Last Fall, all the existing nests died when we had our first hard freeze, but prior to that those nests had raised new queens, that mated in the nest, and then left to go dormant for the winter. Those new queens will be the ones that will start new nests this spring. Normally, a large portion of those queens (one queen = one nest) die during the course of a normal winter, but last winter was the second warmest on record, so more queens probably survived. If we have a normal spring, those queens will start their nests in early-mid May. They develop slowly and are usually not noticed until late July. At that time they reach critical mass and explode in size and population, literally overnight. They can nest in the ground or in the structure of a building (yellow jackets only) or they can build a large gray paper nest similar to the picture above (yellow jackets or bald faced hornets).
So, keep and eye out as we progress through summer, and be aware of your surroundings, because these critters can be truly dangerous, especially if you are allergic to bee stings.