When does a weather day change? Why does Saturday’s forecast low occur on Sunday morning?

This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

Dear Tom,
I know you’ve addressed this before, but I still get confused. When does a weather day change? Why does Saturday’s forecast low occur on Sunday morning?
— Lou Linneweh,  Carol Stream, Jim Eder, Mundelein
Dear Lou and Jim,
It can be confusing, but there is a difference between a day’s observed high and low temperatures and the forecast high and low.
Past weather is always cataloged on a standard time calendar day.
So all of the meteorological variables on a past day — the high, low, precipitation, wind, etc. — would have all occurred within the 24-hour period for the day in question.
In contrast, the high/low temperature forecast for a future date would be the expected high for the day in question and the low for the following morning.

Notice: you are using an outdated browser. Microsoft does not recommend using IE as your default browser. Some features on this website, like video and images, might not work properly. For the best experience, please upgrade your browser.