From extreme rainfall affecting southern Africa to chilly nights and the return of frost risk in the UK, the weather this week highlights just how varied conditions can be at this time of year.
Alongside South African rainfall and the day‑to‑day UK forecast, this week’s Deep Dive will also look back on what April delivered across the UK - a month that brought stark contrasts in rainfall, abundant sunshine for many, and temperatures that behaved very differently by day and by night.
Exceptional rainfall risk in South Africa
While large parts of central Africa are experiencing seasonal rainfall that is not out of the ordinary, attention is focused on South Africa, where exceptionally heavy rain is unfolding over a short period through Wednesday.
The situation involves a deep area of low pressure in the upper atmosphere, known as a cut‑off low, which is positioned close to southern Africa. At high levels, unusually low atmospheric pressure values indicate a very intense system. This feature draws warm, moisture‑laden air southwards, where it wraps back around the low. As this air is forced to rise, it produces prolonged and intense rainfall.
Current forecasts show rainfall totals widely reaching between 75 and 150 mm in around a day, with some locations exceeding this. Over several days, totals build into the hundreds of millimetres. To put this into context, average rainfall for May in some of the affected areas is around 40 mm, highlighting just how unusual and impactful this event is.
READ MORE: Week ahead: Unsettled start but warming towards the end of the week
Impacts enhanced by terrain and strong winds
The impacts are likely to be heightened by local geography. As moist air is forced upwards over higher terrain, rainfall intensity can increase further. This raises the risk of rapid runoff and flash flooding, with water moving quickly through river catchments.
Forecasts also suggest some rivers could experience flows associated with very long return periods, highlighting the potential severity of the flooding risk. Alongside the rainfall, strong winds are also expected. Gusts may reach levels capable of causing power disruptions and structural damage, compounding the overall hazards.
While conditions are expected to ease later in the week, the combination of heavy rain and strong winds makes this a particularly concerning period for the affected regions.
A quieter but cooler pattern for the UK
Closer to home, the weather across the UK is less dramatic but still noteworthy. The UK sits between areas of high-pressure to the west and low-pressure to the east, resulting in a changeable but not especially unsettled pattern.
There will be a mix of cloud and sunshine, with some outbreaks of rain at times, but nothing on the scale of widespread storms. Rainfall amounts are generally expected to be modest. However, colder air is moving south from the north, bringing a noticeable drop in temperatures.
This colder, clearer air is particularly important when considering night‑time conditions, as it increases the risk of frost in some areas, especially where skies clear and winds fall light.
Showers and a risk of thundery downpours
Although much of the UK will stay dry for long periods, there are areas where showers may develop. Parts of south‑west England, for example, may see convergence zones form, helping to trigger heavier showers. In a few places, these could become locally intense, with hail and thunder possible.
Elsewhere, showers may also develop across eastern parts of the UK, though many places will avoid them altogether. Overall, rainfall is expected to be fairly localised, with long dry spells for many.
Later in the week, low-pressure passing to the north‑west may bring wetter and windier conditions to parts of Scotland, but again these are not expected to be extreme.
READ MORE: May Bank Holiday weather extremes
Turning attention to the weekend
As the weekend approaches, forecast attention turns southwards. There are indications that warmer air could begin to move north from lower latitudes. With this change comes the potential for more widespread and heavier rainfall, possibly accompanied by thunderstorms, particularly later on Saturday and into Sunday.
This shift also signals a recovery in temperatures, following the cooler spell through midweek. While confidence in exact details decreases at this range, it highlights a more unsettled and warmer trend developing.
Frost in May: how unusual is it?
With colder air moving in, a common question arises at this time of year: how unusual is frost in May? The answer depends very much on location.
There are two key types of frost to consider. Air frost occurs when air temperature falls below freezing, while ground frost happens when the temperature at the surface drops below 0°C, even if the air temperature slightly above remains positive.
In May, ground frost is more common than air frost. At this stage of the year, the ground has not yet accumulated as much stored warmth as it does later in summer. This means that similar air temperatures can lead to colder ground conditions than they would in early autumn.
Data shows that on average many northern parts of the UK still experience multiple days of ground frost in a typical May, while southern and coastal areas see fewer due to the moderating influence of the sea.
Last frost dates vary widely
Looking at average dates of the last air frost highlights a clear north‑south pattern. In southern and coastal regions, the average last air frost tends to occur earlier in the spring, often before April ends. Further north, and over higher ground, the average last frost can extend well into May, and in a few places even into June.
This means that frost at this time of year is not especially unusual, particularly in rural and elevated areas. Sheltered valleys are also more prone, as colder air drains downhill and pools near the surface overnight.
Frost and a warming climate
Comparing more recent climate averages with earlier periods shows that the average date of the last spring frost has been shifting earlier in the year. This is consistent with a warming climate.
However, this does not remove the risk of late frosts altogether. In fact, they can become more problematic for gardeners and growers, as plants and blossoms may develop earlier in a warmer spring, leaving them more vulnerable when a late cold snap does occur.
READ MORE: Weekly weather quiz: Test your knowledge
A look back at April: dry, sunny and contrasting
April is likely to be remembered by many as a dry month, particularly across southern and eastern parts of the UK. While western Scotland and some north‑western areas saw wetter‑than‑average conditions, much of the southeast experienced notably low rainfall.
Several counties recorded one of their driest Aprils on record, highlighting the strong regional contrasts. When looking at the UK as a whole, April finished drier than average, despite a wetter start to the month.
Sunshine was another standout feature. UK‑wide, April was markedly sunnier than average, with some southern counties recording their sunniest April on record. This abundance of sunshine played a key role in temperature patterns.
Daytime warmth, cooler nights
April was warmer than average overall, but the warmth was not evenly distributed between day and night. Maximum temperatures were well above average, reflecting frequent sunshine. Minimum temperatures, however, were closer to average, due to clear nights allowing heat to escape.
This combination of sunny days and cool nights is typical of dry spring weather, and it explains why frost and large day‑to‑night temperature ranges can still occur, even during a warmer‑than‑average month.
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