Large Hail in Gladstone June 18, 2012

Severe thunderstorms affected portions of south central and eastern Upper Michigan during the afternoon hours on Monday, June 18th. One of these storms produced over 2" diameter hail around Gladstone (see below). The radar loop below shows the evolution of the storms from 1-6 pm EDT. The yellow boxes are severe thunderstorm warnings, the blue boxes are special marine warnings, the green box is a flood advisory, and the red boxes are tornado warnings (none issued in Upper Michigan).

Chuck Bassett forwarded a picture of hail over 2" in diameter from this afternoon (around 3:35pm), just north of Gladstone. 

Below is a radar screen capture from just before the large hail in the above image was observed. Once again, the recent upgrade of the NWS Marquette radar to dual-polarization technology has aided in the identification of large hail. Two key features are evident in the image that reveal a very high likelihood of large hail.

1) Three Body Scatter Spike (TBSS) - This feature is the long, skinny shape extending down and right of the main thunderstorm core (pinks and purples in the upper left image). When the radar beam hits large hail in the storm, it is sent toward the ground. The ground then sends the energy back up to the storm core, where it hits the hail and returns to the radar. The increased time it takes for the radar beam to travel back to the radar makes the radar perceive that the echoes are farther away than they really are. This is because the radar uses the length of time it takes the radar energy to return in order to calculate the distance from the radar. In other words, the longer it takes for energy to return, the farther the object is from the radar. Therefore, a "spike" develops down-radial (away from the radar) from the storm. The radar in this case is located up and to the left, or NNW, of the storm.

The TBSS is also very clear in the correlation coefficient (CC) image (lower right). Since CC measures the similarity of objects within the radar pulse, the random nature of the energy returning to the radar results in poor correlation in object sizes. Therefore, very low CC values are measured.

2) Near-zero Differential Reflectivity (ZDR) - ZDR (upper right) is a measurement of object shape. Flat objects, like large rain drops, are wider than they are tall, so ZDR is positive. Spherical objects, like hail, do not have a favored size orientation, so ZDR is near zero. Note how the pocket of gray (ZDR near zero) in the upper right image lines up with the very high reflectivity in the upper left image. Additionally, the TBSS extends away from this core. This type of feature in ZDR data is a major advantage with the dual-pol upgrade.