Yep, I live in Finland where we are quite far ahead of the curve when it comes to renewables and nuclear. This is one large problem. When it's winter and cold and dark, solar does not help. Wind only helps when its windy.
Batteries are coming, and will solve it, but it will still take time for them to be come cheap enough.
Only once we have ~7 days of storage (~100 GWh of storage) will we be able to get rid of the last fossil plants.
Also fun fact: a lot of solar panels in Finland are mounted vertically to walls rather than rooftops. That's because we're so far north that the sun just hangs around the horizon rather than going directly overhead. You end up catching more sunlight pointing directly south.
Budgeting for over a month without power is unheard of. Extreme scenarios ("dunkelflaute") are measured on the order of hours, maybe days, and they get shorter and less frequent the larger the grid is.
Batteries are coming, and will solve it, but it will still take time for them to be come cheap enough.
Only once we have ~7 days of storage (~100 GWh of storage) will we be able to get rid of the last fossil plants.