The radar loop below shows the emergence of light reflectivities, which appear to be moving to the west. This image is a radar loop from Sunday morning showing the re-emergence of the mayflies. Notice they spread west southwest in the direction of the mean wind flow. It's fascinating to see just how high the reflectivities are with areas of greater than 40 dbz echoes (yellows) which are higher than what is typically observed with insects being detected on radar.
So why did they appear to emerge over the western suburbs? It is believed that a lake breeze carried these mayflies inland on Saturday evening. Looking at the radar loop below from Saturday evening we can see a pronounced lake breeze boundary oriented from northwest to southeast across the western suburbs. Near the end of the loop the boundary begins to dissipate. Notice that this area corresponds well with the area of increasing reflectivity during the early morning hours, shown on the left. Therefore it is likely that the mayflies were picked up by and carried west by the boundary Saturday evening. Then, radar showed blossoming reflectivity around sunrise Sunday morning right in the location of where the boundary dissipated Saturday evening, so this was mayflies taking off early Sunday morning from where the boundary had left them Saturday evening!
The radar loop above is from Saturday evening, notice fine line on radar pushing west across the western suburbs. Notice the yellow and oranges in the fine line, those are unusually strong reflectivity values for an outflow or lake breeze boundary.
The image above is a 4 panel of different variables depicted on radar from Saturday evening from where the insect laden boundary began to dissipate. The upper left panel is reflectivity (much like shown in the loop above). The upper right panel is differential reflectivity which showed extremely large values within the boundary which means the objects being detected were oriented horizontally and were shaped flat like pancakes. The lower right panel is an image of correlation coefficient, the purple and reddish colors are high values, meaning the shape and size of the objects being detected are fairly similar. Values this high are somewhat unusual with insects, but in this case the concentration of mayflies is very high and type of insects is fairly uniform resulting in the unusually high values.
The image above is a 4 panel of radar data much like the one above, but this one from Sunday morning when the mayflies were taking off again. The values again are very similar with very high differential reflectivity values (upper right) and fairly high correlation coefficient (lower right), though lower than the previous evening as the mayflies were more spread out and lower concentration as indicated by the lower reflectivity (upper right panel). The lower left panel is an algorithm of the radar that attempts to determine what the radar is picking up based on the various variable. Interestingly, the algorithm thought it was detecting large rain drops (yellow areas in the lower left panels). The mayflies tricked the radar algorithm into thinking they were large rain drops because the reflectivity values were higher than normal for insects as were the correlation coefficients (lower right). Also, large rain drops tend to flatten out like pancakes when they fall due to friction, so the large differential reflectivity values in the upper right panel were consistent with those that would be expected with large rain drops.
The image above is of a swarm of mayflies, based on the reflectivity values picked up on by radar, there were likely scenes similar to the one above this morning across the Fox River valley and points west into north central Illinois.