In spring, when a young person's fancy turns to . . . tennis, the best kind of love match occurs under sunny skies. For Kristen Moore and Michael McGrath, the 11th Avenue park in Salt Lake City provides the venue for a little racquet action in an unseasonably balmy 69 degrees. The best news of all comes from Bill Alder, meteorologist for the National Weather Service, who has erased an earlier prediction of an upcoming storm. This tennis-biking-gardening-garage-sale weather, with temperatures creeping into the 70s, is supposed to last through the weekend.