Summer Forecast 2024: Episodic with violent heat spikes

It's that time of year once again, summer has arrived. I've had many of you in my inbox ask me for some insight, it is worth stressing these seasonal forecasts are somewhat experimental there is a whole new science which has emerged only in last 15 years. Also in the case of the UK, we are merely a grain of sand in the hemisphere, it is inherently tricky but as an informed weather nerd, this is my attempt.

Summer 2024 headlines:

• Overall, a warmer-than-average summer is expected but it likely will not challenge the hottest summers of 2006, 2018 and 2022. I would not rule out very hot weather in excess of 38C (100F). I'd say this is most likely in the middle of summer in July, these past 12 months have been record-warm globally as such it is likely there will be intense hot air relatively close in Southern Europe.
  

• Rainfall is a tricky one i'd say a wetter summer is more likely than a dry summer. I will outline why this is more probable if you continue reading. Separately, these past 18 months have been the wettest on record in England where we have experienced the highest level of rainfall since Met Office data began in 1836. In the midst of high global temperatures and record high sea surface temperatures even with onset of La Nina, which will begin the cooling influence of oceans and resulting atmospheric response, it will likely be too soon in terms of lowering temperature this summer in Northern Hemisphere.

El Nino transitioning to La Nina

Sea Surface Temperature (SSTs) Anomaly in the equatorial Pacific with blue areas indicating colder than average waters. This is not caused by weather but strengthening of easterly trade winds which results in cold water upwelling off the west coast of South America. We can see the development of La Nina well underway in the east of basin. Weak La Nina conditions are expected to develop by July this summer.

ENSO Pacific SST's anomalies as of June 1, 2024

Now the question might be how are "cool" ocean temperatures thousands of miles away is possibly influencing our weather? ENSO is one of the most important climate phenomena on Earth due to its ability to change global atmospheric circulation which changes the rainfall distribution around world. For NW Europe the effects are less obvious, in contrast to Australia the recent 2020-2023 La Nina event was marked by record breaking rainfall in eastern Australia as warm ocean water piles up in the western Pacific.

On average summers which are transitioning to La Nina as the first year are more prone to being shaken up by westerlies, while summer 2022 which featured 40C was a solidly La Nina summer this was during the 3rd consecutive year of La Nina. A very unusual setting this has only occurred two other times in the last 74 years. I think the oceanic/atmospheric link was stable and constant this is ideal and what you want to sustain fine and warm conditions for a significant period of time. This is very much unlike what we are seeing heading into summer 2024, where significant changes will be occurring from tropical forcing. We experienced the fifth strongest El Nino event in last 74 years over last winter, such a breakdown to moderately strong La Nina later in year within the same calender year is very unusual.

in terms of ENSO evolution this summer +ENSO to -ENSO the following analogues are most in line (1998, 2010 & 2016). This gives the following look for this coming summer. A low-pressure anomaly can be seen in Scandinavia this perhaps suggests the exceptional late spring warmth should be savoured here. Generally, it suggests a summer with increased northern blocking with winds coming more from a northern direction. It is a reasonable outcome for southern Britain but for Scotland, it is not pointing towards much of a summer but broadly across NW Europe it suggests near-normal temperatures perhaps slightly below.

Pacific Decadal Oscillation PDO

The PDO is a long-term climate pattern that affects temperatures of the Pacific Ocean over long periods it typically oscillates warm and cold every 10 years.

This summer the PDO is expected to be extremely negative the Eastern Pacific Ocean becomes cooler and the Western Pacific Ocean becomes warmer. Since, 1981 where these daily records have been kept only 2012 (remember the Jubilee washout?) as reached such a deeply negative phase. While this is more a variable in forecasting in US during -PDO summers we observe often are hot and dry in Europe but northwest Europe more prone to cooler and wetter. Of course, it is not black and white, 2022 was also deeply negative, however as previously discussed that summer coincided with a very unusual backdrop.

