It's always interesting looking back on old ice extent records and we are lucky that, unlike the Arctic, the Baltic area has always been well populated and could piece together a detailed chart back to 1720.
I do wonder if a complete freeze over of the Baltic will ever occur again in my lifetime. Unless something dramatic changes, the odds are increasingly stacked against it.
Looking back over the very cold northern European winters of the past, a big feature was the Scandinavian High Pressure system. Every few years it seems, it would exert a strong influence and import cold air of Russian origin over Europe. When this happened during the winter months, the Baltic Sea ice would grow rapidly. They would often block out the Atlantic air for several weeks eg. February 1985.
I have heard some debate that a strong, persistent Scandinavian High hasn't existed since Feb 1991. I don't know if this is necessarily true - but for whatever reason the "Scandi" High seems to be more short lived, of weaker influence, occurring too late in the season (eg March 2013) and more often centred (pushed?) to the east over western Russia. And a feature of recent European winters, including this one, is for the deep cold then to be channelled SW from Russia down into the Balkans and SE Europe. With NW Europe once again living a charmed life.
Perhaps the steady decline in sea ice over the Barents/Kara seas has had an effect on the potential formation and sustainability of the Scandi High. Even as I type this, the NWP models are predicting yet another Scandi High to build, but it looks like it may be a short lived affair with no real persistent cold to rapidly develop the Baltic Sea ice cover.
I can see at least 3 major issues preventing a future max ice cover event for the Baltic :
1) Synoptic weather patterns. In particular the lack of persistent Scandinavian High as mentioned already.
2) Global warming. In
this study (2007) it is stated that the warming trend for the entire globe was about 0.05°C/decade from 1861–2000, while the trend for the Baltic Sea basin has been somewhat larger, 0.08°C/decade. Warmer global temperatures have meant it's going to be increasingly difficult to freeze the sea.
3) Increased Baltic SSTs. By the end of this century the projected decrease of ice cover is dramatic, with the Bothnian Sea, large areas of the Gulf of Finland and the Gulf of Riga, and the outer parts of the southwestern archipelago of Finland becoming, on average, ice free. This may happen even earlier.