Parts of Guam have been hit by flash flooding, with 40 centimetres of rain reported in 24 hours in some areas.
There have been reports of towns being inundated and more heavy downpours are expected over the next day or so.
Guam National Weather Service forecaster Derek Williams has told the heavy rains have caused numerous problems.
"We've had roads washed away, one family rescued from their flooded homes, numerous trees down, numerous power outages, things that you would see in a tropical storm, however this isn't a tropical storm yet," he said.
"It's not even a tropical depression - it just so happens that the heaviest winds and rain are south of the circulation."
Forecasters say the rain brought on by a developing tropical disturbance centred north east of Saipan.
The Joint Typhoon Warning Centre in Hawaii has issued a tropical cyclone formation alert.
"Right now it's kind of let up as far as the intense rainfall - it's to our south and east now - and west and north - we're kind of in a hole," he said.
"We expect it to pick up later today, and hopefully it won't be another 10-15 inches like we've had in the last 24 hours."
Mr Williams says the weather pattern isn't unusual for this time of year.
"The overall pattern across the Western Pacific is a normal monsoonal flow that we haven't had in the past few years," he said.
"So these events are somewhat normal in a normal season, but we haven't had a real heavy monsoon event for quite some time here."
© ABC 2013
12:35 EST Heavy showers have continued over southwestern parts of Western Australia after yesterday saw some of the heaviest August rain in 100 years.