Arthur was downgraded to a post-tropical storm with maximum sustained winds of 110 km/h as it approached southwest Nova Scotia, said the Canadian Hurricane Centre.
Arthur is expected to make landfall near southwest Nova Scotia near 6:30 am ET, said CBC Meteorologist Peter Coade.
Winds ahead of the storm are already gusting to 95 km/h at Baccaro Point in southwest Nova Scotia.
Heavy rain is falling in southwest New Brunswick, said Coade, adding 46 millimetres of rain in St. Stephen since midnight.
"Rain will continue heavy through New Brunswick today," said Coade.
More than 18,000 Nova Scotia Power customers are in the dark Saturday morning.
Most of them are along the south and Atlantic coast of the province. Several thousand customers in Halifax lost power just before dawn, according to the Nova Scotia Power website. Power outages are also a problem in New Brunswick. NB Power reports about 2,500 customers in the dark in Fredericton and St. Stephen.
Firefighters in Fredericton were tweeting to advise people of downed power lines and for drivers to watch for pooling water and branches down on the streets.
In the Fredericton area, more than 9,200 NB Power customers were without power at 5 a.m. Saturday. That number had been reduced to more than 2,500 by 6:30 a.m.
In Nova Scotia, more than 4,500 customers of Nova Scotia Power were without electricity early Saturday morning, including a cluster of three outages in the Yarmouth, Shelburne, Pubnico area where Arthur is expected to make landfall.
Risk of local flooding
New Brunswick is forecast to see the highest rainfall amounts from Arthur while Nova Scotia will bear the brunt of the highest winds.
The Canadian Hurricane Centre forecasts strong winds for much of the Maritimes, with the stronger winds possibly gusting to 120 km/h over exposed coastal locations of southwestern Nova Scotia. Elsewhere in the southern Maritimes, winds gusting to 90 km/h are expected, with tree damage and downed utility lines expected.
All of New Brunswick is expected to receive heavy rainfall, with total amounts near 100 millimetres forecast for much of the province,exceptthe extreme northwest and southeast.
"The rainfall rates are of particular concern since they could exceed 15 millimetres per hour during a period of several hours," stated the Canadian Hurricane Centre in a tropical cyclone information statement issued at 2:45 a.m. "These conditions could lead to local flooding of small rivers and creeks and possible road washouts."
St. Stephen, in southwestern New Brunswick, received 46 mm of rain since midnight with much more rain expected by the time Arthur clears the region overnight Saturday.
CBC meteorologist Peter Coade said the area of New Brunswick from St. Stephen to Bathurst in the northeast could see 100 to 150 millimetres of rain.
"Generally you can take a swath of rain in New Brunswick, let's say from the southwest down in St. Stephen, Charlotte County up toward the Bathurst area where you're going to see about 100 to 140, or maybe 150 millimetres of rain before this all moves out," said Coade.
"Most of it is during the daylight hours of today — 80 to 110 day in the daylight hours — and then possibly another 20 to 30 in the overnight hours before it comes to an end before daybreak."