I kinda watch the soil temps but mostly follow the spring bloom. Watch the Lilac bloom, it's been pretty fail safe for me for yellow morels. Good rain right around lilac bloom should be good to go. Black morels on the other hand (central/northern MN) appear earlier than yellows. Seems like a couple weeks earlier. Also deep woods will pop much later than open areas with direct sun light. Man I love hunting these damn things!!
Check this site out to follow the leaf and bloom index for each spring.