Ottawa braces for 40 mm deluge as Environment Canada issues heavy rainfall warning

Key facts

• Environment Canada issued a heavy rainfall warning for Ottawa on Monday 4 May 2026.
• Up to 40 mm of rain could fall by the end of the day Monday.
• More rain is expected on Tuesday, according to the federal weather agency.
• The warning was in effect as of 4:00 PM EDT Monday.
• Observations were recorded at Ottawa Macdonald-Cartier International Airport.
• The forecast was issued at 3:30 PM EDT Monday 4 May 2026.

A deluge descends on the capital

Environment Canada has issued a heavy rainfall warning for Ottawa, forecasting as much as 40 millimetres of rain by the close of Monday. The warning, posted at 3:30 PM EDT on 4 May 2026, signals that a potent weather system is poised to saturate the city. The federal agency cautioned that additional precipitation is expected on Tuesday, compounding the risk of localized flooding. Residents are advised to monitor water levels and avoid driving through standing water.

Monday’s downpour and Tuesday’s follow-up

By 4:00 PM Monday, observations at Ottawa Macdonald-Cartier International Airport already reflected the onset of heavy rain. Environment Canada’s detailed forecast, issued simultaneously with the warning, projects that the 40 mm threshold could be reached before midnight. The system is not expected to exit quickly. Forecasters predict a second wave of rain on Tuesday, though the exact amounts remain uncertain. The back-to-back days of precipitation raise the prospect of cumulative totals exceeding 50 mm in some areas.

What the warning means for residents

A heavy rainfall warning is the highest alert level for rain issued by Environment Canada. It indicates that significant amounts are likely to cause ponding on roads, reduced visibility, and potential flash floods in low-lying areas. The agency urges people to stay clear of rivers and creeks, which may rise rapidly. Drivers should reduce speed and turn on headlights. The warning remains in effect until further notice, with updates expected as the system evolves.

Broader weather context and preparedness

Ottawa has experienced a wet spring, and this event adds to the seasonal precipitation totals. The city’s infrastructure, including storm drains and culverts, is being tested by the volume of runoff. Local emergency services have not yet issued a state of emergency, but they are monitoring the situation. Environment Canada’s historical climate data shows that May rainfall averages are often exceeded during such events, though this storm’s intensity is notable.

Looking ahead: clearing expected midweek

The long-range forecast suggests that the rain will taper off by Wednesday, with drier conditions returning for the latter half of the week. However, the ground will remain saturated, and any additional rain could quickly lead to new problems. Environment Canada continues to update its seven-day outlook, which includes daily high and low temperatures, chance of precipitation, pressure, humidity, and wind chill. Residents are encouraged to check the latest forecasts before making travel plans.

The bottom line

• Ottawa is under a heavy rainfall warning with up to 40 mm expected Monday and more rain Tuesday.
• The warning was issued by Environment Canada at 3:30 PM EDT on 4 May 2026.
• Observations at Ottawa Macdonald-Cartier International Airport confirm the onset of heavy rain.
• Localized flooding and hazardous driving conditions are likely.
• Drier weather is expected by Wednesday, but the ground will remain saturated.