Tran Trong Hung, a resident in the Ha Long Bay area told AFP: “The sky turned dark at around 2 pm.”
There were “hailstones as big as toes with torrential rain, thunderstorm and lightning”, he said.
Torrential rain also lashed northern Hanoi, Thai Nguyen and Bac Ninh provinces on Saturday.
In the capital Hanoi, several trees were knocked down by strong winds.
The storm followed three days of intense heat, with the mercury hitting 37 degrees Celsius in some areas.
Mai Van Khiem, director of the National Center for Hydrometeorological Forecasting, was quoted in VNExpress as saying that the thunderstorms in northern Vietnam were not caused by the influence of Tropical Storm Wipha in the South China Sea.
Ha Long Bay is one of Vietnam’s most popular tourist destinations, with millions of people visiting its blue-green waters and rainforest-topped limestone islands each year.
Last year, 30 vessels sank at boat lock areas in coastal Quang Ninh province along Ha Long Bay after Typhoon Yagi brought strong wind and waves.
Earlier this month, a ferry sank off the popular Indonesian resort island of Bali, killing at least 18 people.
