Winter has finally come to Game of Thrones. A commercial for "The Winds of Winter" shows a white raven flying over Westeros, a sign that winter has finally come after a ten year wait. The maesters in Oldtown send white ravens to all the keeps in Westeros to signal a change of seasons, as Westeros's seasons are sporadic and unpredictable. Unlike the real world, summer or winter can last for years in Game of Thrones and change with little warning. When Game of Thrones began six years ago, Westeros was experiencing its longest summer ever, which many believed was an ominous portent of things to come.
So what will winter bring to Westeros? While the North will be the most impacted by the change of seasons, cold weather will hit nearly every part of the Seven Kingdoms. Snow will fall as south as Highgarden and Kings Landing, and travel will be much more treacherous, even on the King's Road. Only one of the Seven Kingdoms will be spared from the bad weather: the southern deserts of Dorne. Snow rarely falls in the city of Sunspear, even during the harshest of winters.
To many, a long summer is a sign of a harsh winter, which may give the White Walkers the opening they need to advance south of the Wall. Legends stated that the White Walkers first invaded Westeros during a winter that lasted a generation, pushing humanity to the brink of extinction. The White Walkers were stopped only by the Last Hero (who may or may not be the Azor Ahai and the prince that was promised frequently mentioned by Melisandre), who founded the Night's Watch and pushed the icy foes back in the fabled Battle of the Dawn. It's unclear just how far south the White Walkers went into Westeros, but Bran the Builder (one of the Starks' ancestors) built the Wall to permanently keep them at bay.
Winter may also bring the Seven Kingdoms to a dangerous tipping point. For several years, the kingdoms have been plagued by wars and rebellions, leaving few lords prepared for the winter. War has touched nearly every corner of the Seven Kingdoms, draining even the fertile Riverlands of their resources. Even the North, which deals with cold weather even during the summer seasons, is probably ill prepared due to years of turmoil after the Boltons drove the Starks out of Winterfell. The Starks are back home, but it's too late to restore real order before winter sets in.
There's also the practical problems that winter brings, such as keeping an army healthy and fed during the winter. This may be particularly troublesome for the Lannisters, who have one of the few armys still standing after the War of the Five Kings. The Lannister army is currently stationed in the Riverlands, which means they'll need to rely on a still hostile populace for food and supplies. Winter also gives guerrilla groups like the Brotherhood without Banners the perfect opportunity to do real damage to a larger, more organized force.
No matter what happens in "The Winds of Winter", the change of seasons ensures that Game of Thrones will be a very different show when in returns next year. The Starks' motto is "Winter is Coming", let's hope someone was listening.