Actually, this is not new. WXSIM has always predicted maximum temperatures in my location above what the GFS or ECMWF model indicates.
I understand that it must be some incorrect setting in the WXSIM configuration.
For example, in GFS output 6Z the estimate for today is 16º and WXSIM tells me 19º. Of course, I understand that this can fail due to unforeseen meteorological causes on any given day but not repeatedly.
He’s a very busy school teacher so contact in term time isn’t easy. He might have a bit more time available in the next couple of weeks due to school holidays.