Layers, Layers, Layers,
We are from Boston, and were on a combination winter/summer vaction on Australia, New Zealand and Fiji, from early June through early July, 2010. June 21 is the start of winter in New Zealand, where it was subfreezing in Queenstown to near 100 with high hmidity in Fiji. Queenstown in late June is not as cold as Boston in January, but there was far more outdoor activity in Queenstown. We rode a jet boat and cruises Milford sound with temperatures in the 20s.
We did not go carry-on, however., but we were gone for twice asa long. We had 42" suitcases and small backpacks. Central America will be the least of your problems, because the clothing should not be very bulky. A good comdination hooded rain jacket/wind breaker and a good fleece jacket, warmer than the GCT ones, along with a sweater should take care of Boston, while the wind breaker/rain jacket or the fleece should take care of Central America. You should also have a wool knit hat and gloves for Boston. Waterproof hiking boots should take care of both. Convertible pants, with zip off legs would be great for both. In general, they are wind and relatively water resistant, light to pack and dry quickly after washing. The North Face makes them that are a bit heavier than most and hold there shape better, REI and Columbia are lighter and thinner.