The start of the month will see an increasingly unsettled theme develop, with a disrupting upper trough over NW Europe week 1 leading to a slow-moving upper low parked close to UK initially before sinking southwest towards SW Europe or just to the west. Showers or longer spells of rain likely in the first 7 days -- more particularly for England and Wales, with Scotland drier and sunnier, temperatures perhaps above average for all. There is increasing model ensemble consensus for the upper low near SW Europe to eventually merge with more large scale upper troughing digging south from Greenland / far North Atlantic around 10-12th, with a more zonal west to east flow establishing, with the jet stream tracking close to southern UK. Upper trough then increasingly dominating the far north Atlantic close to western UK mid and perhaps well into later September, as the polar vortex strengthens from its summer hiatus and expends cold air aloft further south over the N Atlantic. This would mean more widespread changeable conditions, with spells of wet and windy weather, especially toward the north and west, but sometimes for all parts. This may be enhanced by the remnants of ex-hurricanes or tropical cyclones -- with potential for one or two named-storms later in the month. Temperatures mid to late month maintaining slightly above average.
Temperatures overall for the month likely to be slightly above the long-term 1981-2010 average, Rainfall likely to be around average.
There will be spells of unsettled weather at times, perhaps wet and windy, particularly early on in the month. But there are indications of a greater probability, for now, for blocking high pressure to build sometime through October -- bringing more settled and fine weather for one or perhaps two weeks for large parts of the country, possibly bringing an Indian Summer, though any warmth will depend on how any blocking anticyclone positions. Generally temperatures above average with some warm days but also some seasonally cool to chilly nights. Perhaps a more unsettled end to the month, as troughing / low pressure returns from the west.
Temperatures likely to be above the 1981-2010 long-term average. Rainfall more likely to be below average for the whole month, though southern England could return average with higher pressure towards the north of the UK in the means.
As polar regions continue to cool and the polar vortex strengthens and expands south, expect unsettled spells, perhaps with occasional Atlantic storminess to affect the UK, with, wet and windy weather at times in the month, but also there is an indication for a tendency for high pressure to build to the west at times, as troughing / low pressure retreats to the east or northeast, bringing cooler northerly or northwesterly flows with a risk of frost and wintriness to higher ground later in the month. But we think there will be one or two vigorous Atlantic storms during the month moving west to east.
Temperatures likely to return close to the 1981-2010 long term average for the whole month, rainfall around average too.
Continue below for the in depth forecast and explanation of the factors which are expected to influence the weather this Autumn.
A summary of the Netweather forecast for summer 2024 was a follows:
How it panned out:
June - a cool month overall -- particularly the first two thirds. With a CET of 14.0C it was the coldest since 2015. May was warmer than June for the first time since the CET began in 1659. For the UK, the average temperature was 0.4C below average. There was then a short-lived hot spell - with 30.0 C at Chertsey(Surrey) and Heathrow on the 25th, and 30.5 at Wisley (Surrey) on the 26th. A fairly dry month, with 71% of the expected rainfall across the UK around and half for England and Wales.
July - after a cool and unsettled first half there was a brief hot spell midmonth, with 31.9 C at St James Park (London) recorded on the 19th. There was another brief heatwave at the end of the month, with 32.0 recorded at Kew Gardens and Heathrow on the 30th. Despite the first half of the month being the coldest first half since 2004, the heat in the second half meant that overall the month was close to average. The mean temperature was in fact beneath the reference average for 1991 to 2020, but above that of 1961 to 1990.
August - A brief hot spell with the 12th seeing 34.8C recorded at Cambridge, the highest temperature of the year (so far). Since then, the weather has been changeable, with low pressure generally in charge close to the northwest, bringing well-above average rainfall to the north and northwest, with Scotland seeing 151% of the August average rainfall. Though southern and eastern England has been drier than average. This August's UK average temperature likely to end up very slightly above the average, by around +0.4C.
The Met Office 3 month outlook went for only 5% that the summer would be cool, when their own provisional stats that they published Friday 30th suggest it was cooler than average.
It's hard every year to contemplate that summer is nearly over and that we are on the downhill slope towards winter. But meteorological autumn is already upon us. In a week's time the first autumn month of September will start. Sometimes, though, September can be an extension of summer, as we saw last year. So can we expect late summer weather and how wet and cold will it be later in the autumn? In this autumn forecast we tell you what we already know about autumn 2024.
There are many factors, conditions in the atmosphere and ocean, around the globe, that affect our weather several months into the future, and it is always difficult to know which factors will be the controlling ones. The calculated long-term forecasts must therefore always be interpreted as a best guess for the weather in the coming time. In addition, the long-term meteorologist also assesses how other conditions around the globe can affect the weather at our latitudes. It is not always that the forecasts catch these signals in time.
To get an idea of what kind of autumn it will be, we will focus on the weather model seasonal forecasts, the developing La Nina, Atlantic SSTs, the Madden Julian Oscillation, N Atlantic hurricane season and analog years with similar transition into a weak La Nina and with similar N Atlantic SSTs.
