Summary of May 20th Thunderstorms

Strong to severe thunderstorms affected western and central Upper Michigan during the afternoon and evening hours of May 20th, 2012. Low-level moisture and abundant sunshine ahead of an approaching cold front allowed for moderately unstable air to develop across Upper Michigan. Many of these storms were small and short-lived, but produced some large hail, gusty winds, and very heavy rainfall. The first loop below is of the radar from early afternoon to early evening. The cold front induced storms in a somewhat linear fashion.


One of the advantages of the recent dual-polarization radar upgrade was noticed this afternoon on many of the storms. Within the oval on the upper left and lower right images below, a feature called a "three-body scatter spike" is apparent. This feature occurs when large hail inside a storm sends the radar beam to the ground, back to the storm, then back to the radar (hence, "three" bodies). Though this has been visible with multiple radar products in the past, the inclusion of correlation coefficient (how similar the objects are within a storm, for example) with the recent upgrade increases confidence that there is indeed hail in the storm. The very low  values (blue) are a big indicator of a three-body scatter spike (lower right image).

Within the rectangle, we notice that the upper right image, ZDR, has a lower value than surrounding locations. ZDR measures the shape of the objects, with flat objects, like large rain drops, producing high positive values. On the other hand, spherical objects, like hail, result in values closer to zero, as seen here. Thus, confidence is once again increased with regards to whether or not hail is located within this storm.

The second loop below is of the visible satellite from mid morning to early afternoon. Notice how cumulus clouds (small pockets of white) developed across central Upper Michigan during the afternoon. Also notice that there is a lack of cumulus clouds in the east. South winds off of the cool waters of Lake Michigan prevented the air from becoming as unstable as further west, thus inhibiting clouds. The boundary of this air, seen as a denser white line across eastern Marquette county, kicked off a few thunderstorms in the afternoon ahead of the main line of storms to the west.


Below is a summary of reports. In addition, we had numerous reports of pea sized hail with many of the storms, including here at the NWS office near Negaunee.

Time        Event-Magnitude      City, County

1233 PM     Hail-0.50 Inch      Rockland, Ontonagon       

0117 PM     Hail-1.00 Inch      2 E Redridge, Houghton       

0130 PM     Hail-0.75 Inch      3 ENE HoughtonHoughton        

0130 PM     Hail-0.88 Inch      2 E HoughtonHoughton       

0132 PM     Hail-0.50 Inch      Marquette,Marquette      

0134 PM     Hail-0.50 Inch      3 NE Houghton,Houghton      

0139 PM     Hail-0.88 Inch      Ironwood,Gogebic        

0145 PM     Hail-1.00 Inch      Calumet,Houghton      

0357 PM     Hail-0.75 Inch      4 S Mass City,Ontonagon       

            Wind gusts up to 40mph as well.

0410 PM     Hail-0.75 Inch      5 ESE Chassell, Houghton      

0440 PM     Hail-0.50 Inch      Houghton,Houghton     

0450 PM     Hail-0.50 Inch      L`Anse,Baraga    

0530 PM     Tstm Wnd Dmg         6 S Witch Lake, Dickinson

            24 Inch White Pine tree knocked over. Estimated winds of 60mph.

0534 PM     Tstm Wnd Dmg         2 SSE Witch Lake, Marquette 

            Trees down across highway M-95 along the Marquette/Dickinson

            County line.

0546 PM     Tstm Wnd Dmg         5 WNW Big Bass Lake, Marquette 

            Small limbs down. Patio furniture blown across the yard.

            Very strong winds, no estimate.

0600 PM     Hail-0.50 Inch       2 W Ishpeming, Marquette  

0604 PM     Hail-0.75 Inch       Kingsford, Dickinson

0606 PM     Hail-0.75 Inch       1 S Granite Bluff, Dickinson  

0635 PM     Hail-0.50 Inch       2 SSE Norway, Dickinson

0707 PM     Hail-0.75 Inch       4 NE Gwinn, Marquette

0721 PM     Hail-0.75 Inch       1 SW Shot Point, Marquette

0832 PM     Heavy Rain-1.75 Inch 10 E Ontonagon,Ontonagon 

            Total rainfall since noon. Small creek on residence is

            over its banks.


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