Met Office deep dive: Autumn’s stormy spell and tropical influences

This week’s Met Office deep dive explores a complex and dynamic weather pattern. Drawing on the latest satellite imagery, model data, and meteorological expertise, we’ll examine the interplay between Autumn’s advancing low pressure systems, the Jetstream, and the rare influence of tropical cyclones.

Setting the scene: Calm beginnings and a brewing storm

The week began with a relatively calm spell for much of the UK, particularly across the southern half of the country. High pressure dominated, bringing light winds, clear skies, and the typical signs of autumn: chilly nights and foggy mornings. In Exeter and elsewhere, dense patches of fog were a regular feature, a result of moist ground following a wet September and the longer autumn nights.

Meanwhile, further north, the weather was more unsettled. Proximity to the Jetstream and low-pressure systems meant cloudier skies and a greater risk of rain. This north-south split is a classic autumn pattern, with the Jetstream acting as a boundary between settled and unsettled conditions.

Unsettled weather is on the way later this week - here's an overview ⤵️ pic.twitter.com/3BnrNp7laS

— Met Office (@metoffice) September 30, 2025

The satellite perspective: Tracking the storm

Over recent days, the storm track has been pushing low pressure systems towards Iceland, guided by the position of the Jetstream. However, a notable feature has been a small swirl crossing Spain, delivering very heavy rain to eastern regions, including Valencia and the Balearic Islands. This was associated with the remnants of tropical cyclone Gabrielle, which dropped up to 200 millimetres of rain in some areas.

As the week progressed, the Jetstream began to intensify and shift southwards, setting the stage for a significant change in the UK’s weather. This shift would ultimately help spin up a deep area of low pressure, the main feature expected to dominate the weather narrative into the weekend.

The Jetstream’s role: From calm to chaos

The Jetstream is a fast-moving ribbon of air high in the atmosphere, and its position and strength are crucial in determining the UK’s weather. Early in the week, the Jetstream allowed high pressure to settle over southern Britain, but as it dived further south and intensified, it began to drive more active weather systems towards the UK.

By Saturday, the Jetstream had helped to spin up a deep low pressure system. This system, and its interaction with the Jetstream, would become the focal point for heavy rain and strong winds, particularly across western Scotland.

Building rainfall: The approach of the first low

The first significant weather event of the week was the approach of a low-pressure system on Thursday night and into Friday. As the Jetstream intensified and dipped south, it spun up this low, which was forecast to bring heavy rain to western Scotland. The combination of active weather fronts and orographic uplift (where moist air is forced up over hills and mountains) meant that rainfall totals were expected to build rapidly.

READ MORE: Hurricanes, typhoons and tornadoes: What’s the difference?

Model data suggested that some areas could see 100 to 150 millimetres of rain, with isolated spots reaching 200 millimetres, especially over the highest ground. This persistent rainfall over 36 hours raised concerns about flooding and travel disruption, prompting the Met Office to issue a yellow warning for western Scotland, valid until Friday morning.

The north-south split: Autumn’s two faces

While western Scotland braced for heavy rain and strong winds, much of the south and east of the UK enjoyed a different side of autumn. Here, high pressure brought dry, fine weather, with temperatures around or slightly above average. Mist and fog were the main hazards, but otherwise, it was a period of pleasant autumn sunshine.

This stark contrast between north and south is a hallmark of autumn in the UK, driven by the position of the Jetstream and the sequence of low and high-pressure systems.

The tropical connection: Humberto and Imelda

A particularly intriguing aspect of this week’s weather was the influence of two tropical systems: Humberto and Imelda. At the time of writing, Imelda was a tropical cyclone and Humberto had recently been a category five hurricane, the second such storm of the season, a rare occurrence not seen in back-to-back years since 1932.

These two systems were unusually close together in the Atlantic, less than 600 miles apart, placing them among the top ten closest pairs of Atlantic storms in the past 60 years. Their proximity raised the possibility of interaction, a phenomenon known as the Fujiwara effect, where two cyclones begin to rotate around a common centre.

The Fujiwara effect: A tropical dance

The Fujiwara effect, named after Japanese meteorologist Sakuhei Fujiwara, describes the interaction between two cyclones within about 800 to 850 miles of each other. When this happens, the storms can begin to orbit one another, sometimes merging or one absorbing the other. This effect was illustrated with examples from past storms, including Iris and Karen in 1995, and more recently, Siroja and Odette in 2021.

