The reality is…
Your plans are really really spread out across North America… you are going to be encountering 4 very different climate zones.
This would be even more pronounced if you come in the Wintertime… December thru March.
As you will find that North America, especially the USA sees a lot more contrast in Winter temps than Australia gets.
In December for example, it could easily be any of the following Daytime Highs…
Las Vegas = 15 C (60 F)
New York City = 5 C (40 F)
Orlando = 21 C (70 F)
Sint Maarten = 30 C (86 F)
But NONE of these are guaranteed. Mr TTR & I travel a lot. We’ve seen SNOW as far south as Florida in December (infact the Southern USA got hit with a brutal Cold Front last week that saw Snow in Florida… and freezing rain and awful temps elsewhere in what normally should be a “warm” part of the country). So it can happen.
And bad weather in areas can and does bring things to a crawl… be it on the roads, or in the air. Which sucks if you are trying to get somewhere on vacation, and only have so much time (and money) at your disposal.
For our Pre-Wedding Trip last December, we were in New Orleans, and it was cold and rainy. In the 30s and 40s F, in an area where it should have been in the 60s and 70s.
So short of packing a ton of clothing options (which in today’s penny pinching airline world) is difficult & expensive to say the least you’ll be frustrated !!
Things will be somewhat better in March, but that is still technically Winter in the North… snow is still very much a possibility for NYCity in that month as well.
April is the beginning of Spring… and there will still be a contrast between regions but not so drastic as Winter, and you can get away with layers and without boots, hats, scarves & mitts.
May would be even better… I’d say the most ideal month.
Summer is good for everywhere north of the Mason-Dixon Line (39 Degrees North Latitude) but can be unbearably hot & muggy come June, July, August & September. And that is also Hurricane Season in the Atlantic.
Stuff to consider
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As for Spring Break.
Spring Break is set by each State / Province / Territory and sometimes even School Districts. So there isn’t one particular week that is THE ONE
The farther north the location, usually the earlier the Winter / Spring Break happens.
For example, the Schools in Quebec typically have their Winter Break at the end of February, Ontario in March. Many US Schools centre theirs around when Easter Falls.
Basically no matter when you travel February thru April you are going to run into Families with Kids on Vacation.
Hope this helps