I compiled all known summers with strongly -PDO (1950, 1952, 1955, 1971, 1999, 2008, 2011, 2012, 2022, 2023) and the results are quite strongly favouring a deep low pressure anomaly in Atlantic Ocean within close proximity of Iberia, France, Ireland and southern UK. This might cause some concern, it does suggest a heightened chance of intense showers and thunderstorms. However, a stubborn low anomaly off NW Europe near to southwest is also associated with extreme heat in Western Europe. Low pressure drives up very hot southerlies from a very hot North Africa acting like a heat pump, this is part of the reason I believe this summer will see extreme heat spikes, but it will not persist for long periods at a time.

When we look at air temperature we get the following results... what we see there is no signal either way for the UK, but generally these summers are cooler than average in Northern Europe. We can identify a very hot North Africa, and this heat dome largely extends northeastwards into the Mediterranean region and SE Europe away from Western Europe. There is a lot to suggest this summer parts of Europe will experience a very harsh summer on top of existing warming trends Europe is warming twice the global average.

Warm Atlantic

Atlantic SST's as of May 31, 2024

As can be seen across the Atlantic warm anomalies are extensive and are in fact record breaking warm in the tropical Atlantic and Caribbean. It is no wonder a highly active hurricane season is expected in tandem with the developing La Nina which removes wind shear, therefore it is a more hospitable environment for developing tropical storms and hurricanes.

Last summer we also had a very warm Atlantic you might recall a "marine heatwave" around UK. The UK experienced its warmest June on record overtaking 1976 there's little doubt the warm ocean water will make it slightly warmer than it would be, and this is one of the main factors along with elevated global heating which quite strongly favours a warm summer. However, as ocean water warms more water evaporates in the air which results in more intense rain, this greater humidity is likely to be a feature this summer especially later on.

World Climate Service on Twitter (highly recommend a follow) tweeted the following:

“One of the more “interesting” analog signals in our recent summer forecast is the set of years with most similar N Atlantic SSTs. The pattern looks reminiscent of summer 2012 for Europe, although that year isn’t one of the analogs. Probably not what the UK crowd wants to hear.”

This is not what we want to hear in NW Europe, note the signal for a very warm and dry SE Europe once again - confidence is higher than average there. Overall, with the warm Atlantic you win and lose, as I feel last summer demonstrated well.

Solar Activity

I debated whether to include this but why not? It will not be given much consideration or any to the forecast. Summer 2024 finds itself during solar maximum a period of high solar activity, which is measured by the higher number of sunspots they appear darker because they are cooler than other parts of the Sun. We are at the height of the 11-year cycle this occurs due to changes in the Sun's magnetic field every many years it flips.

Current image of Sun's surface:

The sun is very spotty and restless at present activity is high. Sunspot 3697 is back facing the Earth it survived the rotation formerly known as 3664 which caused the exceptional low latitude aurora on night of May 10th, certainly a night that will be remembered for many years to come. It was even visible in central London although the phone camera did much of work. This summer there may well be further opportunity, the next solar max mid-2030s!

When we look at previous solar maximum summers back to NOAA records from 1948 we get the following look this combines (1958, 1968, 1979, 1989, 2001 & 2014). A rather bonkers extreme low-pressure anomaly in central-southeastern Europe. A year without summer for parts of the European continent if there is anything in this solar link I'm very dubious myself.

I believe it is more useful to remove the summers from 40+ years ago in a colder climate which is frankly gone, it yields a rather different result for UK and Ireland. This hints towards high-pressure ridging near Ireland (warm + dry?) interesting also summers of 1989, 2001 and 2014 were also thundery summers. Even not accounting for solar link other variables do appear to be quite supportive of an unstable summer.

Precipitation impacts from Hunga Tonga eruption

On 15 January 2022, the Hunga Tonga an underwater volcano launched 146 teragrams (Tg) of water vapour into the Earth's stratosphere equivalent to over 60,000 Olympic-sized swimming pools. Increasing the water vapour concentration by 10%. I do think this has played a part in the past 18 months being the wettest on record a new climate modelling paper reports the following: "Our WACCM simulations reveal significant surface temperature and precipitation anomalies globally which peak years 3-7 after the initial water vapour was injected into the stratopshere, e.g. years 2025-2029 got HTHH, but can appear earlier." Of course, there is significant uncertainty involved but this extended exceptional wet weather experienced since late 2022 not just isolated to UK but around world could be intensified, water vapour is a greenhouse gas as such Earth is likely warmer than it would be otherwise on the top of El Nino (2023-2024) there are reasons to believe this summer could bring exceptional downpours and rainfall to NW Europe and different parts of the globe. And to add on previous variables makes me believe a wetter summer is more likely than a dry one.

