The slow-moving storm - dubbed Super typhoon Mawar - delivered rainfall of up to 2 inches per hour overnight, the US National Weather Service (NWS) said in its forecast.
Moving northwest at 13 km/h, Mawar could produce landslides, flash flooding and life-threatening storm surge, the NWS said in a series of warnings for the tropical island.
"I'm very worried for our people's safety and very concerned," Guam's Governor Lou Leon Guerrero told National Public Radio during an interview on Wednesday.
She compared the storm with 1962's Typhoon Karen, which flattened much of the island, which has a current population of about 170,000.
Typhoon Mawar's wind speeds placed the storm in Category 4, the second-strongest designation on the Saffir-Simpson Hurricane Wind scale.
The growing frequency and intensity of such storms around the globe are symptomatic of human-driven climate change, according to scientists who study weather patterns.
According to early reports, Mawar damaged houses and forced the rescue of eight people, said Guerrero, who expects a more complete assessment of the destruction after winds tamper down.
Video footage on social media showed strong winds, rain and trees swaying as the typhoon hit Guam.
On Tuesday night, President Joe Biden approved an emergency declaration, authorizing federal assistance for Guam, where the United States has a military presence.
Mawar is expected to bring unseasonal rains to Thailand this week
Thailand is expected to see more rain at the end of this month despite not being directly in the path of the evolving Super typhoon Mawar, the Thailand Meteorological Department has forecast.
The department reported on Wednesday that the storm was still in the Pacific Ocean and was moving northwest rather than towards the South China Sea.
Once the maximum sustained wind of the tropical storm exceeds 117 kilometres per hour or more, it would be categorised as a typhoon, hurricane, or cyclone, depending on where the storm starts on the world map.
Two weather agencies -- Joint Typhoon Warning Centre, and Japan Meteorological Agency -- forecast that Mawar could intensify into a "super typhoon" by Friday as a result of the Pacific Ocean’s sea surface temperature rising to 29 to 30 degrees Celsius.
The term "super typhoon" is used when a typhoon’s sustained wind strength reaches 240 km/h.
As the storm is heading from the eastern part of the Philippines to Taiwan, it is projected to slow down due to cooler sea temperatures there, they said.
Mawar may hit the Philippines by the weekend: Weatherman
The tropical cyclone expected to enter the Philippine area of responsibility (PAR) has intensified into a super typhoon, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (Pagasa) said on Tuesday.
Pagasa weather specialist Rhea Torres said “Mawar” intensified into a super typhoon as of 2:00 pm.
(As of 2:00 p.m., Mawar has intensified into a super typhoon,” Torres said in a public weather forecast.
Torres also reiterated that the super typhoon could enter PAR by Friday or Saturday.
“Sa ngayon wala pa itong direktang epekto sa anumang bahagi ng ating bansa ngunit habang tinatahak nito ang karagatan ay posibleng lumapit ng ating PAR by Friday or Saturday,” she said.
"Currently, it does not have a direct effect but while moving over the sea, it could come near PAR by Friday or Saturday,” she said.
Once it enters the PAR, the typhoon will be given the local name “Betty.”
It would be the second storm to hit the country this year and the first this month.