September 7, 2021 - An Early Morning Late Season Large Hail Event In Northeast Wisconsin - Updated 9/10/21

Environmental Summary

In the mid-levels of the atmosphere (Fig.1), a seasonably strong 500 mb trough was deepening over Wisconsin that was accompanied by winds of 70-75 knots and temperatures of -14 to -18 degrees C.  Severe thunderstorms typically develop in areas where strong vertical wind shear (related to storm updraft maintenance/persistence) and sufficient buoyancy (related to the updraft strength/intensity) coincide.  This was the case early Tuesday morning September 7 (Fig. 2).  The white rectangle denotes the area where the HREF ensemble forecast mean deep layer wind shear was greater than 60 knots AND where mean MUCAPE (Most Unstable CAPE) was forecast to range from 1500 to 2000 J/kg. A RAP analysis estimated sounding near Oshkosh, WI (valid at 10 am CDT), indicated MUCAPE values were likely greater than 2000 J/kg (Fig. 3B).  Forecasters can examine the amount of available CAPE in the region where hail typically grows within a thunderstorm updraft.  This region is referred to as the Hail Growth Zone (HGZ) and refers to the layer in the atmosphere where temperatures range from -10 C and -30 C.   Figure 3A indicates that the largest CAPE values (~ 600 J/kg) in the HGZ was located right over Wisconsin. Large CAPE in the HGZ favors very strong updrafts that can support/suspend large hail stones.

Other factors that can also contribute to enhancing the updraft in a thunderstorm, include whether or not the updraft is rotating.  The storms that dropped the largest hail on September 7th were supercell thunderstorms characterized by a persistent cyclonically rotating updraft.  Rotating updrafts generate vertical pressure perturbations that can further strengthen an updraft. Research suggests that thunderstorm (supercell) updraft speeds of 100 mph would be required to support the 4.5" hail stone that fell just north of Appleton! Two rounds of severe storms rapidly moved across the forecast area during the early to mid-morning hours.  As can be seen in the enhanced GOES-16 satellite imagery (Fig. 3), The first round impacted areas north of Green Bay before about 630 am CDT with wind damage reported across the northern portion of Door County.  The second round of storms was more intense and developed very rapidly as the first round of storm moved out over Lake Michigan.  These storms developed ahead of the aforementioned mid-level short wave trough and an accompanying cold front that was surging southeast across Wisconsin during the morning. 

 Fig 1.  HREF mean 500 mb heights, temperature and wind speed                    Fig 2. HREF mean wind shear vectors and most unstable CAPE (shaded)

 Fig 3. Enhance GOES Imagery with CAPE and surface plots overlaid.              Fig 3A.  RAP HGZ CAPE (J/Kg). Values over WI were ~ 600 J/kg

  Fig 3B.  RAP estimated sounding near Oshkosh, WI valid @ 10 am CDT 

Climatology

Let's take a look at the environment on September 7, 2021 and see how it compared to early September climatology.  We know that some of the ingredients that favor large hail include large CAPE, especially in the hail growth zone (typically in the -10 C to -30 C layer), steep 700-500 mb (mid-level) lapse rates and low wet bulb zero (WBZ) heights.  In Figure 5 below, we see that the Most Unstable CAPE, based on nearby RAP estimated sounding profiles near and south of Green Bay was somewhere between 2000  and 2500 J/kg (black dot).  This is close to the 90th percentile (red line) for early September (Fig. 5).  Note that all this instability was above 850 mb with substantial amounts in the hail growth zone region (Figs. 3B & 4).  Also note the strong winds in this layer.  Mid-level lapse rates (700-500 mb layer) were also steep and were also near the 90th percentile for early September (Fig. 7). The steep temperature lapse rates (how quickly the temperature decreases with increasing height) contribute to large values of CAPE in the mid-levels of the atmosphere.  Large mid-level CAPE is one factor that contributes to strong updrafts in thunderstorms capable of supporting large hail stones. Note that the wet bulb zero heights (similar to the freezing level but also accounts for evaporative cooling effects)  were not very low compared to climatology (Fig. 6), but were likely lower to the west of Green Bay as colder (and drier) air aloft, associated with the upper-level trough, was advancing eastward toward the Fox Valley (see Environmental Summary tab, Fig. 1).

