Communities across southern New Brunswick are being blasted by a major rain storm, with forecasts calling for as much as 100 millimetres of rain on Wednesday.

The rain is causing reduced visibility and flooding in some areas.

Motorists are being advised to use caution.

In Fredericton, more than 50 millimetres had fallen by noon, most of it in heavy downpours, resulting in some accidents.

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Several streets in Saint John were closed, including McAllister Drive, from Westmorland Road to Golden Grove Road. (Bobbi-Jean MacKinnon/CBC)

"I felt my car hydroplane," said Mike Tidd, who was on his way to Oromocto when the car in front of him swerved.

"I know there was just spray in front of me and I saw this grey truck go sideways and over into the ditch and come to a rest."

Tidd said he stopped his car to try to help the driver.

"I got out and he was very white and I didn't think it was going to be very good, but he's going to be OK," Tidd said.

Saint John received nearly 100 millimetres of rain in just six hours.

Work crews have closed some roads to traffic and police are telling motorists to obey the barricades and not drive through the water-logged streets.

On the east side, McAllister Drive is closed from Westmorland Road to Golden Grove Road, while Majors Brook Drive is closed from McAllister Drive to the entrance of Parkway Mall.

On the city's west side, Fairville Boulevard is also closed from Kierstead Road to Manawagonish Road.

Warnings issued

A low pressure system moved in overnight and it started to pour just before dawn.

Environment Canada issued heavy rainfall warnings for southern New Brunswick on Wednesday. The forecast projects a low pressure system to move over the Maritimes by Thursday evening.

"Rain at times heavy will continue over southern New Brunswick [Wednesday] and taper to showers this evening. Generally, rainfall totals of up to 75 millimetres are expected, but there is potential for local amounts of up to 100 millimetres," according to a weather warning issued by Environment Canada.

Saint John issued its own rainfall warning on Tuesday and said its municipal operations unit would be monitoring the weather conditions.

"Residents are advised to check eaves troughs and drainage swales, as well as catch basins or ditches on or near their property for debris," according to the city’s statement.

Meanwhile in Moncton, the city issued its own statement, advising citizens that crews would be on standby in case the rainfall caused any problems.

"The city does not expect any major issues, but is being proactive so that the impacts of any issues as a result of localized flooding are minimal," according to City of Moncton’s statement.