In the current scenario, Humberto and Imelda were not expected to perform a classic Fujiwara “tango”, but their proximity was already causing some interaction. This added complexity and uncertainty to the forecast, as computer models can struggle to predict the precise outcomes of such rare events.

READ MORE: When is a hurricane not a hurricane?

The impact on the Jetstream and UK weather

As Humberto and Imelda drifted northwards, they began to influence the Jetstream. Normally, the Jetstream steers low-pressure systems, but powerful tropical cyclones can also affect the jet, creating kinks and troughs that alter the downstream weather pattern.

In this case, Humberto’s transition from a hurricane to an extra-tropical system was expected to create a ripple in the Jetstream, which in turn would affect the development and track of the next low-pressure system approaching the UK. Even subtle shifts in the Jetstream’s position could move the heaviest rainfall by 50 miles or more, with significant consequences for which areas would be most affected.

A number of weather warnings have been issued. A yellow rain warning is currently in place for heavy rain which is continuing to affect western Scotland throughout today and tomorrow (Thursday) allowing large rainfall totals to build up. The heaviest rain will be over hills and mountains. A further 30-50 mm of rain is expected to fall, with as much as 60-80 mm over west-facing mountains from late Wednesday through to Thursday night.

Ensemble forecasting: Embracing uncertainty

Given the complexity of the situation, with multiple interacting systems and a highly dynamic Jetstream, the Met Office relied on ensemble forecasting to assess the range of possible outcomes. Rather than depending on a single deterministic model run, ensemble forecasts involve running the model multiple times with slightly different initial conditions, producing a range of scenarios.

This approach helps to quantify the uncertainty in the forecast and provides a better sense of the probability of different outcomes. For example, the ensemble forecast from the European model, run 50 times, showed a variety of possible tracks for the low-pressure system, with most scenarios taking it across Scotland but some further south or north.

Analysing the scenarios: Wind and rain risks

The ensemble and deterministic model outputs were used to assess the risks of strong winds and heavy rain. The most likely scenario was for the low to track across Scotland, with the strongest winds on its southern flank, affecting Northern Ireland, Northern England, North Wales. In this scenario, the Northern Isles might escape the worst winds, which would be concentrated in the southwestern quadrant of the low.

Alternative scenarios included the low tracking further south, bringing damaging gusts to Northern England, Wales, and Northern Ireland, or drifting further north, affecting the north of the Great Glen. Each scenario carried different implications for wind and rainfall distribution, and the ensemble approach allowed forecasters to communicate the range of possibilities and the associated uncertainties.

Another feature to watch for was line convection, where a narrow but intense band of rain and gusty winds develops along the front of the system. This can lead to sudden, torrential downpours and rapid increases in wind speed, adding to the hazards associated with the storm.

READ MORE: A look back at the 2024/25 storm season

The combination of persistent rain from the initial low and the potential for further heavy rain from the weekend storm raised concerns about flooding, particularly in western Scotland. Even though these areas are accustomed to heavy rainfall, the accumulation over 36 hours, followed by additional rain, could overwhelm rivers and drainage systems.

The Met Office’s yellow warning highlighted the risk of travel disruption and localised flooding, urging people to stay up to date with the latest forecasts and to take precautions if travelling in affected areas.

The interplay between the Jetstream, low pressure systems, and rare tropical influences creates a challenging forecasting environment, with significant uncertainty about the exact track and intensity of the approaching storm.

Ultimatley the deep area of low-pressure has today been named Storm Amy by the Met Office and is the first named storm of the 2025/26 season.

Storm Amy is expected to develop rapidly as it approaches the UK on Friday 3 October, bringing very strong winds and heavy rain to many northern and western areas. The storm’s evolution is being closely monitored, with its development influenced by the remnants of Hurricanes Humberto and Imelda over the tropical Atlantic. These systems have accelerated the jet stream, contributing to the formation and intensification of Storm Amy.

By using ensemble forecasting, advanced visualisation tools, and the expertise of meteorologists, the Met Office provides the best possible guidance to help people prepare for whatever the weather may bring.

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