During this autumn, the global weather phenomenon La Niña is forecast to gradually make its comeback following a year or so of El Nino then a transition to neutral ENSO conditions since spring. The influence of La Nina extends beyond the areas around the equator, but the influence on the weather during autumn in our country is limited. There is a connection between La Niña and the weather in our spring. In a La Niña autumn, more hurricanes are often active in the Atlantic Ocean, which makes the weather forecast for the UK extra uncertain. We may have to deal with remnants reaching Europe more often. Depending on the track of these remnants, this gives us either warm and sunny weather if they track way north over the Atlantic or stormy and cool weather if they track directly at UK. However, La Nina isn't imminent, but expected to develop this autumn, so its effects on global weather patterns may not manifest until late autumn at the earliest.
Although La Nina will likely not be as strong as the peak of the last event, which spanned three years, it may still be strong enough to impact both the ocean and the atmosphere.
Seasonal forecasts are correct about 60% of the time and the forecast for one month ahead is of course more reliable than for the third month. It is also important to realise that seasonal models purely outline a rough trend for monthly average air pressure, temperature and precipitation. Even an averagely warm month can easily have a cool week and vice versa.
Monthly and seasonal forecasts for autumn and spring tend to be reliable less often than those for summer and winter. This is because the predictability of the atmosphere is smaller in the transitional seasons, where cold and heat often alternate. Ex-hurricanes in the autumn can also make the forecast extra uncertain.
The most recent ECMWF weeklies forecast which goes out 46 days ahead, suggests lower pressure towards the south and higher pressure to towards the north during the first half of September, so more unsettled conditions towards the south drier in the north. As September progresses, pressure pattern reverses to higher pressure to the south and lower to the north, albeit a weaker signal. So more changeable conditions for all but with rainfall greater towards the north and west. Temperatures generally around the average.
EC46 for 500 hPa over Europe/N Atlantic for next 3 weeks:
With regards to this model's current seasonal output, this may change in the September update, but we will only use the predictions for October and November, as with other models, as the EC weeklies can give a more reliable outlook.
October - features lower heights near Iceland, higher heights over continental Europe, indicating a typical changeable SW flow leading slight above average temps and average rainfall (not shown). November has the mean trough to the NE of UK, so perhaps a greater chance of average temperatures, perhaps with cool/cold spells in the mix, average rainfall, more to the east.
For October, mean trough over Scandinavia, but slightly above average temperatures and average rainfall. November -- lower heights towards arctic circle to the north, higher heights over UK and much of Europe -- weak heights tendency; temperatures well-above average in the south to slightly above in the north. Rainfall average.
October -- tendency for higher heights over western Europe, lower towards NE and E Europe; temperatures slightly above average; rainfall (not shown) around average. November -- notable low height / trough mean anomaly over Scandinavia, higher heights over UK/W Europe; temperatures slightly above average; rainfall average.
Drawing conclusions from all three models above, October and November have a greater chance of temperatures being slightly above average, though ECMWF does lean towards an average November, perhaps with cool/cold episodes given the lower heights to the northeast signal. UKMET and NCEP more dominated by higher heights signal, both months. Historically, using the analog years with onset of weak La Nina in autumn, has favoured higher pressure / heights both these months, but the sample size is small, so not a strong signal, especially given other drivers -- as will be discussed.
But as always, it may not be so clear-cut, with other drivers also at play -- such as the MJO, ex-tropical storms and Atlantic SSTs, with more short-term variations likely as some other drivers of our weather will also play their part. The Atlantic hurricane season, after an active start, has been quiet recently, but is showing signs of awakening, so may affect patterns over the North Atlantic and in turn, our weather, through September into early October, which along with a developing weak La Nina. Also, it is worth taking into consideration the impacts of the MJO (Madden Julian Oscillation), who's cycles can affect weather patterns across the globe too, but can only be predicted up to a month ahead at the most. I will also take-a-look at Numerical Weather Prediction model seasonal forecasts, analogs of previous autumn months with similar global patterns, particularly with La Nina conditions present.
Sea surface temperatures have been record warm across the North Atlantic Ocean since March 2023. Temperatures have been so extreme that much of the North Atlantic has been experiencing a marine heatwave. This particular event has already brought devastating heat stress to nearly all of the Atlantic's tropical reef areas and contributed to a very active 2023 Atlantic hurricane season. The very warm sub-tropical Atlantic is also been forecast by the US NOAA to fuel a very active hurricane season this year. This could mean there is a greater chance of remnants of hurricanes or tropical storms impacting UK weather in early to mid-Autumn, both directly with unsettled weather or indirectly by boosting high pressure systems to build north if the remnants track to the north of the UK.
However, further north, a cool anomaly has developed west and northwest of the UK covering the far north Atlantic -- this may lead to a tendency to bring more average temperatures this autumn if the flow is off the Atlantic rather than anything excessively warm, while also perhaps influencing the storm track and pressure patterns over Europe. We will later look at close matches of years with late August SSTs that had a similar SST anomaly pattern.
