The Met Office has issued yellow weather warnings for a significant outbreak of heavy rain and thunderstorms sweeping across large parts of the UK from Wednesday through Thursday and into the weekend. The alerts cover east Wales, the Midlands, northern England, East Anglia and southern England, including London, marking one of the most widespread heavy rain events across southern and central regions this summer. The combination of hard, parched ground and intense downpours poses a serious flash flooding risk, with lightning, large hail and gusty winds also expected across the affected zones.
Southern England was first to experience the rain overnight on Wednesday, with Herstmonceux in East Sussex recording 21.4mm of rain in six hours. A yellow warning continued until 08:00 BST on Thursday in the south-east, with a wider warning area covering all of Wales and large swathes of England lasting until 22:00 BST. According to the Met Office's forecast update, heavy showers, thunderstorms and locally significant rainfall are expected, with a more organised area of thundery rain approaching southwestern areas.
For some areas that have seen barely any rain so far this August, the next few days could bring the wettest weather in many weeks. Brize Norton in Oxfordshire has recorded just 0.4mm of rain so far this month, less than 1% of what it would normally expect during August as a whole, while Kew Gardens recorded no rain at all in July and only 1.8mm so far in August. The unsettled spell follows a period of dry conditions in parts of the UK, which meteorologists have identified as a factor that can increase surface-water flooding risk when rain arrives suddenly.
When will the storms arrive and move?
The first band of thunderstorms developed as warm and humid air moved northwards from France, initially bringing a few showers to Cornwall and the West Country during Tuesday evening. Heavier downpours with thunder and lightning arrived later that night, with the first band of heavy showers reaching the southeast coast of England around midnight before crossing Greater London and moving northwards into the home counties and East Anglia.
The first yellow weather warning came into force at 03:00 BST on Wednesday and remained in effect until 22:00 BST. While not all locations within the warning area experienced thunderstorms, those that did faced flooding risk, with 20-30mm of rain possible in less than an hour. The hardest hit locations accumulated 40-50mm within two to three hours. During Wednesday morning, the storms spread further north into the Midlands and the Peak District.
On Thursday morning, downpours accompanied by lightning, hail and squally winds moved north across Wales, the Midlands and parts of eastern England, arriving in northern England by the early afternoon. As this initial line of downpours cleared northwards the skies brightened and sunny spells developed, but further scattered showers that could again be very heavy and thundery remained possible. Some rain arrived in Northern Ireland and Scotland later in the day, although these northern areas of the UK avoided the very wettest weather.
What rainfall totals are expected?
Within the Met Office yellow warning area, 20-30mm of rain could fall in just a few hours, with up to 50mm over a 12-hour period. The wettest areas could potentially see 75mm of rain, accompanied by large hail, lightning and gusty winds. Hills exposed to the easterly wind could possibly see 75mm. Lightning may be frequent during the heaviest downpours, and hail of around 1-2cm could fall in the most intense cells.
The weather pattern is linked to low pressure moving in from the southwest, bringing an unsettled period with heavy downpours and thunderstorms likely each day through the period. Northern Ireland will also see some showers but these are likely to be less intense with less thunder. Many parts of Scotland will remain drier with sunny spells, although some showers are likely in the southwest.
Why are thunderstorms so difficult to forecast?
Thunderstorms present a significant forecasting challenge because they can develop very quickly and operate on a relatively small spatial scale. Tiny variations in temperature, humidity, wind and atmospheric instability can determine whether a storm forms at all or the precise direction it will travel. Forecast models can often identify the risk of thunderstorms across a wider area, but pinpointing exactly where and when an individual storm will strike remains extremely difficult. Even a difference of a few kilometres can mean the distinction between a torrential downpour with thunder and lightning and an area that remains completely dry.
Forecasters know that the conditions are ripe for storms to form, with warm humid air across the UK interacting with a zone of colder air pushing in from the west, higher in the atmosphere. This generates low pressure and instability. However, the computer weather models that drive weather apps will always struggle to pinpoint exactly where and when the biggest storms will spring up. It is a bit like heating popcorn kernels in a pan: you cannot know precisely which kernels will pop and in which order, while some kernels may not pop at all. Some of the storms will also occur at night, which may mean they do not show up in the daily weather icons.
What are the expected impacts?
Where the rain does fall, flash flooding is a real concern. This will be especially problematic where the ground is hard after the prolonged dry spell, making it difficult for the water to soak in. Hazardous driving conditions are expected with a chance that roads will be closed and communities cut off. Large hail and frequent lightning are also possible during the heaviest downpours, with lightning strikes potentially leading to power cuts.
The wet weather will be welcome news for the large areas of England and Wales that remain in drought, however there is increased risk of flash flooding where the ground is very dry and hard. Not enough rain will fall to alleviate the recent dry spell, with experts warning that months of persistent wet weather is needed to replenish water supplies.
What about the weekend and major events?
The coming weekend brings a number of festivals and high-profile events. Reading and Leeds Festivals both run from Thursday until Sunday, as does Creamfields in Cheshire. Manchester Pride takes place from Friday until Bank Holiday Monday, while Saturday marks the start of Notting Hill Carnival in London, which also continues until Monday.
The weather looks decidedly mixed for these events, with a fresher feel, brisk winds—especially in the south—and the likelihood of rain at times. A zone of wet weather will move eastwards across England and Wales on Saturday, although drier and brighter conditions may spread from the west later. Northern Ireland and Scotland will be a little drier with some sunny spells, but there is still the chance of a few showers.
More heavy showers will move eastwards on Sunday, this time affecting most parts of the UK. Some of these could be heavy and thundery, with wind gusts touching 40-45mph in the far south-west of England later. Temperatures will tend to range from 16°C in northern Scotland to 23°C in southern England, which is about normal for late August. It will also feel rather humid for many.
Low pressure remains in charge on Monday, meaning strong winds and further showers, perhaps heaviest across the north of the UK. Spells of rain are set to continue over the weekend and beyond, accompanied by windy conditions in the south. There is no sign of a return to the dry and hot weather that has dominated this summer.
What happens next?
According to weather forecasts, Friday is flagged for further heavy rain and thunderstorms across the UK, with the risk extending into the bank holiday weekend. The Met Office has issued a separate active thunderstorm warning for Northern Ireland running from 11:00 on Thursday to 06:00 on Friday. The current Thursday thunderstorm warning is scheduled to end at 22:00 BST on 27 August.
Residents in affected areas should monitor official weather alerts closely and prepare for potential disruption from flooding and severe weather. It is worth keeping an eye on the hourly weather symbols, as well as the percentage chances of rain, if making outdoor plans. The arrival of this significant rainfall system marks a notable shift from the recent dry spell that has affected parts of the country, though the intensity and speed of the downpours mean that the ground's reduced ability to absorb water quickly will remain a critical concern for flood risk management.
