Soil Temperature Map
Generally, soil temperatures must reach 50 degress F before morels will begin to fruit in your area. This is a good rule of thumb for when to start looking, not an exact number to hold fast to. Kuo (2005) suggests morels can be found at soil temperatures as low as 47 deg. F. He does not specify whether this is for yellows or blacks. I would suspect blacks can begin forming at even lower soil temperatures. This map provides soil temperatures that are updated daily.
NOAA Climate Prediction Center - Long Range Forecast - Temperature and Precipitation
State Specific Maps:
Another method that some researchers use to predict when morels will begin to fruit are called degree-days. This looks at the number of days that have been above freezing so far in the year, and combines that with the number of degrees it has been above freezing - forming a single metric: degree-days.
Researchers have found that yellow morels fruit when somewhere between 365-580 degree days have been attained in the year. The more degree-days are attained before yellow morels are found, the longer the fruiting season will be.
A degree-days map for the upper Midwest is shown below:
Degree-day data across the United States.