This is the 976th most frequent Icelandic word.
veðurstofu
Weather office or meteorological office.
In this sentence, 'veðurstofan' is the nominative case of the definite form of 'veðurstofa,' used as the subject of the sentence.
Íslenska veðurstofan spáir stormi á morgun.
The Icelandic weather station predicts a storm tomorrow.
Here, the word 'veðurstofuna' is in the accusative case form of 'veðurstofa' and is used to denote the place being visited.
Ég fór á veðurstofuna í gær.
I went to the weather station yesterday.
'Veðurstofunni' in this instance is the dative case of the definite form of 'veðurstofa,' indicating the source (the station) providing the report.