This plot shows a cross-section of the atmosphere at the equator we can see there is still significant and anomalous amounts of water vapour, especially in the lower atmosphere.

Seasonal models

Multi-model forecast for June-August 2024

The CS3 multi-system seasonal forecast is slightly supporting a wetter summer in Scotland and far north however, overall no real overriding signal for UK and much of Europe. Nonetheless, in terms of temperature, there is virtually unanimous agreement for a warm summer with the strongest and most frequent heat episodes in Southern-Southeastern Europe and weakest in Northern Europe.

My prediction

I'm expecting frequent and at times severe heat in southern and southeastern Europe, occasionally this is likely to move further northwards up into France and SE UK especially. These plumes from North Africa are likely to be intense and could bring very high temperatures but probably more short-lived heat in contrast to the Mediterranean region. I do not expect the summer to be great in northern Europe, I expect fine and warm weather to be punctuated by fresher and cooler oceanic air, which could give rise to thunderstorms.

June

It is very unlikely the summer will peak as early as it did last year, the first half of June is likely to be on the cooler side with winds frequently from the northern quadrant. This will be more unsettled for northern UK, I favour rainfall to be below average further south but temperatures are not likely to be spectacular. Late in the month, I see the potential for some hotter and drier weather from the south perhaps bringing the first 30C weather.

July

I foresee this will be the hottest summer of summer, indeed on average it is the hottest month of year, no shocker there with August in second, but I believe this summer month will stand out more than usual. Possibly the most prolonged settled spell of the summer. I also think this is when we are most likely to experience the hottest weather of the summer, extreme heat is favoured. A very warm summer is anticipated in the land areas of the Northern Hemisphere.

August

I think this will be a quite divided summer month the first half could see an extension of July's hot and settled spell, I do though think by this point northern Britain will see the seasonal deterioration has already kicked in well there's always September? Overall, the final month of summer has potential to be quite useable but it will be under threat.

Final comments

Overall, a warmer-than-average summer is likely in UK, however, northern Britain could see more average-below average temperatures and more frequent rainy spells. A hot and dry summer appears rather likely in S/SE Europe. In some way, this might be a more traditional summer with typical peak in midsummer, but the heat could be much more atypical. And downpours are likely to be heavier than usual a wetter summer is favoured over a drier summer, but regional disparities could be great. Thank you for reading.

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Winter 2023/24: A Winter to Remember?

Winter Forecast Headlines:

• Overall, over the course of the meteorological winter encompassing the months of December, January and February. Temperatures are favoured to be colder than normal to 1991-2020 winter climatology and perhaps even significantly so. I am the most bullish of a colder winter since I've been producing these winter forecasts the reasoning will be laid out in this report.

• Generally, near normal winter rainfall, I believe the most anomalously wet weather will tend to be seen in southern UK. The jet stream being dispaced further south with subtropical high pressure system known as Azores High which brings warm winter weather to Europe will be more withdrawn. I believe this will be a source of continuity through this upcoming winter.

I'd like to begin this forecast with a caveat; seasonal forecasting is very challenging this forecast is somewhat experimental but scientific. It combines a number of key drivers to produce this winter forecast. I will outline them one by one, it is these "background signals" and their interrelationship that drive numerical weather prediction (NWP).

ENSO (El Nino Southern Oscillation)

There are 3 states to ENSO, in recent winters we have seen La Nina conditions, associated with colder than normal sea surface temperatures (SST) in the central and eastern tropical Pacific; neutral conditions suggesting close to average SSTs or El Nino which is the opposite of La Nina, where much warmer waters in equatorial Pacific dominate.

