A typhoon warning is in effect for Guam and Rota Island in the Northern Marianas, while a tropical storm warning is in effect for Saipan and Tinian in the Marianas.
RNZ Pacific reports sustained winds are expected to reach 225km/h with gusts reaching 282km/h during Wednesday.
Landon Aydlett from the weather service said the typhoon had dropped in strength from maximum sustained winds of 250km/h to 225km/h.
The decrease in wind speed changed Mawar's status from a super typhoon to a category four typhoon.
"All of Guam would be feeling typhoon force conditions, with the strongest conditions particularly over southern Guam and central Guam where the eyewall will be passing over," Aydlett said.
"Whoever's under the eyewall expect torrential rainfall for a period of time as this thing trucks slowly over Guam later this afternoon."
Aydlett told people in Guam who are at risk of flooding to get to higher ground and for everyone to seek solid shelter.
He said tropical storm force winds were likely to persist from Wednesday night through to early Thursday morning, but conditions should start improving around sunset tonight.
Aydlett said there was a triple threat to Guam.
"That's torrential rains that may result in landslides and flash flooding, catastrophic winds and life-threatening storm surge."
The National Weather Service is predicting wave heights to reach 9 metres and for the potential of 50 centimetres of rain in some locations.
Storm could be one of the worst in recent history
Brandon Aydlett, who is also with the National Weather Service office in Guam, said Mawar would compare to typhoon Pamela in 1976 and Pongsona in 2002 - which caused $US730 million of damage.
A worst-case scenario would be similar to typhoon Karen in 1962, which was the strongest typhoon on record, he said.
Aydlett said it was "absolutely vital" for people in Guam and Northern Marianas to find a fully reinforced concrete shelter to wait out the storm in.
"Everybody in the Northern Marianas should be in shelter right now.
"Tropical storm force conditions will be on setting across the CNMI (Northern Mariana Islands) especially for Guam, if you're not in a fully reinforced concrete structure, you are at risk," he said.
Meanwhile, a price freeze on gasoline, all foods, bottled water, beverages, ice, and all clothing was implemented on Wednesday by Marianas Governor Arnold Palacios.
US President Joe Biden also approved the Governor's request for the declaration of a pre-landfall emergency in the Marianas in response to the approaching typhoon.
Biden declared an emergency through a Presidential Emergency Declaration, which frees up federal resources that could be tapped to assist the country.
Through the declaration, Biden ordered that federal assistance be used to supplement the CNMI and local response efforts.