Forecasting snowfall amounts is one of the most difficult challenges for meteorologists. Yesterday's storm certainly confirms that statement. Snowfall amounts varied from nearly a foot in Lake County (10" in Beach Park) to just 1.4" officially at O'Hare. The graphic below from the National Weather Service office in Romeoville shows the wide snowfall gradient across the area.
The difficulty with a storm like the one we saw Thursday is determining when the change from rain to snow will occur. The "Ping Project" can help. Ping stands for positive identification near the ground. The NOAA National Severe Storms Laboratory has partnered with the University of Oklahoma to launch a free "Ping" app that allows users to report precipitation. The app is availabe for both Iphones and Android mobile devices. The Ping Project website is another way to report precipitation. Ping users began to report snow late yesterday morning in Lake and McHenry counties indicating the change from rain to snow was occurring there first. That early transition to snow helped in part to produce the heavier snow totals there. The graphic below shows how the ping reports helped identify the rain/snow line.