The Canadian Hurricane Centre in Dartmouth, N.S., is warning flash floods are possible if tropical storm Gabrielle continues to drench Nova Scotia.
Program supervisor Chris Fogarty said areas that have had problems in the past, such as Truro, are at risk.
Fogarty said the cloud formations and lightning flashes offshore have his team thinking there could be heavy bursts with 20 millimetres of rain in any hour.
The areas of concern stretch from Halifax through the middle of the province to Truro and Amherst and include central and eastern Prince Edward Island.
Morning commuters in Halifax felt the effects of tropical storm Gabrielle Friday morning as a heavy swath of rain pelted the region.
"It's only going to get harder," said CBC meteorologist Peter Coade.
Environment Canada has already issued a series of rainfall warnings for Nova Scotia and P.E.I.
The storm was expected to dump up to 70 millimetres of rain in parts of the Maritimes before moving on to Newfoundland later Friday evening.
Gabrielle was downgraded to a depression but regained tropical storm status earlier on Thursday.
Wind gusts could hit 70 kilometres per hour, but Coade said the storm is expected to be more of a rain event.
Rainfall warnings are in effect for parts of central and northeastern Nova Scotia as well as Queens and Kings counties in Prince Edward Island.
Forecasters said if there is a slight change in the storm's track or intensity, the rainfall warnings could be extended to other areas.
The strongest winds are expected along western Cape Breton, where winds known as Les Suetes are expected to peak at 90 kilometres an hour.
Halifax police were warning motorists of hydroplaning on wet roads.
"Treat it as a snowstorm," said Const. Pierre Bourdages. "Don't rush things."
"If you find yourself hydroplaning, take your foot off the gas pedal, disengage the cruise control if using it and let your vehicle slow down until the tires make contact with the road surface," read the police release.
Inspectors with Nova Scotia’s environment department are hoping there will be no more siltation in the Little Sackville River. Last week, rain sent silt from construction sites in to the river
Crews said they spread as much straw as they could on Thursday at Twin Brooks construction site in Lower Sackville before Friday’s forecasted rainfall to reduce the risk of silt running into the river.
But Walter Regan of the Sackville Rivers Association said he doubts it will work.
"Anything is better than nothing, but behind me is not enough," he said. "It's too late. We're getting between two and four inches of rain, and I guarantee you the majority of this site will be washing into the Little Sackville River."
Ramar Construction set up a containment pond and put filters over the storm drains, but nothing could contain last Tuesday's rainfall.
"Sometimes during a short duration, in intense rainfalls, some of these controls can fail," said Robert Cuthbert, an inspector with the department.
"Fair enough," said Regan. "But the thing is, if you have good planning in place, you anticipate the worse storm."
Environment Canada does not consider silt pollution, but it can still be deadly for Atlantic salmon and trout that live in the Sackville River.
Meanwhile, Hurricane Humberto — this season’s first hurricane — is maintaining its strength far out over the Atlantic Ocean heading northward.
The category 1 hurricane, centred 705 kilometres over the Cape Verde Islands, has maximum sustained winds near 140 km/h. The U.S. National Hurricane Centre expects it to weaken over the coming days.