In technical writing it is important to be precise. If you want to state something, it should generally be stated in the clearest language possible. In a comment you say:
I very specifically need to refer to the timeframe of June-July-August.
and since you want a replacement for "Summer 2019" I suggest one of the following:
* June-August 2019
* June, July, and August 2019
* June through August 2019 (which I have been told in a comment might sound strange to non-American readers)
Why not state a season?
You say you want to be specific, but using a season in place of the specific months is not very specific. If you state the months, readers know exactly what you mean. If you say "summer" it is open to interpretation. This is true even if your audience is local. Does summer mean the date range starting the day after spring ends, and ending the day before fall begins? If so, whose definition of those dates should be used? It isn't the same everywhere. Or maybe it means the period of time when it feels like summer, which may or may not match up reasonably well with the specific date range? It isn't clear.
Why not say "northern summer"?
This, in my opinion, is even worse. It has the same specificity problems of stating a season, but with additional confusion. First, it relies on the reader knowing when summer is in a different part of the world. 873
Maybe they can figure it out, but its an unnecessary obstacle for them to understand your document.
On top of that, without reading the other answer, I wouldn't know what "northern summer" means. I live in the northern United States. If I had to guess, I would have guessed it means summer in Canada, which maybe is supposed to mean a shortened summer? It is unnecessarily confusing, and doesn't even convey the information to a fully informed reader.
To further emphasize this point, here are some alternate interpretations that I don't think are too much of a stretch:
* Summer in the northern region of the reader's country
* Summer in the northern region of the author's country
* An alternate thing the reader has never heard of called "northern summer"1
Even if the reader eventually settles on the correct interpretation, too much time was spent trying to determine what was meant when a simple June-August would have been much clearer.
When can I use a season?
I don't think stating a time period as a season is always to be avoided. It can be used when a characteristic of the season is what is important. If you were writing documentation about a solar panel, which will provide more power during the long daylight days of the summer, it would be perfectly reasonable to say "The highest power output can be expected during the summer". In this case what is important isn't the particular date range, but instead a characteristic of the season. This makes using the season name more appropriate. I can't at the moment think of any reason I would want to use "northern summer", but if I did, I would instead phrase it as "summer in the northern hemisphere", since, as previously mentioned, I find "northern summer" to be unclear.
1. Maybe someone can help me better express this. I'm referring to what happens when a term that looks like adjective-noun does not modify the noun, but instead refers to a different noun. 591