I would take it to be a transition from conditions-of-traditional-autumn directly to conditions-of-traditional-spring, skipping the winter that's usually very distinct.

Which has happened in large parts of Europe this year, as the traditional conditions for announcing the start of "meteorogical winter" (which, for my area, is the daily mean temperatures being below 0 °C (32 °F) for five consecutive days) have not happened at all; we've had weather that matches "late autumn" all through from October till now.