of the UK later in the weekend while the main system seems to merge with a trough to the NW.  The UKMET sticks the LOW offshore for longer and sweeps the system to the NW of the UK with any wind impact mostly limited to the far west. This disagreement between models shows uncertainty so … keep watching forecasts if weekend weather (especially later) is important.


Ex-Joaquin location by Saturday Oct 10

Prior to this the pressure across the country is set to build after an unsettled start to the week.  The charts below show a purposeful rise in pressure to the end of the week, with the Joaquin uncertainty kicking in thereafter, although Saturday looks good on most charts at present.

The Atlantic satellite view shows the impressive swirl of Hurricane Joaquin tracking near Bermuda today and the UK top right.