Flooding due to heavy rainfall requires 2 conditions to be met: 1) heavy rainfall, and 2) the inability of the soil to absorb this rainfall. Only the first of these is addressed with the data and information provided on this website (http:// www.hazards.ca).

Land slope, elevation, the type of land surface (i.e. vegetation, pavement) and soil conditions (e.g. soil type, antecedent moisture content, frozen or non-frozen) will strongly influence the location and severity of flooding. For example, an identical rainfall total occurring with different soil conditions can lead to very different flood potentials. Within Ontario, the Ministry of Natural Resources and Conservation Authorities are responsible for forecasting where and when flooding is likely to occur and issuing flood warnings as required. The flood forecasts and warnings are based on guidance provided by the Meteorological Service of Canada weather forecasts and warnings. For more information on flood forecasts and warnings in Ontario, please refer to the Ontario Ministry of Natural Resources website:
http:// www.mnr.gov.on.ca/ MNR/ water/ p767.html.

In Ontario, Environment Canada issues the following types of weather warnings related to heavy rainfall:

Severe Thunderstorm Warning

One of the three criteria for issuing a Severe Thunderstorm Warning in Ontario is related to heavy rainfall. This type of Severe Weather Warning is issued in Ontario when one or more of the following is expected to occur (Environment Canada, 2001):
  • Wind gusts of 90 km/ hour or more;
  • Hail of 2 cm in diameter or larger;
  • Rainfall of 50 mm or more in one hour OR 75 mm or more within 3 hours.

'DAYS WITH', 'EXTREME DAILY' AND 'DATE OF LAST OCCURRENCE' RAINFALL DATA

Data for the 'days with', 'extreme daily' and most recent 'date of extreme daily' rainfall graphics on this website ( http:// www.hazards.ca ) was extracted from Environment Canada's National Climate Data Archive. Only climate and weather observing stations with at least 20 years of rainfall record were used in the data analyses. Not all climate stations used in the analyses are still operating. For the 'date of last occurrence' maps, stations with data records ending prior to 1998 are indicated. The data is also available from the Environment Canada Climate Normals website (Environment Canada, 2003a):
http:// climate.weatheroffice.ec.gc.ca/ climate_normals/ index_e.html

Note: The variation and frequency of heavy rainfall amounts over short distances is largely related to the track and scale of storm systems but can also be influenced by local factors which include topography and proximity to large moisture sources (i.e. the Great Lakes). The density of stations in the existing rain gauge measuring network may not be sufficient to capture all of these local variations in daily rainfall.

"ATMOSPHERIC HAZARDS IN ONTARIO" MAPS

FLOOD#1 - 24 HOUR PEAK (25 YEAR RETURN PERIOD) ESTIMATED RAINFALL AMOUNTS (mm)

This contoured map refers to the estimated 24 hour rainfall amount (mm) that can be expected to be reached or exceeded once every 25 years, on average, for areas of south-central-eastern Ontario (Auld and MacIver, 2000). The 25 year return period estimated rainfall amount may also be described as the estimated 24-hour peak estimated rainfall amount that has a 1 in 25 chance of being reached or exceeded each year. The analysis is based on Ontario climate station data for each station's period of record up to and including 1993.

FLOOD#2 - TOTAL NUMBER OF HEAVY RAINFALL EVENTS BY ENVIRONMENT CANADA PUBLIC FORECAST REGION (1979-2004)

For the purposes of this Environment Canada analysis (MSC-Ontario Region, 2005d), a heavy rainfall event was considered to have occurred when rainfall of 50 mm or more in one hour or 75 mm or more within 3 hours was observed. This corresponds to the same rainfall thresholds included in Environment Canada's Severe Thunderstorm warning criteria. The heavy rainfall events for the period 1979-2004 were plotted by occurrence within each of the individual Environment Canada public forecast regions (for additional information on these regions, please refer to the Environment Canada (2005d) website:
http:// www.on.ec.gc.ca/ weather/ regions/ intro_e.html ).

Note: Care must be exercised when using this map as regional differences in heavy rainfall occurrence are not merely a function of severe weather climatology, but can also be greatly influenced by population, road density and distribution of volunteer severe weather observers. It is therefore more likely that the number of heavy rainfall events reported in a public forecast region with a higher density population/ road and observer network will be greater than in a region which has a lower density population/ road and observer network.

FLOOD#3, #4, #5 - AVERAGE NUMBER OF DAYS PER YEAR WITH DAILY RAINFALL EQUAL TO OR GREATER THAN 25 MM (1971-2000)

These maps show the average annual frequency of days with 25 mm or more rainfall at climate stations in Ontario (#3 provincial overview, #4 southern Ontario, #5 northern Ontario) based on the 30 year climate normals period, 1971-2000. Only climate stations with rainfall records greater than 20 years within this period were included. Some stations may no longer be in operation.