Will St Swithin Be Right?

St Swithin's Day has arrived and the dry and very warm conditions which have characterised July so far across much of the country are continuing. After the first 14 days of the month the Central England Temperature (CET) is running at an exceptional 20.4°C, which is a remarkable 4.5°C above the 1961-1990 long-term average. So, will the St Swithin's Day weather lore hold true this year, or is a change on the horizon?

St Swithin's Day: Folklore vs. Physics

St Swithin's Day is a traditional British weather folklore festival celebrated annually on 15th July. According to the legend, whatever the weather is like on this day, whether persistent rain or blazing sunshine, will continue for the next 40 days and 40 nights.

In reality, meteorological records show no instances of a continuous 40-day streak of either rain or sunshine in the UK. However, there is arguably a grain of truth in the underlying idea: by mid-July, the UK's summer weather patterns often become "blocked", or stuck in a routine. The position of the jet stream frequently becomes relatively static at this time of year, meaning prolonged spells of similar weather are more likely than during the transitional seasons of spring or autumn.

Despite this historical context, we are better served by looking at what modern computer models suggest for the medium term. These models provide a far more objective guide, having no understanding of ancient weather lore. Instead they rely on complex atmospheric physics and, increasingly, artificial intelligence (AI). These AI systems are trained on decades of historical weather data, learning to recognise patterns and determine the most likely evolution from our current starting point.

Will the heat continue?

There is strong agreement among forecasting models that temperatures will dip in the short term. Cooler air is expected to filter down from the north, circulating around an area of high pressure positioned to the west of the UK. This shift means the highest temperatures will be pushed towards the west and south-west, but even there they are expected to remain below the strict heatwave thresholds defined by the UK Met Office.

However, there are indications that temperatures will rise again during the final third of July. There is some divergence between the various ensemble models regarding exactly how high temperatures will climb and how long the heat will last. (Note: An ensemble model works by running a weather simulation multiple times with slightly different starting conditions to calculate the probability of different outcomes. To learn more about how this works, see our guide: Why one weather forecast is never enough.)

Despite variations between the models, the background conditions are stacked in favour of high temperatures returning. The seas around the UK are currently warmer than average, and the ground is exceptionally parched. Dry soil heats up much faster than damp soil because the sun's energy goes directly into warming the air rather than evaporating surface water. Given these factors, it would not be a surprise to see the mercury breach the 30°C mark once again in southern and central areas.

Will it stay dry?

In the short term, there is high confidence across the forecast models that drier-than-average conditions will persist across most of the UK. There could be localised exceptions. For instance, isolated thunderstorms could be triggered in the south, delivering a sudden deluge to a few locations. However, the potential for widespread, meaningful rainfall appears very limited.

In the far north and north-west, there is a slightly greater chance of patchy rain or drizzle moving in from the Atlantic, but even here accumulated rainfall totals are expected to remain relatively small over the next 10 days.

Looking further ahead into late July and early August, the forecast becomes more speculative as the accuracy of computer models naturally decreases at longer ranges. However, there are growing signals that the stagnant weather pattern across the North Atlantic and western Europe may finally begin to break down, allowing atmospheric pressure to fall.

Lower pressure opens the door to weather fronts moving in from the Atlantic, which would significantly increase the chance of widespread rain. Notably, recent updates from the ECMWF (European Centre for Medium-Range Weather Forecasts) ensemble models are pointing towards "positive rain anomalies" by the end of July and into August. In simpler terms, this means the models are hinting at wetter-than-average conditions replacing our current dry spell.

Change is possible, but not certain

The weather over the past few months has been nothing short of remarkable. Both May and June delivered record-breaking heat, and July has followed suit, with a long succession of days on which temperatures have reached 30°C somewhere in the UK. Because this pattern of anomalous warmth and extreme dryness has become so entrenched, particularly across southern Britain, calling a definitive end to it is fraught with forecasting risk.

Until the computer models align, present a consistent signal, and maintain that signal as the lead time draws nearer, confidence in a major breakdown of the heatwave remains low. Meteorologists look for consistency over multiple model runs before issuing a firm forecast.

While a transition to a wetter, more mobile weather pattern would be a huge relief for farmers, gardeners and water companies, others may view the timing as unfortunate, given that it could coincide with the start of the main school summer holidays.

For now, the best approach is to keep up to date with the short-to-medium-range forecasts. Be aware that a significant change in the weather may well be brewing on the horizon, regardless of what today's St Swithin's Day sunshine or showers might suggest.