Bem, no Epílogo do ADWD é
Snowing heavily in King's Landing and a white raven arrives from the Citadel indicating that winter has come
Assim, pelo menos, pode ficar abaixo de zero nessa parte de Westeros.
Há mais alguns detalhes em esta resposta , em particular:
Mr Martin, why does Westeros seem the only place effected by the Others and the long winters? The other parts of the world seem not to care.
George_RR_Martin - Westeros is not the only place affected, but it's affected most strongly, because it's the only landmass that extends that far north. The other continent is bounded to the north by an icy polar sea.
O que parece indicar que os lugares mais ao norte são mais afetados pelo inverno.