Thursday morning brought widespread rain across northern Illinois. Over far northeast Illinois (Lake and McHenry Counties) this began to mix with and then eventually change over to all snow late in the morning. This changeover was occurring due to cooling in the atmosphere, mainly from diabatic processes as a result of strong dynamics. Meteorologists were able to detect the changeover through various means as shown below.
The NWS Doppler Radar provides dual-polarization data which can tell a great deal about the radar sample, including precipitation type and composition of the sample at the radar beam height. Correlation coefficient (CC) is one of the dual-pol data sets. This provides magnitudes between 0 and 1 and basically tells how dissimilar or similar, respectively, the sample is. In other words, values of 0.97 to 0.99 indicate the precipitation is very homogenous at the radar beam height, likely all one type. Lower values under 0.97 indicate a mix. Parts of the atmosphere that have a rain/snow mix, such as the melting layer, will almost always show up as low values of CC.
During Thursday morning, the melting layer "ring" as seen in the CC data would begin to contract inward over Lake and McHenry Counties as the atmosphere was forced cooler and precipitation began to change over to one type...snow. In the two-hour late morning loop below, reflectivity (power of the returns) is shown to the left while CC is shown to the right. Yellow and orange pixels indicate low CC values indicative of a heterogeneous radar sample and a mix. Keep your eye on the far northeast two counties in Illinois.
There is nothing better to meteorologists than ground truth reports and the NWS is fortunate to have spotters and key partners which are always reporting to us. The public can also report through social media as well as now the PING Project. Below are hourly maps of precipitation type from public reports during Thursday morning. In the red outlined area are Lake and McHenry Counties and one can note the reports of mix followed by snow.
To report precip type where you are at, check out the PING project web site. There even is a smart phone app available!