The hurricane season got off to an early and violent start with Hurricane Beryl, the earliest category-5 Atlantic hurricane on record. In the Atlantic basin, a typical season will yield 14 named storms, of which seven become hurricanes and three become major hurricanes. However, the US NOAA forecast was that atmospheric and oceanic conditions would support an above-normal 2024 Atlantic hurricane season, with a 90% probability of this result, with this season only having a 10% chance of a near-normal season and a negligible chance of a below-normal season.
However, since the last named storm Hurricane Ernesto bothered Bermuda, it's been an oddly quiet couple of weeks, things are starting to change in what's been long projected to be a frenzied Atlantic hurricane season. However, long-range models are becoming increasingly confident that the deep Atlantic tropics will soon spawn a system worth tracking, and there's plenty of reason to believe that a busy September and/or October may still lie ahead -- as per NOAA predictions
The Atlantic ocean basin is expected to be remarkably active due to warmer-than-average sea surface temperatures in the tropical Atlantic Ocean and Caribbean Sea; reduced vertical wind shear due to La Nina; weaker tropical Atlantic trade winds and an enhanced west African monsoon. These conditions are expected to continue into the autumn.
The remnants of these hurricanes of tropical storms or hurricanes can alter the path of the jet stream, causing high pressure and warmth to build north over western Europe -- if the remnants track north of the UK. But if these remnants are directed towards NW Europe, they could bring unsettled, wet and windy weather. We expect that the reawakening of the hurricane season soon over the N Atlantic will likely lead to a wetter but warm September and perhaps well into October too.
Although the MJO is a wave of enhanced convection that circum-navigates the global tropics, it does impact the upper patterns at higher latitudes of the northern hemisphere, so can be a useful tool to use for predictions for 3-4 weeks ahead, but no more than that really.
The Madden--Julian Oscillation (MJO) is currently moving through the Maritime Continent through phase 4 and likely then 5, before then carrying on into in the western Pacific (Phase 6) into the 2nd half of September. There is usually a lag of around 10 days for each phase to impact weather patterns, but composites of phase 4 and 5 in neutral ENSO tend to promote low pressure south of the UK, which is what we are seeing in the models for the next few weeks. Beyond that and MJO forecasts become more unreliable, but there is a continued signal for lower pressure to the south and high pressure to the north if the MJO moves into the western Pacific as currently forecast.
Looking through ENSO ONI (Oceanic Nino Index) records, there are 5 years since the start of the 1980s (bearing in mind the global climate has changed rapidly since then) that had a transition into a weak La Nina by and during the autumn. These were 1983, 1995, 2005, 2016 and 2020.
September analog -- troughing dominates to the northwest with higher heights over mainland Europe, temps average in the north to above in the south
October analog -- above average heights over NE Europe, lower heights west of UK, slightly above average temps -- perhaps due to dominance of flows from the south.
November analog -- low heights / troughing over NE Europe -- higher heights anomaly close to west UK. Average temperatures. Significant cold anomaly over SE Europe.
Analogs conclusion: a mixed picture comparing ENSO and late August N Atlantic SST analog composites, especially for September and November, but what stands out is that October is dominated by high heights in both ENSO and late August SST composites. Heavily leaning to the SST analog years for September. Other 2 months: October looks most likely to see high pressure at times; November could be unsettled and greater chance of cool/cold -- with trough over NE / E Europe.
We've looked at the weak La Nina autumn analog year composites, but as already alluded to, the influence of La Nina on the weather during autumn in UK is limited. So, it is worth looking at the potential influence on N Atlantic SSTs -- which can influence pressure patterns over adjacent Europe.
This is the SST anomalies for the N Atlantic for 30th August 2024 -- there is a notable cold anomaly to the west and northwest of the UK all the way to Greenland and a notable warm anomaly from the NW Atlantic / New Foundland areas SE to the Azores area. Generally the sub tropical area is anomalously warm too.
Going through the SST anomalies that best match this pattern on 30th August back to 1985: the years 1989, 1994, 2009, 2015 stand out. Particularly 2015. But not all these years had the same ENSO status starting these autumns. However, this is the composites for all three months. Note that September closely matches the 500 hPa we are expecting in at least first few weeks!
September -- low heights to the south, higher heights close to the NW.
October -- notably a strong upper high anomaly over NW Europe, with low heights over SE Europe.
November -- low heights/troughing close to the west and over E and NE Europe, higher heights over southern Europe.
Analogs conclusion: a mixed picture comparing ENSO and late August N Atlantic SST analog composites, especially for September and November, but what stands out is that October is dominated by high heights in both ENSO and late August SST composites. Heavily leaning to the SST analog years for September. Other 2 months: October looks most likely to see high pressure at times; November could be unsettled and greater chance of cool/cold -- with trough over NE / E Europe.
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