Fig.4 (above) - 7am Observed Green Bay Atmospheric Sounding              Fig.5 (above) - Est. MUCAPE near GRB (dot) .vs. Climatology

Fig. 6 (above) - Observed Wet Bulb Zero (WBZ) Height .vs. Climatology      Fig.7 (above) - Observed 700-500 Lapse Rates .vs. Climatology

Radar Overview

A brief radar discussion highlighting the most impactful storms of the event is provided below.

We will take a look at the storms that produced the 2-4+ inch diameter hail that fell from New London and Hortonville, east across the I-41 corridor, to near Wrightstown. Figure 8 (below) is a storm-relative animation (we are moving along with the storm) of the primary hail-producing storm. The left-hand panel is the standard reflectivity (Z).  The right-hand panel is the differential reflectivity (ZDR).  ZDR essentially measures the shape (diameter) of the target illuminated by the WSR-88D Doppler radar beam. The radar is dual-polarized, such that it will send out energy both in the horizonal plane (parallel to the ground) AND in the vertical plane (perpendicular to the ground).  By taking the difference between the horizonal diameter of the target - the vertical diameter of the target, we can determine the shape of the target! Values of ZDR that are large positive (red colors) indicate big pancake-shaped water droplets.  Hail on the other hand is typically more round in shape, thus the ZDR values will be small or near zero (greenish-blue or gray colors). So, forecasters can identify where hail exists by looking for areas where near zero ZDR values are collocated with very high reflectivity (Z) values (Fig. 8). The circled areas on the radar animation identify the persistent hail cores within the primary thunderstorms that produced swaths of large hail across east-central Wisconsin.  As you examine the radar animation, note that the first hail core develops and moves east across Waupaca County, while a second core develops to the south that moves east across southern Outagamie County, dumping hail of 3 to 4+ inches in diameter just north of the city of Appleton.  Radar snapshots when the supercell was located just west of Hortonville (Fig. 10) and west of Mackville (Fig. 11) are shown below.  Note the strong rotation associated with this storm.  This rotation was only observed aloft, generally above 10, 000 FT AGL, thus any tornado threat was minimal.  However, as mentioned in the Environmental Summary section, strong mid-level rotation acts to further enhance storm updrafts than can support very large hail. 

Fig. 8 - Storm relative animation denoting the evolution of the hail storms.      Fig. 9 - Preliminary hail reports across the Fox Valley area.

Fig. 10 GRB radar data - Reflectivity (left) and Storm Relative Velocity (right)    Fig. 11 - Same as Fig. 10 except as storm was passing north of Appleton

Preliminary Storm Reports

000
NWUS53 KGRB 071936
LSRGRB

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE GREEN BAY WI
236 PM CDT TUE SEP 7 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0616 AM     TSTM WND DMG     3 W EPHRAIM             45.15N 87.22W
09/07/2021                   DOOR               WI   PARK/FOREST SRVC 

            TREES DOWN IN PENINSULA STATE PARK. REPORT 
            FROM THE DNR. DELAYED REPORT. TIME ESTIMATED 
            BY RADAR. 

0630 AM     TSTM WND GST     4 N NEWPORT STATE PARK  45.29N 86.98W
09/07/2021  M42 MPH          DOOR               WI   BUOY             


0632 AM     TSTM WND DMG     JACKSONPORT             44.98N 87.18W 
09/07/2021                   DOOR               WI   LAW ENFORCEMENT 

            TREES AND WIRES DOWN AROUND AND NORTH OF JACKSONPORT.


0810 AM     HAIL             ABRAMS                  44.78N 88.06W
09/07/2021  M0.75 INCH       OCONTO             WI   BROADCAST MEDIA  



0825 AM     HAIL             1 W NEW LONDON          44.38N 88.77W
09/07/2021  M1.00 INCH       WAUPACA            WI   TRAINED SPOTTER  



0828 AM     HAIL             NEW LONDON              44.38N 88.75W
09/07/2021  M0.75 INCH       WAUPACA            WI   TRAINED SPOTTER  



0838 AM     HAIL             2 NNE STEPHENSVILLE     44.38N 88.57W
09/07/2021  M1.00 INCH       OUTAGAMIE          WI   EMERGENCY MNGR   



0839 AM     HAIL             NEW LONDON              44.38N 88.75W
09/07/2021  E1.75 INCH       WAUPACA            WI   TRAINED SPOTTER  

