Similarly, a NASA-affiliated source says that

The final warming is the last stratospheric warming of the season. After this warming, the stratosphere never recovers to its previous state and the vortex breaks up and dissipates. The final warming often occurs in March or April. Sometimes the stratosphere never recovers from what would otherwise be a mid-winter major warming in January or February, and that warming becomes the final warming.

As far as I understand it, once the polar vortex breaks up, one cannot have sudden large temperature dips (like we are having in the northeastern part of the US this weekend of April 2-3, 2016). Thus it would seem that the date of final warming is a good candidate for the official date of the end of "meteorological winter".

However, NOAA defines seasons according to calendar months (see here). Meanwhile, the Swedes define the start of spring as "when the average daytime temperature exceeds zero degrees Celsius on seven consecutive days" (see here).

So: purely on the basis of current best science, would it make sense to define the end of winter as the date of the final warming?

Yes, one could not know that it happened until some time after it happened, but my understanding is that this lag is on the order of days, not weeks.

Yes, such a definition would have the consequence that some years, meteorological winter would end in January. But, so what? Some winters do seem to end in January... while the present one lasts into April.

Yes, one cannot define the demarcation between the spring and summer or summer and fall using the polar vortex as a reference. But, so what? There is no reason why each demarcation couldn't be a separate story. Even the fall/winter boundary need not necessarily be defined as the date of the formation of the polar vortex.

(And while we're at it--and I know this is off topic, but still---why don't the media report the date of the breakup of the polar vortex? It seems it would be of interest to people, given the notoriety the concept has received especially the last couple of winters... In fact, is the date reported anywhere?)