temperatures will be in the upper twenties with a possible high of 30 degrees Celsius.

e.g.

Temperatures can reach the 30 deg mark.

Hört, hört! Wann denn?!

Kommt drauf an, wo du bist. Bei uns sollt's heute angeblich bis auf 34 Grad (Celsius) raufgehen. :-)

Gart - HR says 23° on Tuesday (Rhein-Main), smile!

30° Celsius [Centigrade] = 86° Fahrenheit

The weather forecasters have not only been promising us sun and NO more RAIN/SNOW for months, now, but also stating that we in fact have it...ha ha ha...lying through their teeth, they are!

@ # 7, Hi Carly, those weather guys and gals are generally a bunch of sorry clowns, but not this time. It's going to be 95 F at my house this afternoon. :-))

Helmi, the weather report is never wrong, it's the date which they don't get right

Hi, Helmi! They keep reporting that we have 22°C and sun - well, 12°C and torrential rain...oh, pardon me, "weather guys and gals": Over the past few years, your weather forecasts here in Germany range from: 8°C - 25°C, sunny to heavy rain, and tend to change on an hourly basis. Do you just look out your window, and perhaps place a few long-distance phone calls??

edit: Exactly, Klaus :-))

Back to the OP, I would say:
It's supposed to go up to 30°C.