When we are talking about a weather forecast or prediction we say, "The high temperature today will be 100 degrees and the low will be 69 degrees." We often eliminate the word "temperature" and just say, "Today's high will be 100 degrees." The exact temperature is always plural.
I, personally, would use "lowest" and "highest" when talking about extremes over a period of time in the past. "Last week, the highest temperature was 105 degrees!"
Hope this helps!