Temperatures across the whole state will jump by between six and 10C.
Mr Claassen said an afternoon or early evening storm midway through the week would give residents some relief from the heat.
“It starts to cool on Tuesday but, at 34C, is still well above average,” he said.
“We might see a late storm, which would break the heat but it’s possible it will be severe.”
Temperatures are expected to have dropped by Wednesday.
“We’re forecasting 29C – a 5C drop from Tuesday and 8C drop from Monday,” Mr Claassen said.
“It should feel much nicer on Wednesday.”