Runners should ensure they are well hydrated and ready for the heat and humidity on Ottawa Race Weekend as the temperature reaches the 30s on Saturday and Sunday.

Friday's forecast calls for a mix of sun and cloud with a 30 per cent chance of rain in the late afternoon and a risk of a thunderstorm, as a southwest wind hits 20 kilometres per hour near noon.

The high will be 30 C with a very high UV index of 8.

The low on Friday night will be 17 C before a humid weekend, which could bring some rainfall for the thousands of runners hitting the pavement.

Saturday's forecast calls for a mix of sun and cloud with a 30 per cent chance of rain late in the afternoon and a risk of a thunderstorm. The high is 31 C.

Rain could fall overnight Saturday, with a low of 19 C, so Sunday morning's run could be on wet paths and roadways.

Sunday's forecast also calls for a mix of sun and cloud with a 40 per cent chance of showers and a high of 31 C.

The rain should continue overnight Sunday with a low of 19 C, and into Monday where the high is currently set at 27 C.