Tropical Pacific Sea Surface Temperature Anomaly (°C) Nov 27, 2023

As can be clearly identified by the above SST chart we are firmly in warm ENSO conditions warm waters span the entire basin, but the warmest waters are in the east closest to South America. This is known as an Eastern Pacific (EP) El Nino it's important to know there are many "flavours" of ENSO there are different types of La Nina and El Nino with different influences to global weather patterns. This El Nino event is expected to peak from now to January beginning to weaken by late winter and spring, nearly all models are showing a moderate to strong El Nino and they transition to ENSO-neutral by summer 2024.

Model Predictions of ENSO from Nov 2023

• "El Nino winters have a tendency to have a mild and westerly start to winter (Nov-Dec) and a colder, drier end to winter (Jan-March) across most of Northern Europe," Professor Adam Scaife (2023)

It's interesting to me the upcoming disconnect of colder weather patterns in Europe with normal expectations from El Nino winter. I believe the MEI index reinforces the atmosphere is not yet truly connected with strongly developing oceanic El Nino. After a very unusually persistent La Nina event 2020-2023, perhaps it is unsurprising that the atmosphere could be lagging behind. Hence, our weather patterns in early winter are not yet being greatly influenced by El Nino, which would oppose colder weather patterns earlier in winter.

Multivariate El Nino/Southern Oscillation (ENSO) index against strongest El Nino years

I think among many other factors yet to be discussed this points to a winter which has the potential to be well-rounded in terms of cold risk with similarity to winter 2009-10. It is likely El Nino will connect more strongly to the atmosphere later in winter, but by then El Nino (albeit not super strength), is known to increase the probability of late winter wintry patterns. Whilst the high global temperatures are clearly unsupportive for a cold winter and there is some concern, not everywhere can be mild simultaneously. I theorise Northern Europe could well be the cold anomaly this winter.

North Atlantic Sea Surface Temperatures

Atlantic Sea Surface Temperature Anomaly (°C) Nov 27, 2023

Owing to our exposed geographic area in NW Europe to edge of the Atlantic Ocean, sea surface temperatures (SSTs), are really quite an important area of focus in producing a seasonal forecast. As can be seen warmer than average SSTs dominate Atlantic, but there is a band of cooler waters off Labrador and East Coast which cross mid North Atlantic and up to Bay of Biscay. Often we see serious Arctic cold spill out of Canada and this collision with much warmer waters greatly strengthens the Atlantic jet stream bringing wet and windy weather to NW Europe. The cold SST anomalies off Labrador therefore are a good thing to see if looking for a weaker Atlantic jet stream, the fact the band of cooler waters is displaced further south is also indicative of the storm track (jet stream) being further south.

The warmer waters at higher latitudes of Greenland & Iceland are also interesting, this is indicative of a lack of stormy wind in this region which upwells much colder waters from depths. The North Atlantic Oscillation (NAO) is a highly influential atmospheric system in North Atlantic that can greatly influence wintertime weather in Europe. The positive phase of NAO corresponds with below-average sea level pressure (SLP) across high latitudes of North Atlantic often the strength of Icelandic Low is used, and above normal SLP over central North Atlantic. Positive NAO results often in a mild, wet Europe. The behaviour of NAO can be influenced by multiple factors but SSTs are also important. I believe the SSTs this upcoming winter season are more supportive of negative NAO atmospheric circulation in North Atlantic. This is associated with above-average sea level pressure across high latitudes of North Atlantic. Therefore, in my opinion, colder weather patterns this winter have a greater chance of occurring than in many recent years where +NAO has dominated.

Northern Hemisphere snow cover

Snow area extent in Northern Hemisphere 2005-2006 to present 2023 indicated by red line

After a rather sluggish growth to snow extent in the Northern Hemisphere during November with very mild patterns dominating in North America and Eurasia, we have seen a strong rally in the last week with colder temperatures. This has taken 2023/24 as of Nov 26th to climb above this point in 2022/23 which was slightly above the latest 18-year average. Overall, this is a good indicator to colder weather while I will say with this factor it is more tenuous one I believe it is important. When snow covers the ground the incoming radiation is reflected back to space. The albedo of fresh snow cover can be as high as 80-90%, therefore, snow cover makes it colder than it would be otherwise, the fact large swathes of Northern Hemisphere have a healthy snowpack could prove beneficial later on. An easterly flowing through a snow rich Russia/Scandinavia towards UK will be colder, and therefore, more fruitful in reducing cold modification from warmer waters surrounding us.

