Answer: Laburnum x Watereri `Vossii` will bloom for just a short time, so enjoy it while it lasts. You are lucky that you have kept it alive long enought to see blooms twice; this is not an easy tree to grow well in your area. In my experience, this tree does best in areas with cool summers and milder winters such as parts of the Pacific northwest or in the United Kingdom.
I think the mild winter we had combined with maturity probably would explain the display this year. This tree is considered hardy into zone 6, and your zip code places you in 6A or the coldest part of the zone. (Depending on your microclimate it might be as cold as zone 5.) Since you have blooms you must have planted it in a reasonably favorable site. So apart from hoping for mild winters and cool summers, there isn't much you can really do with the possible exception of providing a windbreak in winter and watering in times of drought.
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