            2 MILES WEST OF HIGHWAY M AND S. 

0845 AM     HAIL             MACKVILLE               44.34N 88.41W
09/07/2021  M1.75 INCH       OUTAGAMIE          WI   TRAINED SPOTTER  



0845 AM     HAIL             4 W MACKVILLE           44.34N 88.49W
09/07/2021  M4.00 INCH       OUTAGAMIE          WI   TRAINED SPOTTER  

            CORRECTS LOCATION AND SIZE OF TENNIS BALL SIZED
            HAIL FROM PREVIOUS REPORT. HAIL OF 4.1 INCHES 
            WAS MEASURED. SIX MILES NORTH OF THE APPLETON
            AIRPORT. HAIL HITTING THE GROUND AND BREAKING. 

0846 AM     HAIL             4 N APPLETON            44.32N 88.39W
09/07/2021  E3.00 INCH       OUTAGAMIE          WI   PUBLIC           

            REPORT FROM FOX VALLEY LUTHERAN HIGH SCHOOL. 
            TIME ESTIMATED BY RADAR. 

0847 AM     HAIL             HORTONVILLE             44.33N 88.63W
09/07/2021  M1.00 INCH       OUTAGAMIE          WI   TRAINED SPOTTER  

            IN DOWNTOWN HORTONVILLE. 

0847 AM     HAIL             4 NNE APPLETON          44.32N 88.38W
09/07/2021  M3.00 INCH       OUTAGAMIE          WI   TRAINED SPOTTER  

            NORTH OF APPLETON ON THE INTERSECTION OF JJ 
            AND E. 

0847 AM     HAIL             2 NNW KAUKAUNA          44.29N 88.28W
09/07/2021  M3.75 INCH       OUTAGAMIE          WI   PUBLIC           



0847 AM     HAIL             5 NNE APPLETON          44.33N 88.37W
09/07/2021  M4.50 INCH       OUTAGAMIE          WI   PUBLIC           

            CORRECTS SIZE OF THE PREVIOUS HAIL REPORT 
            FROM 5 NNE APPLETON. 4.5 INCH HAIL MEASURED. 
            REPORT VIA SOCIAL MEDIA. TIME ESTIMATED BY 
            RADAR. 

0848 AM     HAIL             2 ENE MACKVILLE         44.35N 88.37W
09/07/2021  M2.75 INCH       OUTAGAMIE          WI   PUBLIC           

            BROKE HOME WINDOW. 

0850 AM     HAIL             5 SSE BLACK CREEK       44.39N 88.42W
09/07/2021  E1.00 INCH       OUTAGAMIE          WI   AMATEUR RADIO    

            ONE INCH OR GREATER HAIL NEAR HIGHWAY S AND 
            47. 

0850 AM     HAIL             2 S MACKVILLE           44.31N 88.41W
09/07/2021  E2.50 INCH       OUTAGAMIE          WI   LAW ENFORCEMENT  

            NORTH SIDE OF APPLETON. 

0853 AM     HAIL             5 SE NEW LONDON         44.33N 88.67W
09/07/2021  E2.00 INCH       OUTAGAMIE          WI   AMATEUR RADIO    

            HAILED FOR 2 MINUTES. HIGHWAY T AND TT. 

0855 AM     HAIL             FREEDOM                 44.38N 88.28W
09/07/2021  M2.75 INCH       OUTAGAMIE          WI   TRAINED SPOTTER  



0857 AM     HAIL             WRIGHTSTOWN             44.33N 88.17W
09/07/2021  E2.00 INCH       BROWN              WI   PUBLIC           



0859 AM     HAIL             WRIGHTSTOWN             44.33N 88.17W
09/07/2021  M2.50 INCH       BROWN              WI   COCORAHS         



0902 AM     HAIL             1 SE LITTLE RAPIDS      44.37N 88.11W
09/07/2021  M1.50 INCH       BROWN              WI   AMATEUR RADIO    



0904 AM     HAIL             1 N GREENLEAF           44.32N 88.09W
09/07/2021  E1.50 INCH       BROWN              WI   TRAINED SPOTTER  



0906 AM     HAIL             2 N APPLETON            44.29N 88.40W
09/07/2021  M1.00 INCH       OUTAGAMIE          WI   AMATEUR RADIO    

            LOCATED APPROXIMATELY 1 MILE EAST OF MEADE 
            STREET AND NORTHLAND AVENUE. TIME OF REPORT 
            ESTIMATED. 

