That's true, but there is very little variance in weather reports. So the translation can be of high quality. The more variance increases, the more fields are involved, the more difficult it becomes to maintain the same level of quality.
What do the translation companies that use automatic translation do? The best practices involve limiting the translation tool's learning to the specific field in which the translator is working and to edit texts afterwards.
I will use the example of manuals for military members. Translators would learn from previously translated manuals. Afterwards, when they started translating the manual and revising everything using automatic translation, that information would be put back into the tool every week. So the translation tool would improve its translation of that manual, since the beginning was already understood. That is one example of the best practices.