Long Range Weather Forecast | Summer 2024 | Netweather.tv (2024)

NWP seasonal forecasts

There is a range of seasonal models that have forecast charts of mean temperature, precipitation and pressure /heights available for the each of the 3 summer months. These get updated every month, the latest was issued earlier in May.

Multi-model seasonal forecast charts

June

For June, it’s a mixed picture from the models, but what most stands out is that ECMWF looks on its own with a signal for high pressure covering northern Europe, including the UK, while other main models (UKMO, NCEP & Meteo France) are showing low pressure over NW Europe. The latest update of the long-range EC46 run of ECMWF, which looks 46 days ahead, goes against the seasonal outlook too and suggests an upper trough over western Europe and ridging to the east and southeast.

The ECMWF seasonal update suggests an average to slightly drier signal from its precipitation forecast, while the other models are average to slightly wetter than normal signal, thanks to low pressure nearby. Meteo France looking the ‘wettest’ across the whole of the UK followed by UKMO.

With regards to temperatures, general picture across the model output for June is for temperatures to be above average, 0.5-1C above. NCEP the warmest of the bunch. Generally, the picture is unsettled at times, with rain for most, but warm.

July

A look at July, again ECMWF looks more on it’s own, with signal for lower pressure over northern and eastern Europe, while UKMO, NCEP and Meteo France have lower pressure towards Iceland and southern Europe, average or slightly higher pressure tendency in between.

All models showing temperatures above average, NCEP warmest, with 1-2C above average. Rainfall signal more mixed, but no particularly dry signal from any of them. No signal to slightly above.

August

The cross-model signal for August looks like July, lower pressure towards Iceland and southern Europe, no signal in between. But individually some differences, ECMWF and UKMO have a stronger signal for lower pressure to the north and northwest. NCEP a strong signal for lower pressure to the west.

All models look warmer than average for August, generally 0.5-1C. Precipitation signal looks more mixed, but generally no signal for wetter or drier.

Summary of the seasonal model output

In summary, for all three summer months, there is a unanimous signal from all models for above average temperatures in most of the UK over June, July and August. The rainfall signal is more mixed and less clear, with generally no signal for wetter or drier, so on balance, average across all three months.

ENSO

One of the first things we look at to construct a seasonal forecast is the forecast for ENSO (El Niño Southern Oscillation). El Niño, the warm phase of ENSO, is the warming of the equatorial Pacific. It has been one of the main players in influencing global weather patterns over the last year, exacerbating background warming from climate change by adding additional warmth to the atmosphere. 2023 was the hottest year on record and the record warmth was boosted by El Niño conditions, on top of long-term human-caused climate change, and that warmth continued into 2024, in fact, the last 11 months have been record-breaking warm individually.

El Nino is rapidly weakening and will all but vanish by the time the summer season kicks into gear. But El Niño’s disappearance and likely arrival of La Nina, a cooling of the equatorial Pacific, doesn’t necessarily mean we won’t experience more record-breaking warmth this year, including this summer. Especially when the world is heating up due to human-driven climate change.

A transition from El Niño to ENSO-neutral is likely in the next month, La Niña may develop in June-July-August period (49% chance) or July-August-September period (69% chance).

The phase of El Niño Southern Oscillation (ENSO) that drives tropical weather patterns can also have some influence on the extra-tropical weather patterns at higher latitudes by forcing large-scale patterns across the globe.

But the strength of El Niño or La Niña’s influence on global weather isn’t uniform and varies greatly based on the strength of the phenomena and the season itself. The influence of El Niño or La Niña on weather patterns isn’t as clear-cut in the summer as it is in the winter, especially during a transition between the two phases.

Temperature differences between the tropics and mid to higher latitudes are more extreme in the winter – which allows the jet stream to become quite strong and influential in driving weather patterns. In the summer, the difference in temperature isn’t as significant and the obvious influence on weather patterns wanes.

There have been some record-breaking warm summers during La Nina in recent years, 2022 was the hottest on record during the 3rd consecutive year of La Nina, the summer which saw 40C reached for the first time in the UK. So, we can’t reliably suggest that we won’t have record-breaking warmth this summer, because El Nino influence has gone. So, we need to look at other factors that may drive the season’s weather and could point towards a hot one, an average one or a cool one (unlikely!).

Anomalously warm sub-tropical North Atlantic

Another thing we are looking at, that could influence and shape the weather patterns over the North Atlantic and Europe through the second half of the summer season, more particularly August, is strengthening La Niña conditions coupled with sub-tropical North Atlantic Ocean temperatures which have been at record highs for over a year. There has been plenty of speculation that this will supercharge the Atlantic hurricane season – which starts in June but peaks from mid-August through to mid-October.