Some researchers use the Siberian snow cover extent in the autumn for more accurate winter predictions. Cohen et al (2007) discuss that increasing snow cover particularly in Northern Asia, leads to stronger diabatic cooling and a strengthened Siberian high. This further leads to an increase in upward propagation of planetary waves resulting in a weakened polar vortex and westerlies, with a strengthened meridional flow. Associated with a wavy jet stream and high lat blocking with an increase of cold Arctic outbreaks in mid-latitudes like in Europe. Whilst a low snow year would result in a strong polar vortex with strong, mild westerly flow.

Stratospheric polar vortex (SPV)

The stratosphere is in the layer of the atmosphere situated between 10km and 50km above the Earth's surface, being above the troposphere the first layer of atmosphere responsible for the weather we receive on the surface. it is important to understand how the stratosphere may affect the winter tropospheric pattern.

How does it form?

Every autumn the stratosphere cools drastically as less UV radiation is absorbed by ozone content in the stratosphere. The increasing differential between the Arctic and warm latitudes further south combine to form and strengthen the wintertime stratospheric polar vortex. Rather easy to understand the colder the stratospheric vortex the stronger the polar vortex becomes with stronger upper westerlies associated with trapping the cold in North Pole. The stratospheric vortex has a strong link with the tropospheric vortex (remember where weather happens). A strong polar vortex is associated with a positive Arctic Oscillation (AO) increasing the probability of mild and unsettled southwesterly winds in NW Europe and mid latitudes.

AO graphic in positive and negative phase

What's happening now with the SPV?

The latest zonal wind forecast from ECMWF as of 28/11 is eye-catching. A weakening in zonal westerly wind in other words stratospheric polar vortex (SPV) is expected into early December, further into December, there is a strong signal for the weakening to go further. On the week 19-26 December, the ensemble mean is close to 10% percentile. This suggests a weak polar vortex is expected mid-late December when climatology often supports the polar vortex at its most powerful. A sudden stratospheric warming (SSW) would be defined by <0m/s zonal wind essentially meaning upper westerlies have reversed to easterlies. While current forecasts are not showing that currently. There has been an increase in members as seen in below chart which reach this threshold later in December, this clearly even among non-SSW runs a sign of something anomalous happening in the stratosphere. A weaker vortex more often equates to a weaker jet stream and a greater likelihood of northern blocking and cold spells in mid-latitudes. I ponder over Christmas period into early 2024 there could be an opportunity to see a cold spell which brings widespread wintry weather. In the case of a major disruption to polar vortex this winter, I think it is unlikely to have infulence until January, potentially right in the heart of winter, it is a frightful prospect for mildies.

ECMWF ensemble zonal wind forecast at 10hPa at 60N issued November 28, 2023

QBO

The previous discussion on stratospheric polar vortex ties quite nicely into QBO. The QBO refers to the regular variation of winds in the stratosphere high above the equator in the 1950s UK scientists discovered winds above tropics in stratosphere oscillate between westerly and easterly in 28-month cycle. Now you may ask how can something so distant in tropics, influence our weather? The QBO is important as it can affect the polar jet stream.

• When QBO is negative (easterly), this indicates greater potential for a weaker jet stream with a greater chance of weaker polar vortex and an increase in cold outbreaks in Europe and parts of North America.

• When the QBO is positive (westerly), there is a greater chance of a stronger jet stream with a mild and stormy wet winter, with a strong polar vortex the cold is locked at pole.

What is QBO doing this winter?

The QBO this winter will be in easterly phase these are quite strong easterlies as a matter of fact typically we look at 30mb level.

During easterly QBO winters as mentioned previously stratospheric westerly circulation tends to be weaker, hence there is a greater risk of a colder winter in Europe and N America. It has been identified that the most receptible environment for a major disruption to the stratospheric polar vortex is when QBO is in an easterly phase with El Nino. We will have both this winter, it is a rare combination, this last occurred in winter 2009/10 which was a very chilly, cold dominated winter. Therefore, one can deduce there is a heightened chance of major SSW this winter, this was part of a catalyst to 2018's Beast from the East but all SSWs are unique. Some even fail to have a response on the tropospheric pattern, I, however, think this is less likely due to mature eQBO. I do think we will see an SSW this winter most likely into January and this could have a profound influence on the late winter period. If it comes together like this, I'd say we could be on for something truly memorable this winter, there is clearly a chance, the vast majority of winters do not present an opportunity like this.

