In the second half of September 2026, Belgium is heading for a spell of unusually warm and dry weather next week, with temperatures potentially reaching 25°C. But when Oscar, a Denver resident, was told an Indian summer was on the way, he did not recognise the expression.
Anyone who has already packed away their summer clothes may have done so a little too soon. According to the European weather model, Belgium is heading for a distinctly late-summer spell as a high-pressure area moves from Spain towards France and eventually the Benelux.
“A high-pressure area typically means more stable weather. Temperatures will easily rise above 20 degrees,” Nicolas Roose, a weather analyst at MeteoSupport, told Het Nieuwsblad.
For central Belgium, forecasts indicate temperatures of between 20°C and 23°C next week, with some model calculations pointing towards 25°C from Tuesday. The warm spell is expected to last at least until Saturday 26 September.
That would put temperatures around 2°C to 3°C above normal for the time of year. Some individual model runs even show 30°C or 31°C, although these are considered exceptions and are not currently regarded as a reliable forecast.
Rain should remain limited. “It will be more of a dry week. Perhaps we will get a local shower here and there,” Roose said.
What is an Indian summer?
An Indian summer is generally understood as a spell of unusually warm, dry and calm weather during autumn in temperate parts of the Northern Hemisphere. There is, however, a technical catch: traditional definitions say the warm spell should occur after the first killing frost.
That distinction matters in the case of Oscar, who comes from Denver, Colorado. Indian summers are a recognised part of Colorado’s autumn weather, traditionally referring to a warm, dry spell following the first killing frost. In Denver, the first frost typically arrives around 7 October, making late September and early October a common period for such a transition.
So why did Oscar not know the term? The answer may be that its use varies considerably between regions and even between speakers of English.
In Belgium, Indian summer is often used more loosely for a warm spell in autumn, even when there has been no frost. “Late-summer weather” or ‘nazomer‘ is therefore a more precise description of what Belgium is expecting next week.
A regular September phenomenon
Warm spells at this time of year are nothing new in Belgium. MeteoBelgique identifies 17 to 25 September as a traditional period for a return of warm, settled weather, with temperatures around 25°C.
According to its analysis, this type of spell occurs roughly once every three years during that particular period. If the period is extended from mid-September to mid-October, similar conditions occur in roughly two years out of three.
Belgium has also experienced considerably more extreme September heat. On 20 September 2003, temperatures exceeded 30°C at several weather stations, reaching 32.2°C in Angleur and 32.0°C in Wasmuel. On 19 September 1961, temperatures reached 31.3°C at Kleine-Brogel, 30.4°C at Zaventem and 30.0°C at Koksijde.
The forecast for next week is therefore unusually warm for late September, but far from unprecedented.
From North America to Europe
The expression Indian summer originated in North America and was already in use by the late 18th century. One of the earliest known references, from around 1778, described a short spell of mild weather between autumn’s first frost and the onset of winter.
The term crossed the Atlantic in the 19th century and became established in British English. The United Kingdom Met Office‘s 1916 glossary defined an Indian summer as a “warm, calm spell of weather occurring in autumn, especially in October and November”.
Europe also has its own names for the phenomenon. In Germany, Austria, and Switzerland it is known as ‘Altweibersommer‘, while several other European countries have names referring to saints’ days, including St Martin’s summer in parts of Western and Southern Europe.
Not caused by hurricanes this time
Indian summers in Western Europe can have different causes. Roose says they are often associated with former Atlantic hurricanes or other ex-tropical systems, which can help pull warm air northwards from Portugal and Spain.
“That is not the setting for next week,” he said. Instead, Belgium’s warm spell is expected to result from the proximity of the high-pressure area.
The warm weather is also not entirely unexpected. Long-range forecasts had already pointed towards a warmer summer, while signals in August suggested that September could also turn out relatively warm.
For now, the forecast is simple: temperatures in the low twenties, potentially reaching 25°C, little rain and several days that should feel more like summer than autumn.
