By the weekend, large swaths of Britain are expected to tip into an official heat wave, with temperatures surging well above the seasonal average. Highs are forecast to reach 33 degrees Celsius (just over 91 degrees Fahrenheit) in some areas, prompting health alerts and reminding many that, yes, British summers can get serious.
The culprit? A high-pressure system parked over southern England, bringing calm and sunny days and allowing temperatures to steadily climb.
“Temperatures will then rise further towards the end of the week as winds turn more southerly and even warmer air over continental Europe will be drawn across the U.K.,” Tony Wisson, a deputy chief meteorologist at the Met Office, the national weather service, said on Wednesday. “We could very well see several places exceeding 30 degrees Celsius, which will be the highest temperature of the year so far.”
For people in hotter climates who are used to sweltering summers, those temperatures might not sound particularly extreme. Many parts of the United States are also expecting their first bout of unhealthy heat this week, with temperatures climbing above 90 degrees Fahrenheit, combined with high humidity.
But in Britain, where the average high for June hovers around 64 degrees Fahrenheit (18 degrees Celsius) and where air-conditioning is still uncommon in homes, the heat could feel uncomfortable and, for some, could pose a health risk.
The U.K. Health Security Agency issued a yellow-level heat health alert — the lowest of the three alert tiers — for much of England, including London, through Sunday.
A yellow alert means that heat may affect vulnerable groups, including those over the age of 65 or with pre-existing health conditions, such as respiratory and cardiovascular diseases.
“Our findings show that even moderate heat can result in serious health outcomes, especially for older adults,” said Dr. Agostinho Sousa, the head of extreme events and health protection at the agency. “It is therefore important that everyone takes sensible precautions while enjoying the sun.”
The heat is expected to build day by day. Wednesday could see highs around 82 Fahrenheit (28 degrees Celsius), climbing to 86 Fahrenheit (30 degrees Celsius) by Thursday. By Saturday, the high may hit over 91 degrees Fahrenheit (33 degrees Celsius) in officially qualifying the run of high temperatures as a heat wave in several regions.
Britain’s highest June temperature on record stands at just over 96 degrees Fahrenheit (nearly 36 degrees Celsius), set in Southampton in 1976. Current forecasts suggest that the record is unlikely to be broken this time.
“While the warmest temperatures are likely across London and the east of England,” Mr. Wisson said, “by Saturday heat wave thresholds could be reached across much of the Midlands, low-lying areas bordering the Peak District and even parts of east Wales.”
Very warm nights are also expected over the weekend. The Met Office said a few places were likely to experience a “tropical night,” when temperatures do not drop below 68 degrees Fahrenheit (20 degrees Celsius).
In the United States, the National Weather Service defines a heat wave loosely — “a period of abnormally hot weather generally lasting more than two days.” The Met Office, however, uses a more formal set of criteria: An official heat wave is declared when a location records at least three consecutive days of temperatures meeting or exceeding the local heat wave threshold. This varies from 77 degrees Fahrenheit (25 degrees Celsius) in the north and west, including Scotland and Northern Ireland, to 82 degrees Fahrenheit (28 degrees Celsius) in Greater London and the surrounding counties.
While the warm spell is above the June average maximum for Britain, meteorologists also point out that such temperatures have become more common in recent years.
“In the last 10 years since 2015 we have reached 30 Celsius at some point during the month of June pretty much every year,” Luke Miall, a meteorologist at the Met Office, said. “So, reaching 30 Celsius in June is not that uncommon anymore.” He noted that the exceptions were 2016 and 2021.
Relief may arrive early next week, as the high-pressure system begins to shift away. But Mr. Wisson said that temperatures could briefly rise even higher before they fall.
“Some forecast models even allow temperatures to rise into the mid-30s of Celsius by Monday,” he said. “While this is feasible, it is considered a lower-likelihood scenario.”
Nazaneen Ghaffar is a Times reporter on the Weather team.
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