Seasonal Models

ECMWF

The European (ECMWF) model is often regarded as one of the top-performing seasonal models. There is a weak signal for -NAO, it is not the most conclusive forecast. Note the above-average mean sea level pressure to south of Greenland which suggests a less cyclonic activity exiting eastern seaboard. A stormy and active pattern is expected in southern US, this is typical of El Nino which causes Pacific jet stream to move south and east. Some of the strongest low-pressure anomalies in North Atlantic region are found in UK/NW Europe, this might suggest a wet winter is more likely. It is noticeable though how these more unsettled anomalies are looking quite widespread across Europe even into Med. The model expects Azores High to be less influential this winter, this brings mildest winter weather to Europe, an indication the jet stream will be further south. Overall, for a 3-month mean, this points to a mixed winter with potential for cold spells while probably not severe.

C3S multi-system seasonal forecast Mean MSLP anomaly December + February

The Copernicus Climate Change Service (C3S) seasonal forecast is based on data from numerous leading prediction systems including ECMWF, Met Office, Meteo-France and more. Therefore, it gives a more holistic look at what best guidance is most strongly favouring. The latest November update reveals a rather strongly contrasting winter with a major difference in atmospheric circulation patterns between early and late winter. Early winter features much cyclonic activity with stronger westerly winds. In late winter we see this flip with cyclonic focus going to southeast Europe with higher atmospheric pressure in North Atlantic and Pole suggesting a reduction in westerly winds it is a classic -NAO/-AO pattern. This is associated with a weak polar vortex. It is clear models are going strongly with ENSO influence with more classic response to El Nino, with the potential for the most harsh winter weather in the second half of winter. In the UK it has been over 30 years since we have seen a very cold February this is something which is significantly overdue. The signal in guidance is interesting and robust perhaps this could be the winter where the 'drought' ends.

Overall thoughts on winter 2023/24

I expect December will be overall close to average, perhaps colder than normal further north expecting a great start to ski season in Scotland. I do not see a shortage of snow this winter. Into the end of week 1, we're likely to see unsettled and milder Atlantic weather with strong winds possible. Overall, I cannot see December being a particularly cold month, I think it will be a very cold month in Scandinavia. The signal is strong for deep cold to be really entrenched to our northeast. It is possible we see another attempt later in the month around Xmas to New Year as tropical forcing comes in.

My thoughts are that January into February is primed to feature the coldest weather of winter, with a major disruptive event to SPV, I think it has the potential to bring significant and persistent cold, with deep -NAO. This block in the North Atlantic could prove very resilient we know in easterly QBO winters, northern blocking where it develops, is able to gain a more secure foothold against westerlies. As discussed earlier, I also believe the Atlantic sea surface temperatures are also rather supportive of a blocking regime in North Atlantic. Seasonal models are also rather bullish on cold wintry risks later in winter, as is more typical from El Nino winter. However, early winter could be disappointing with much rain.

I'm feeling bold the background drivers as discussed are the most supportive they've been in a number of years. This does not guarantee a cold winter, but I believe the risk is higher than in many other recent years.

It is possible this could be the coldest winter since 2009-10 in UK. If the stratospheric warming fails and blocking fails to materialise then it likely will be a wet winter, but probably not overly mild. The dreaded Euro high seems less likely to be a foe this winter.

Thanks for reading, I apologise if much of it went over your head! And as ever keep up with @TheSnowDreamer for the latest shenanigans.

Summer Forecast 2023: Unstable

Well, here we go again. Summer has arrived and many of you are eager to hear my thoughts, strangely. Following on from last summer, which experienced the worst drought in Southern England since 1976 and the highest temperatures in likely centuries. There is some apprehension we will see similar again this summer, as with any British summer not famed for their predictability, one year can vary significantly from the next.