Enhanced Tropical Storm activity forecast over the tropical N Atlantic could influence mid-latitude weather patterns later in the summer.

But the anomalously warm sub-tropical North Atlantic Ocean and warm seas around the UK / N Europe can also more directly influence Europe’s weather through enhancing surface temperatures and also driving stable pressure patterns. A marine heatwave in the Atlantic Ocean in June and early July 2023 was thought to be a contributing factor in bringing the European heatwave in July last year. The marine heatwave likely affected atmospheric circulation patterns and warmed up the airmasses above them. With the anomalously warm ocean temperatures between northwest Africa and the Caribbean likely to persist into the summer, further marine heatwaves could trigger ‘heat domes’ to expand over NW Africa and in across southern Europe, perhaps expanding north to the UK at times too. This weather pattern allows a warm air mass to build up under a high-pressure system, creating stable and dry conditions.

1995 best match to this May out of all the analog years with strong El Nino to La Nina transition for N Atlantic SSTs in May. After a shaky start to June with cool and cloudy conditions in the first half of the month, summer 1995 turned out to be hot and dry:

Soil moisture

Dry soils encourage anomalously high temperatures during the summer months, because when the ground is wet the sun’s heat is mostly taken up by evaporating the moisture rather than going into heating the ground up. Whereas dry ground readily absorbs heat and radiates it back to the air above.

Across much of Iberia, away from the north, the ground is already dry following a dry winter. However, the rest of western and central Europe has moist soils, locally excessively wet and this looks set to continue into early June.

Iberia with the dry ground may see extreme heat easily build again this summer, if heat domes build in from NW Africa, this potential increased by anomalously warm SSTs over the North Atlantic to the west of NW Africa. Some of this heat may push north towards the UK, though wet soils over France following a very wet spring may moderate the heat that builds here compared to the ground being dry. So, we may see extreme heat confined to SW Europe, so perhaps more likely we don’t see a repeat of the July 2022 record-breaking heatwave.

Other drivers affecting mid-latitude weather patterns that affect UK and Europe in summer.

Several studies have shown that Rossby waves emanating out of the tropics can influence the extra tropics in summer that bring warm temperatures to the UK. Also, it has been suggested that changes in sea ice can also affect midlatitude summer circulation patterns, including over the North Atlantic/European sector. Sea ice anomalies in the spring season prior to hot summers show higher sea ice than normal in the Sea of Okhotsk and lower sea ice than normal in the Barents/Kara seas. Recent analysis shows sea ice in the Sea of Okhotsk is well-below normal, while in Barents/Kara seas is slightly below normal.

The average Arctic Sea ice extent for April 2024 was 14.12 million square kilometres (5.45 million square miles), placing it sixteenth lowest in the passive microwave satellite record (Figure 1a and 1b). As of the beginning of May, extent is well below average in the Sea of Okhotsk, and slightly below average in the Bering and Barents Seas and off the coast of Labrador. Ice is near the average position along the eastern coast of Greenland.

Source: National Snow & Ice Data Center

It is likely that influences from different remote regions are active or dominant in different years. The influence from the tropics has been demonstrated to be linked to tropical precipitation anomalies via extratropical wave trains. Circulation anomalies associated with wet conditions in the Caribbean project particularly strongly onto the hot UK summer conditions, with a weaker signal from the tropical Pacific consistent with developing La Niña. There is a strong signal for wet conditions in the Caribbean through the summer from the seasonal models, which ties in with the predictions for a very active hurricane season, so this indicates an increased chance of hot weather in the UK this summer.

Madden Julian Oscillation (MJO)

The MJO is a large-scale coupling between atmospheric circulation and tropical convection. It differs from ENSO in being a travelling pattern across the warm tropical oceanic areas of the globe rather than the standing pattern of El Nino / La Nina over the tropical Pacific. It is characterised by an eastward progression of large regions of either enhanced or supressed tropical rainfall, this anomalous rainfall area is mostly evident over the western Indian Ocean, then the warm tropical western and central Pacific as the MJO progresses eastward. The MJO wave of enhanced or supressed tropical rainfall is usually less evident when it moves over the eastern tropical Pacific and tropical Atlantic.

The MJO affects the Indian and Monsoon, plays a role in the onset of ENSO events, has an impact on tropical cyclogenesis but also has more far-reaching impacts on northern hemisphere extratropical weather through Rossby Wave propagation. The Rossby Wave propagation depends on the longitude of where the enhanced convection associated with the MJO wave takes place. As the MJO has a significant impact on northern hemisphere weather patterns, including the North America, Atlantic and Europe, there are correlations that can be made between the 8 different phases of the MJO and the lagged impacts on the upper air patterns that might be expected over the North Atlantic and Europe based on composites of previous events.

