The capital's biggest rainstorm of the year could be coming.

Rain could begin this morning in Ottawa-Gatineau, with a stronger chance of it beginning after lunch and a risk of thunderstorms.

Once it starts falling it may not dry up for good until Thursday evening, perhaps even Friday morning.

It could be falling quite heavily tonight.

The threshold for a rainfall warning is 50 millimetres in 24 hours — for the capital, Lanark County, Brockville and Cornwall areas and Prescott and Russell, the forecast calls for 40 to 50 millimetres by the time the storm is done.

(Environment Canada)

Ottawa hasn't received more than 20 millimetres of rain in a day since May 10, the Friday before Mother's Day.

Its biggest two-day rainfall total this year was 41.4 millimetres between April 19 and 20.

It got 62 millimetres in all of September, 53 in August and 52 in July.

The record for Oct. 17 rainfall in Ottawa is 26.8 mm, set 13 years ago.

Not quite as rainy elsewhere

The high is in the low teens and eastern winds holding at 20 km/h could gust up to 40 this morning.

Tonight's low is 7 C and it could only get up to 10 C tomorrow.

Elsewhere, Kingston is expected to get slightly less rain (in the 25-35 mm range) that doesn't fall as heavily as further north.

The Pembroke and Maniwaki areas could see between 10 and 20 mm.

Winds could gust up to 50 km/h in Kingston, causing waves for boaters along the St. Lawrence Seaway and Lake Ontario, and in Maniwaki.