Emergency shelters have been set up in Guam as Tropical Depression 14W intensifies.
Guam's National Weather Service had forecast the depression would become a Tropical Storm overnight in the Commonwealth of the Northern Mariana Islands (CNMI).
Its science and operations officer Brandon Aydlet said there were two things the weather service was "really concerned about".
"One is the rainfall, which we're going to be expecting heavy, torrential rains, starting tonight, continuing through Wednesday night.
"And then another wet pattern sets up through the weekend. So one is the concern for flooding."
Guam Governor Lou Leon Guerrero has placed Guam under Condition of Readiness 3 (COR3), which means every resident should be preparing for a storm.
Residents had been asked to prepare for heavy rain and runoff.
"The second is the wind," he added.
"Winds even from a tropical storm will be severe to some folks, especially the poorly structured buildings and houses. So to that end, the Guam Civil Defense, they have established a couple of shelters."
He said the CNMI were familiar with the threats posed by tropical cyclones, with last year's Typhoon Mawar still front of mind.
"Typhoon Mawar was about 130 mile [209km] per hour category four typhoon as it passed over the north tip of Guam," Aydlet said.
"This one, we're looking at a 40 miles per hour Tropical Storm passing over or very near Guam overnight. So both had heavy rainfall. Though, in the case of this storm, 14W, we're not anticipating as much widespread and prolonged significantly heavy rainfall.
"But with that said, eight to 10 inches of rain can still have dramatic impact, especially on the mountain streams across southern Guam so that is a concern."
Although the storm was expected to be "short lived", it would still pack a punch.
Tropical storm force winds of at least 39mph were expected overnight for Saipan, Tinian, Rota and Guam.
Tropical Depression 14W was moving west at 9mph.
It was expected to make a turn toward the northwest with a slight increase in forward speed through Wednesday.
This current forecast track sent 14W over or near Guam early Wednesday morning.
Once to the northwest of Guam, 14W was expected to continue intensifying over the Philippine Sea.
Updates could be found on the Facebook page for US National Weather Service Guam.