Compound noun: Heat warning.
Helle is a noun that means ”swelter,” ”heat” or ”hot weather.” In Finnish weather forecasts the term helle is used for temperatures above +25 °C (+77 °F).
Let’s translate the rest of the headline from yesterday’s Metro, ”Odotettavissa on tukalan lämmintä.” A literal translation of the phrase might be, ”It is to be expected tricky warm.” Better English would be ”It is expected to be uncomfortably warm.”
Odotettavissa is the passive present participle of the verb odottaa (see odottaa conjugated here) in its inessive plural form. The passive present participle functions like an adjective and is often called the ”verbal adjective.” Odotettava is the basic form, which has then been declined in our example. Odotettavissa describes something that ”is being expected.” When combined with olla (”to be”) it means ”is to be expected.”
[It’s not necessarily clear from this phrase why odotettavissa is in inessive plural, but from the text below the photo it appears to be because the places where the heat is expected are in the south and in the center of Finland: ”Ilmatieteen laitos varoitti eilen, että lähtivuorokauden aikana on odotettavissa tukalan lämmintä maan etelä- ja keskiosassä.” (The Bureau of Meteorology warned yesterday that during the day it is expected to be uncomfortably warm in the south and center.) If you have a better explanation of why odotettavissa is used here in inessive plural, please leave it in the comments. I understand this is the form of the word you typically hear in Finnish weather forecasts.]
On is our verb, the 3rd person singular of olla (”to be”). It means ”is.”
Tukalan is the genitive singular of tukala, an adjective which means ”hard” or ”tricky” when talking about a situation. Here it modifies the final word of the phrase, lämmintä, partitive singular of lämmin, an adjective which means warm.