The Madden-Julian Oscillation (MJO) is currently entering phase 4 in the Maritime Continent and is expected to continue into phase 5 toward the last few days of May and first week of June. It may briefly reach phase 6 in week 2 of June before collapsing into the Circle of Death sometime during that week. Phase 5 composite for June tends to promote low pressure over western Europe, so something we’ll consider.

Analogs

Years with a similar El Nino to La Nina transition: we can use previous years (analogs) where similar Oceanic Nino Index (ONI) decreases were noted between the previous winter and the upcoming summer. 1998 is a strong analog year: going from a strong El Nino to strong La Nina in the same year, the Nino last winter was very strong/strong. We can add 2016, 2010, 1995 for a drop from Nino to Nina in the same year too. We may use years prior to the 1990s too, but to problem is going too far back is that global warming has increasingly skewed atmospheric circulations, making analogs less useful. But prior to the 1990s, 1983 and 1973 are good analog years of a transition from strong El Nino to La Nina.

Analog years of strong El Nino to La Nina transition since 1970 & since 1990 for each month:

June

The June analog composite features a low height anomaly over southern Europe and high-pressure anomaly to the west of the UK and also over NW Russia. This is a shift further east of the 500mb pattern that has prevailed so far in May this year, with low height anomaly over SW Europe much of May further east over southern Europe, while ridging is more prominent close to the west of the UK. This suggests the west of UK drier but generally dry and settled for much of the month, though with low heights close to the south – perhaps risk of showers here. All the months, bar 1998, were generally dry and warm.

July

Low heights are prominent to the north of the UK and higher heights over or south of the UK over the analog years. This suggests flow often from the southwest, the north wetter, the south drier. Mixed outcomes for the analog years, 1973 and 1998 were cool and wet, 1983 and 1995 were hot and dry, 2010 and 2016 were warmer than average but drier in the southwest, while wetter in the NW.

August

The analog years composite features a strong signal for high heights (ridging close to western UK) with low heights over Scandinavia. August in 1973 was very warm and wet, 1983 and 1995 were hot and dry, 1998 warm and sunny in SE, wet and dull in the NW. 2010 cooler than average and wet in the south, while 2016 was very warm and sunny.

Summer Forecast Month by Month

June – unsettled & cool first half or two-thirds, but heat may arrive later in month

This month it may take a while for any real heat to arrive, with low pressure tending to limit progress north of heat that may build over the continent, but it could turn hotter late in the month.

There is reasonable confidence that high pressure will be located the west or southwest of the UK and low pressure close the north or northeast in the first 7-10 days. This suggest the flow will be from the west or sometimes northwest. Temperatures remaining around average, perhaps slightly below in the south, occasional spells of rain, but generally bright with showers at times.

For 10th-20th, confidence drops, but the greatest signal is for low pressure to be close by or over the UK and much of NW Europe. This suggests unsettled conditions predominating, with spells of rain or showers, perhaps heavy with thunder, but also some drier warm interludes. Temperatures around average,

For 20th-30th, there is no clear signal, so confidence on any outcome is low, potential for a spell of higher pressure but also could be lower pressure influence at times. Greatest chance in the month for more settled weather, becoming warmer than average, perhaps with a hot spell.

July – very warm or hot month, with high pressure often in charge

Overall signal from seasonal models (albeit not strong) and best analog years is for high pressure to be close to or over NW Europe and lower pressure towards Iceland. How far north and east high pressure influences uncertain and may fluctuate. But it appears high pressure may be a dominant feature of the weather bringing dry spells through much of the month, though Scotland could be wetter closer to lower pressure. Likely a very warm month overall, perhaps hot at times in the south. There will be occasions where all parts will be unsettled for a few days as breakdowns of the settled weather occur, but the exception rather than the rule we think.

Confidence is quite good concerning the temperatures being above average, perhaps 1-2C above. Rainfall below average for England and Wales, but perhaps above in Scotland, especially towards the northwest.

August – very warm month, unsettled & wetter towards second half

The evolution for the month of August, as would be expected two months away, is less clear than for June and July. The cross-model signal for August is for lower pressure towards Iceland and mid-North Atlantic and higher pressure over mainland Europe. With overall signal for pressure to not be as high as July over UK, perhaps less settled that July, with more chance of rain, but more humid too with the flow from the southwest or, as times, south. The expected active hurricane season may bring quite an unsettled second half to the month, as ex-tropical storms drift towards northwest Europe, but if they head towards Iceland, potential they could bring brief plumes of hot weather too. Heatwaves possible over mainland Europe appear greatest this month. Given the predominant wind direction from southwest or south, temperatures will be above average, but not as much as July, given greater chance of unsettled weather off the Atlantic, but still 0.5C-1C above the 30-year average. Rainfall could equally be average or above, but less likely to be below.

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