According to the National Center for Hydro-Meteorological Forecasting, a mass of cold air to the north of Vietnam was moving southward on Oct. 2. Scattered showers and thunderstorms were already occurring in parts of the northern region, with temperatures at 7 a.m. generally ranging from 27-29 degrees Celsius.

The cold air is forecast to reach the northeastern region around the evening and night of Oct. 4 before spreading to other parts of northern Vietnam and the area from Thanh Hoa to Hue.

Winds will shift to the northeast at force 2-3, strengthening to force 3-4 in coastal areas.

From the night of Oct. 4, the northeastern region and Thanh Hoa will turn chilly, with cold conditions possible in some high mountainous areas. Minimum temperatures are forecast at 19-22 degrees across northern Vietnam and Thanh Hoa and 14-17 degrees in high mountainous areas.

The latest forecast follows an earlier assessment that northern Vietnam would experience two phases of cold air in early October, with an easterly cold air mass arriving from the evening and night of Oct. 1 before strengthening more significantly around Oct. 4-5.

Nguyen Duc Hoa, deputy head of the Climate Forecasting Division at the National Center for Hydro-Meteorological Forecasting, had forecast that temperatures would begin declining across northern provinces from Oct. 5.

He said minimum temperatures would generally range from 20-23 degrees Celsius, while some high-altitude areas in Lang Son and Lao Cai could fall below 18 degrees, bringing noticeably cooler conditions at night and in the morning.

Hanoi to turn chilly after thunderstorms

In Hanoi, rain, moderate rain and thunderstorms are forecast from the night of Oct. 4 through the morning of Oct. 5, with heavy rain possible in some locations.

The capital is expected to turn chilly from around early morning on Oct. 5 as the effects of the cold air become more pronounced.

Across northern Vietnam, the cold front is expected to bring rain, moderate rain and thunderstorms from Oct. 4 through the morning of Oct. 5, with locally heavy to very heavy rainfall.

From the night of Oct. 4 through Oct. 6, the area from Thanh Hoa to Hue is also forecast to experience moderate rain and thunderstorms, with heavy to very heavy rain possible in some places.

Forecasters warned of tornadoes, lightning, hail and strong wind gusts during thunderstorms.

Hoa said the risks are heightened because the cold air is arriving after a period of hot weather. The interaction between warm and cold air masses could destabilize the atmosphere, potentially producing severe thunderstorms, strong gusts and even hail.

Heavy thunderstorms could also cause localized flooding in low-lying areas.

Cold front follows unusually hot late September

The change in weather comes after a period of unusually intense heat in northern and central Vietnam in late September.

Hoa said the heat was caused by the southern edge of a low-pressure trough over southern China being compressed by continental high pressure to the north, combined with the circulation of a subtropical high at upper levels.

As a result, the northern midlands and plains, southern Phu Tho and the region from Thanh Hoa to Da Nang experienced hot weather, with severe heat in some places. Maximum temperatures generally reached 35-36 degrees Celsius, with some locations exceeding 37 degrees.

On Sept. 29, eight maximum temperature readings in northern and central Vietnam exceeded previous historical records.

In Hai Duong, Hai Phong, the temperature reached 37.3 degrees, surpassing the previous record of 37.2 degrees set on Sept. 5, 2024 and Sept. 2, 2020. Nam Dinh in Ninh Binh recorded 37 degrees, above the previous record of 36.6 degrees on Sept. 2, 2020, while Nho Quan in Ninh Binh reached 37.7 degrees, exceeding the 37.4-degree record set on Sept. 3, 1988.

Northern Vietnam typically experiences one or two heatwaves in September, each lasting two to four days. However, late-September and early-October heat has occurred before, including in 2009, 2010, 2016 and 2023.

The latest heatwave was expected to end on Oct. 2.

For October as a whole, average temperatures nationwide are forecast to be 0.5-1.5 degrees Celsius above the long-term average, although hot weather in the northern plains and midlands and central Vietnam is expected to become largely localized.

Strong winds and rough seas expected

The approaching cold air will also significantly affect maritime conditions.

From around early morning on Oct. 5, winds in the Gulf of Tonkin are forecast to shift to the northeast and strengthen to force 6, occasionally force 7, with gusts of force 8-9. Waves are expected to reach 2-3 meters, producing rough seas.

From Oct. 5, northern waters of the East Sea are forecast to experience northeast winds of force 6-7, gusting to force 8, with waves of 2-4 meters and rough seas.

From Oct. 6, the area of strong winds is expected to expand across the northern East Sea, including the Hoang Sa special zone.

Tropical storms could still affect Vietnam in October

Despite the arrival of cold air, forecasters said tropical cyclone risks remain during October.

Hoa said rainfall in mountainous northern Vietnam this month is forecast to be 5-20% above the long-term average. Other parts of the northern region and the area from Thanh Hoa to Hue are expected to receive 20-40% less rain than average, while rainfall in the south-central coast, Central Highlands and southern Vietnam is forecast to be 5-20% below average.

Widespread heavy rainfall may still occur during the month, with central Vietnam the area most likely to be affected.

The number of storms and tropical depressions operating in the East Sea in October is forecast to be below the long-term average. However, forecasters have not ruled out the possibility of a storm or tropical depression affecting Vietnam's mainland.

The long-term October average is around two storms or tropical depressions in the East Sea, of which 0.8 make landfall.

Further cold air outbreaks are also expected to affect northern and central Vietnam during the month.

Hoa stressed that weather patterns are becoming increasingly complex amid climate change and advised authorities and residents to closely follow forecasts and warnings so they can adjust production plans, prepare appropriate response measures and ensure safety.

Bao Anh