What does high not valid after 2pm mean?

Answer 1

You are not first to ask this question, however, there is no reliable information available to explain this with guaranteed accuracy. All that is possible, is to make an inference, and I can use my long-time experience in atmospheric sciences to do so. On average, the warmest time of day is around 2:00PM local time. It could most certainly vary, from 1 pm to 4 pm, although unusual conditions can change this further, such as if an overnight, early-morning, or late-day frontal boundary passes. Despite that the sun's rays are strongest at noon, due to the laws of inertia, it will take time to heat the atmosphere, causing a delayed effect. Since their [weather forecast provider's] forecasts are not sophisticated enough to selectively remove the high temperature after it occurs (thus expires), they have simply taken the "easy way out" and automatically engineered their forecasts to eliminate the high temperature after a fixed time.

Disclaimer: This is simply an inference that I am providing for informational purposes only, and therefore, a guaranteed accurate conclusion is impossible to make.