'It's chilly — where's my blanket?' Fall feeling won't last: heat wave not over yet

The extreme heat that pushed temperatures in the Greater Seoul area close to 40 degrees Celsius last week eased over the weekend as cold, dry air moved in — but forecasters warn the respite will be brief, with the heat expected to intensify again later this week.

Korea Meteorological Administration automated weather station data show a sensor in Nowon-gu recorded 40.2 C at 3:28 p.m. Friday — the first time any monitoring point in Seoul has logged a temperature above 40 C since Aug. 1, 2018. The official Seoul temperature, measured at the KMA observatory in Jongno-gu, also climbed to 38.0 C that day.

The picture changed over the weekend. The nationwide high Sunday fell to around 36 C, while Seoul stayed near 33 C. A tropical night advisory for Seoul was lifted Saturday night — the first break in 17 days — and a noticeably cool breeze swept through the city, just one day after Friday marked the traditional solar term of Ipchu, the start of autumn. Residents said they switched off their air conditioners and reached for blankets in the cool night air.

The KMA said the shift was driven largely by an influx of cold air from the north. "When we were under the influence of southwesterly winds, hot and humid air from the south was intensifying the heat wave," said Lee Won-gil, a KMA forecaster. "Now, relatively cold and dry northeasterly winds are flowing in from a high-pressure system to the northeast. The character of the air itself has changed, and that is causing temperatures to fall."

Increased cloud cover has also played a role. Cold, dry air combined with clouds blocking sunlight has reduced surface heating during the day. At night, the same air mass causes temperatures to drop more quickly, which lifted the tropical night advisory and brought the cool breezes, the KMA said.

A widening temperature gap between the east and west has also emerged. The eastern coast, directly exposed to the cold, dry easterly winds, is seeing relatively lower temperatures. But as those winds cross the Taebaek Mountains, the air warms through a foehn effect, pushing temperatures higher on the western side of the peninsula.

While the extreme highs have eased, the heat wave is far from over. Rising humidity is expected to push the apparent temperature higher, and dangerous heat conditions will persist, the KMA said.

Morning lows across the country this week are forecast at 23 to 26 C, with afternoon highs of 28 to 35 C — above seasonal norms. The apparent temperature is expected to exceed 33 C across most of the country and top 35 C in some inland areas, with heat wave and tropical night conditions forecast to continue.

Over the weekend of Aug. 15 to 16, skies will be mostly cloudy nationwide, but morning temperatures are still expected to range from 23 to 26 C and afternoon highs from 30 to 35 C. Hot, humid air moving into the lower atmosphere could destabilize conditions and bring scattered showers, particularly inland.

Forecasters say it is too early to rule out another round of near-40 C heat. The KMA said the pressure pattern that produced the extreme temperatures this summer could return through late August.

"After the 13th, temperatures look set to climb back to around 34 C, with some areas forecast to reach 35 C," Lee said. "It is difficult to say the heat wave has fully retreated."