However, it's hard to ignore the run of summers since mid 2010s fierce heat spikes have become almost an annual occurrence.

Now enough of that, let's get into the forecast!

Summer Forecast 2023 headlines:

• Overall, a warmer-than-average summer is highly favoured likely not as hot as 2022.

• An unstable thundery summer featuring dry spells but also episodic intense showers and thunderstorms and occasional Atlantic influence. This summer I expect to see a low-pressure anomaly in Atlantic to our W/SW, it's more prevalent in El Nino summers as last seen in 2015, where strong El Nino conditions developed, it will try to progress east breaking down the heat in NW Europe. The west is likely to see the wettest conditions while the east is likely to maintain the hottest and driest weather for longest.

• I consider it very unlikely we see 40C temperatures this summer, but temperatures above 35C are favoured by me, unlike recent years I expect humidity to be more of a feature, which would make the heat more unpleasant with sticky nights being a challenge.

Numerical Weather Prediction (NWP) Models

NWP computer models are used by forecasters and hobbyists to forecast weather in short and long timescales. Typically strong reliability is limited to 5-7 days out but with the help of teleconnections the "background signals". They commonly have more permanency and there are known patterns to our weather patterns in certain regimes. We can look beyond, sometimes weeks and even months ahead with some success. Of course day to day weather cannot be forecasted, but a broader feel for a season can be achieved. Some of the important teleconnections are the ENSO state (El Nino and La Nina); the MJO; the QBO; the Jet Stream; Stratospheric Weather; Sea Surface Temperatures; Sea Ice and Snow Cover extent. It's these telecommunications which are ultimately fed into NWP models and build these skilful forecasts.

• The C3S seasonal forecast this blends and creates an average of top NWP models including ECMWF, UK Met Office and Meteo-France. The signal for this summer (June-August) is compelling. Warm summer. Above average temperatures are strongly favoured in Western Europe, there is a particularly strong signal for a hot weather in North Africa and Iberian peninsula, suggesting southerly flows will be capable of delivering very high temperatures to our shores with temperatures exceeding 35C.

• There is an enhanced risk of above average rainfall across Southern Europe. Indeed, for a forecast which was initialised at start of May the anomalies have accurately identified the early summer pattern at least. As a whole though, across summer the wetter signal is not particularly strong, perhaps suggesting it will weaken as summer progresses.

• A dry summer is not really favoured in UK/Northern Europe in contrast to last year's prediction, which was highly accurate identifying it to be very warm and dry, suggesting the risk of drought is lower this year, rainfall is favoured to be close to longterm average.

El Niño–Southern Oscillation (ENSO)

The El Nino/La Nina Southern Oscillation has a considerable influence on climate patterns across the world. Summer 2023 is expected to have El Nino conditions, this is a warm ocean phase, which results in higher global temperatures. Whereas, La Nina is the opposite, resulting in cooler global temperatures, I will stress last summer was a La Nina summer, yet it was record-breaking hot summer in UK and Europe. Not everywhere can be simultaneously hot or cold this is why global averages are not without scrutiny, and for tiny countries like ours compared to global field, the connection is not obvious and there is greater uncertainty.

It has been a rather long time since we last had a fully El Nino summer, the last was in 2015 which was last cooler than average summer in UK, and overall it was quite unsettled. In SE temperatures were near normal, but school holidays in August were a bit of a washout. While June was very dry. A varied summer, it is often remembered for its one-day heatwave at start of July which set a July record at the time despite much of month being cool especially in west.

A table of El Nino summers is provided below with SST data since 1950.

El Nino summers >0.5C SST anomaly based on Oceanic Nino Index (3-month running mean (JJA) of ERSST.v5 SST anomalies in the Nino 3.4 region