0910 AM     HAIL             5 NNE APPLETON          44.33N 88.35W
09/07/2021  M1.00 INCH       OUTAGAMIE          WI   TRAINED SPOTTER  

            NORTHEAST SIDE OF APPLETON AT JJ AND FRENCH 
            ROAD. 

0910 AM     HAIL             2 N APPLETON            44.29N 88.40W
09/07/2021  M2.00 INCH       OUTAGAMIE          WI   AMATEUR RADIO    

            OCCURRED AT NORTHLAND AVENUE AND MEADE 
            STREET. 

0912 AM     HAIL             2 N APPLETON            44.29N 88.40W
09/07/2021  E1.50 INCH       OUTAGAMIE          WI   TRAINED SPOTTER  



0913 AM     HAIL             3 NNE KIMBERLY          44.33N 88.29W
09/07/2021  M4.00 INCH       OUTAGAMIE          WI   PUBLIC           

            CORRECTS THE TIME OF THE PREVIOUS HAIL 
            REPORT FROM 3 NNE KIMBERLY. 

0918 AM     HAIL             KAUKAUNA                44.28N 88.27W
09/07/2021  E1.00 INCH       OUTAGAMIE          WI   TRAINED SPOTTER  



0918 AM     HAIL             KAUKAUNA                44.26N 88.26W
09/07/2021  M1.50 INCH       OUTAGAMIE          WI   TRAINED SPOTTER  

            NEAR HWY 55 / CE. 

0920 AM     HAIL             5 SE AURORAVILLE        43.99N 88.94W
09/07/2021  E1.00 INCH       WAUSHARA           WI   TRAINED SPOTTER  

            JUST NORTH OF BERLIN. 

0920 AM     HAIL             WRIGHTSTOWN             44.33N 88.16W
09/07/2021  E0.75 INCH       BROWN              WI   COCORAHS

0927 AM     HAIL             4 NW TWO CREEKS         44.34N 87.61W
09/07/2021  E1.75 INCH       KEWAUNEE           WI   PUBLIC           

            THE HAIL CAUSED DAMAGE TO CARS. 

0932 AM     HAIL             MARIBEL                 44.26N 87.80W
09/07/2021  M1.25 INCH       MANITOWOC          WI   TRAINED SPOTTER  

            HIGHWAY NN AND HIGHWAY Z. 

0937 AM     HAIL             2 NNE WAYSIDE           44.27N 87.92W
09/07/2021  E1.25 INCH       BROWN              WI   PUBLIC           

            REPORT FROM MPING. 

0939 AM     HAIL             2 S OSHKOSH             43.98N 88.55W
09/07/2021  M1.00 INCH       WINNEBAGO          WI   TRAINED SPOTTER  

            AT EAA. 

0949 AM     HAIL             1 N STOCKBRIDGE         44.07N 88.30W
09/07/2021  E1.00 INCH       CALUMET            WI   PUBLIC           

            REPORT FROM MPING. 

0958 AM     HAIL             5 SE WITTMAN FIELD      43.93N 88.47W
09/07/2021  E0.75 INCH       WINNEBAGO          WI   TRAINED SPOTTER  



1004 AM     HAIL             1 WSW CHILTON           44.02N 88.18W
09/07/2021  E1.25 INCH       CALUMET            WI   PUBLIC           

            REPORTED AT CHILTON HIGH SCHOOL. 

1015 AM     HAIL             CHILTON                 44.03N 88.16W
09/07/2021  E1.00 INCH       CALUMET            WI   PUBLIC           

            REPORT FROM MPING. 

1025 AM     HAIL             8 ESE VALDERS           44.02N 87.73W
09/07/2021  M1.00 INCH       MANITOWOC          WI   PUBLIC           



1048 AM     HAIL             1 SE CLEVELAND          43.90N 87.74W
09/07/2021  M1.25 INCH       MANITOWOC          WI   PUBLIC           

            REPORT RECEIVED VIA SOCIAL MEDIA. 

Photos

Final Note:

Although the morning of September 7, 2021 started out quite stormy with many battered by large hail, there is always a silver lining.  This beautiful photograph below (Courtesy Patrick LaPrise, Marinette, Wisconsin) was taken over the Marinette/Menominee area looking north-northeast over the Menominee Bay at 6:13 pm CDT early that evening.