Warnings and alerts were sent out in a variety of ways on April 9th, from weather radios and outdoor warning sirens to text messages and weather apps relaying alerts on cell phones and smartphones. All of those warnings and alerts were issued by the National Weather Service office in Chicago, where teams of meteorologists were working feverishly to give out warnings ahead of the advancing thunderstorms.
The NWS staff were preparing days ahead of the storms. They worked out a schedule to bring in extra staff members needed to cover the storms, and developed how they were going to get the word out about the significant weather event looming in the next few days.
By the day of the storm, the National Weather Service meteorologists were adjusting the staffing schedule to have all hands on deck right before the storms struck. Multiple teams were set up to monitor the radar and track individual storms, while other meteorologists were manning the phones and computers to send out messages to emergency responders, local airports, and out over social media.
As the storms erupted, the radar teams watched each storm cell, and looked for signs of a tornado like a hook-echo on the radar (a curved part of a supercell thunderstorm where rotation is on-going), watched the velocity data to see where the storm was rotating, and looked at other layers of radar data that showed where particles were different from rain and hail. This is a sign that debris may be being picked up by a tornado.
From there, a few computer mouse clicks can send out a warning, complete with a message tailored specifically to that storm. In the case of the Fairdale tornado, text such as "damage threat- considerable" and "a particularly dangerous situation" helped emphasize the violent nature of this tornado. This wording is reserved only for the most dangerous tornadoes, which applied to the size and strength of the Fairdale tornado.
In total, 11 tornadoes touched down across Illinois on April 9th, making for a very chaotic situation at the National Weather Service as they tracked multiple tornadoes, many at the same time. The trained professionals, however, know how to communicate effectively during these loud and rapidly changing situations, and were able to effectively give warnings about the many dangerous storms that afternoon. Once the storms had cleared, survey teams from the Chicago and Quad Cities offices went out to asses the damage, and rate the tornadoes based off of their findings.