You're conflating the literal and figurative meanings for "winter".
Winter is already here.
By this, we mean the season of winter. This has been confirmed by the maesters. I forget which episode it is (I'm looking for the quote now, but have not found it), but the maesters specifically confirmed the advent of winter.
To paraphrase what was said (in absence of a direct quote).
The maesters all agree, winter has come.
In other words, winter is already here. It will of course still grow stronger, but the current season is now winter, not summer.
Winter is coming.
This is a much more figurative statement. It is the words of house Stark. This isn't just talking about the season. When a Stark says "Winter is coming", there are a lot of possible interpretations:
- Summer is not permanent, it will be winter at some point.
This does address the meteorological season. However, the emphasis is on the need to prepare for winter.
- This happiness will not last. Life can become harder in the future.
This similarly emphasizes the need to prepare for hard times. Though not speaking about the season, but figuratively using "winter" to mean "hard times", the intention is very similar.
There is a third reason, which is the most applicable for your example, but it requires a lot of ground work to be laid to justify its meaning:
- The Walkers will rise up against us. They have not been permanently defeated, and this peace will not last.
This interpretation isn't actively observed by the Starks when the show starts. Ned Stark is clearly shown to not believe the Ranger who claims to have seen the Walkers. Ned ends up executing him, if he believed the threat to be real, he would have kept the eye witness around for testimony.
However, the Starks have historically been the defenders of the North, and therefore also the main line of defense against the Walkers.
- Brandon Stark built the Wall. Though there is an argument to be made for the Wall being created magically, Brandon "the builder" Stark was at least involved (maybe building the castles rather than the wall itself) and is credited for doing so.
- The Starks have always been the major house in the North. Even if not kings (due to bending the knee to the Iron Throne), they have functionally always ruled the North. The North is the main defense against the Walker threat. Therefore, the Starks will be expected to lead the defense against the Walkers, and will logically be incentivized to keep reminding people that the Walkers are real and not permanently defeated.
It makes perfect sense for the Stark ancestors to have chosen their words to be "Winter is coming". Maybe they acknowledged that people will stop believing in the Walkers if they haven't been seen for centuries, and therefore chose "Winter is coming" to (hopefully) always remind their descendants of the very real Walker threat.
The fact that "Winter is coming" is good advice for three separate but very real reasons, makes it highly commendable as words of wisdom.
Thematically, it also paints the Starks as good guys. All the other houses are focused on describing themselves:
- House Baratheon - Ours is the Fury.
- House Greyjoy - We Do Not Sow.
- House Martell - Unbowed, Unbent, Unbroken.
- House Tully - Family, Duty, Honor.
- House Tyrell - Growing Strong
But the Starks' saying is not meant for self-aggrandizement. It isn't meant to paint the Starks as great people. It is a selfless warning to all people that they must unite.
Comparatively, the Starks are the only house whose saying inspires unity rather than distinguishing themselves from others.
That's a lot of ground work. But here's the application:
When Jon and the Northmen speak about "Winter is coming", they mean to say:
The Walkers and their army of wights are coming for us, and will be on our doorstep soon.
They are not talking about the season, they are talking about the Walkers, who thematically embody the season.
Summary and direct answer to your question
"Winter" has many meanings:
- The actual season of winter
- Hard times
- The White Walkers
But in Season 7, they keep talking about preparing for the winter which is coming. So... Is the Winter here or not?
Meteorologically, winter (the season) is here. But the White Walkers are still marching for the Wall, and are therefore not there yet.
In that sense, "hard times" are also still on the way, since an attack from the White Walkers will somewhat obviously bring hard times.