1951	​Cool summer June-July rather dry but August wet near 100mm at Heathrow.
1953	Unremarkable summer cool for modern climate, drier than average.
1957	June warm for period 23.6C average max, dry and sunny. July-August unsettled and cloudy.
1963	Another quite cool summer lacking in heat drier than average, August cloudy.
1965	Cool summer average Tmax of 19.3C in July at Heathrow significantly below average. Rainfall bit above average.
1969	One of the better summers of 1960s, August though cloudy and unsettled.
1972	June exceptionally cool average Tmax 17.5C modern average is 21.6C. A dry summer in London but often cloudy.
1982	Near normal temperatures nothing overly warm, all summer months deficit in sunshine. Largely forgettable but remarkably thundery June some locations had 14 days of thunder in June alone.
1987	Poor summer and often cloudy. June disappointingly cool, very dull and wet the rest of summer did not redeem itself.
1991	June cool, dull and wet, however July and August performed strongly.
1997	A decent summer August was particularly warm with warm humid nights. Most thundery summer since 1982.
2002	OK but quite forgettable as was wider year.
2004	All 3 summer months had over 200 sun hours at Heathrow not a common occurrence. August warmest month a month of two halves Newton Aycliffe in County Durham saw 210mm wettest month on record.
2009	June best month much of summer unremarkable a quite thundery and wet July but not cool. Coldest winter since 1978/79 would follow.
2015	A fairly good summer in south of England but poor elsewhere with westerly airflow, start of July saw intense but very fleeting plume. Setting a new July temperature record of 36.7C at Heathrow, It wouldn't last July 2019 saw 38.7C a new temperature record in UK exceeding 38.5C in 2003 in Kent, and then on 19 July 2022 would smash it apart with 40.3C at Coningsby, Lincolnshire . August though disappointingly wet and cloudy.

From list above of previous El Nino summers, it's notable to me that none of the greats are listed there, many of these summers lacked significant heat there is also a pattern for these summers to have best weather in June or July, only for August to deteriorate.

Although occasionally August provides hottest weather of the summer, but it is unstable. Also quite noticeable a few of these summers were memorable for thunderstorms. Some might argue it is just chance, but I noticed a tendency for our classic British summers to cluster together in neutral ENSO conditions. Including: 1976, 1983, 1990, 2003, 2006, 2018. To me it strikes to me as more than a coincidence, my theory when ENSO is rather inactive weather patterns globally become more static, and dry and warm patterns lock in. This summer El Nino conditions have formed and are expected to strengthen to moderate/strong status indeed this is also expected to be a similar intensity to 2015. I believe this suggests the summer will be more prone to being mixed up, with not one pattern dominating for too long unlike in 2022.

Atlantic Sea Surface Temperatures

May 2023 global sea surface temperatures were the highest on record, relevant for UK and of influence, the east Atlantic warmth from tropical Atlantic to UK are warmest on record. This suggests southwesterly flows are prevailing wind direction will be warmer than average. It also suggests more water vapour in the air, with the higher temperatures of ocean water this signals to me a humid summer. Potentially prone to heavy rainfall events, perhaps also more prone to torrential thundery downpours. I get the impression heatwaves this summer will be more unpleasant, less of the dry heat in recent years which has made things a bit more bearable.

Summer Forecast 2023 recap:

• Overall, a warmer-than-average summer is highly favoured likely not as hot as 2022.

• An unstable thundery summer featuring dry spells but also episodic intense showers and thunderstorms and occasional Atlantic influence.

• High temperatures are likely but the risk of pushing 40C is very low.

• A changeable summer but predominantly warm. Likelihood of drought low/moderate, driest in southeast but downpours could muddle the statistics.

Conclusion

I typically give a monthly breakdown to my forecasts but ultimately it's a pointless exercise. My aim is more to present a flavour, this is more likely to be successful, and I'd rather not spend more time on this, and instead enjoy the lovely weather we're having. After what seemed a very long spring. I would say though, I favour the best weather of summer, perhaps the most lengthy spells of settled weather in first half of summer in June and July.

It's difficult to see into August potentially the hottest weather of summer may occur with unpleasant, high humidity, but also featuring the most disturbed and changeable weather. I don't expect a repeat of last summer, but we must be vigilant to extreme weather, it seems nowadays through very little build-up or effort we see terrific heat. Looking at historical El Nino summers you can't help but feel underwhelmed for our summer prospects, but in the year 2023 the climate has changed quite significantly and what was familiar then is not familiar now.

Thanks for giving some of your time! As always I will be posting regular updates this summer and beyond on my Twitter page @TheSnowDreamer :)