For licensed hams who want to participate you will basically need a 2m rig and should attend one of the local canwarn training session held every spring. At the training sessin, you will be instructed on what to look for during a severe weather event, and you will be briefed on the radio net protocol. You will also get a canwarn map atlas so you can provide the net control station with your co-ordinates.
For others who want to listen to CANWARN radio operations, you will need a scanner that is capable of tuning the 144-148 Mhz VHF ham band. Idealy, your scanner should also be able to cover the 440-450 Mhz UHF ham band. Scanners are available at a number of radio shops (eg Radioworld in Toronto, or any Radio Shack store).
In the Toronto area, listen to VE3ULR on 442.025. This is the HUB of the CANWARN system for South Central Ontario, operating out of the Toronto Weather Office. Listen for the "Canwarn Net Control" Station with the callsign VE3YZW. 442.025 is located near Uxbridge, northeast of Toronto so you may not be able to hear it depending on your location. Also, remember, it will only be active with CANWARN traffic if a severe weather event exists. The Toronto Canwarn office uses the following frequencies in Central Ontario which can be linked using the 442.025 frequency:
Other frequencies in use in the Greater Toronto Area:
Huron Shores Canwarn:
Manitoulin Island:
South Western Ontario:
Niagara Region:
Dufferin County ARES & Canwarn:
Eastern Ontario:
Other good local weather frequencies are as follows: 162.400 Toronto CN Tower; 162.475 St Catherines/Niagara; 162.550 Buffalo NOAA station. Marine frequencies are 161.650 and 161.775 for continuous marine and weather broadcasts.
Canwarn nets operate under three (probably self evident) states of alert.
Condition green: Severe Thunderstorm Watch in effect. This is the basic state when the net is in operation and taking various weather reports from spotters. The frequency is available for regular traffic but conversations should be kept brief so the Net Controller or spotters can break in if new information becomes available. This is the time to ask "silly" questions.
Condition yellow: Severe Thunderstorm Warning or Tornado Watch in effect. This is an alerted state of the net with the NCS only requesting and receiving specific reports from certain spotters such as Hail larger than marble sized; damaging winds; wall clouds; funnel clouds or any other unusual weather events. The frequency is under Net Control.
Condition red: Tornado Warning in effect. This is the highest state of alert, usually prompted by confirmed sightings of active wall clouds or funnels. No general radio traffic is allowed on the frequency under condition Red. Only reports of wall clouds, funnel clouds, or tornados are accepted by net control.
More information can be found here on the official EC canwarn page.