Environment Canada issued a heat warning for the Howe Sound region on Aug. 16 for Wednesday and Thursday. Temperatures are expected to reach daytime highs of 29 to 35 C inland, and near 27 C near the water.
The agency says temperatures will moderate around Friday.
According to Environment Canada, there will be a mix of sun and cloud, with temperatures reaching a high of about 26 C on Saturday and Sunday, which are the days the Squamish 50 will be taking place.
Runner and Squamish meteorologist Jason Ross also said there is a chance of showers and thunderstorms for the race.
On Aug. 15, he said the latest guidance suggests the ridge of high pressure will break down late Thursday and into Friday as an upper trough pushes onto the South Coast.
The unstable southerly flow aloft through the weekend could bring the possibility of some convection firing up thunderstorms.