Your Forsythia look alike could be winter jasmine, Jasminum nudiflorum, one of the only two species with opposite leaves and yellow flowers. A native of China, it survives outdoors in sheltered places as far north as Boston, Massachusetts. In mild climates it blooms throughout much of the winter; in harsher ones, it blooms in late winter, usually a little ahead of forsythias and before its leaves expand. It grows from 3 feet to, when grown against a wall or support, nearly 15 feet tall, producing  slender branches and rigid, hairless, four-angled branchlets.

There are other early-Spring blooming shrubs that also flower in yellow and might be confused with forsythia. Lindera benzoin (Northern spicebush) is a U.S. native plant with greenish yellow flowers appearing in March. It grows in USDA zones 4 to 9. Ribes odoratum (clove currant) blooms in bright yellow in April and can be grown in USDA zones 4 to 8.

Courtesy of the NYBG Plant Information Service