Overview
On the morning of April 12, 2014, a strong surface warm front lifted north across northern Illinois, but very warm and moist air several thousand feet above the surface had already rushed into the area. Thus the atmosphere was characterized by warm and unstable conditions aloft and relatively cool and dry conditions at the surface.
Rockford Airport at 9am had a temperature of 58 degrees and a low dewpoint (measure of moisture content in the atmosphere) of 34 degrees.
MUCAPE is Most Unstable Convective Available Potential Energy- A measure used to quantify the total amount of instability in the atmosphere especially when the surface is more stable as it was at the time the storms formed. 2000 J/kg is high instability for northern Illinois in mid April and supportive of strong to severe storms if other factors are favorable.
The warm and unstable air 3 to 5 thousand feet above the surface was driven into the area by what meteorologists call a low level jet. Deep layer moisture convergence on the nose of this low level jet with 40 to 45 mph winds at the 850 mb level (4kft-5kt) along with additional lift provided by a midlevel disturbance caused a cluster of showers and thunderstorms to form over northeast Iowa and northwest Illinois.
South-southwest winds of 40-45 mph at 3 to 5 thousand feet above the surface into northwest Illinois indicates the strong low level jet pumping elevated warmth and moisture into the region.
The red contours over northern Illinois indicate moisture flowing together in a deep layer of the atmosphere, which helped provide the lift to form the initial cluster of thunderstorms.
Regional Radar at 8am CDT
Strong westerly winds of 50-60 mph were present at about 15,000 above the surface, with 40-45 mph southerly winds at 4 to 5,000 feet up in the atmosphere. This change in wind direction and speed is referred to as wind shear, and high values of it (like were present during the morning) are a key ingredient in the development of severe thunderstorms.
500 mb (~15,000 ft AGL – dark blue) and 850 mb (~4-5kft AGL- red) wind arrows overlaid with radar data at 10am CDT. The red arrows pointing north (winds out of the south at 4-5kft) and blue arrows pointing east (winds out of the west at 15kft) show the large change in direction with height in the atmosphere and the presence of high levels of deep layer wind shear.
The combination of strong deep layer wind shear and high levels of atmospheric instability aloft allowed thunderstorms to acquire rotating updrafts, known as supercells. These storms often produce the most significant severe weather that can impact our area, including very large hail, damaging winds and tornadoes at times when lower level atmospheric conditions are supportive. A significant clockwise turning of the winds with height, also known as veering, is even more supportive of rotating storms, which is what we had today. Furthermore, while the original storm motion of the storm cluster west of our area was toward the northeast, supercells often take on a deviant motion. Typically, they become right movers, meaning they take on a path right of the overall storm motion. In the case of today’s storms, that meant towards the due east or even just south of east as shown in the image below.
The Event
A pair of supercells formed from the initial cluster over northeast Iowa and northwest Illinois and moved due east instead of northeast because they were right movers. These supercells went on to produce a swath of hail and wind damage in Winnebago, Boone and McHenry counties. Then the southern cell became more dominant, merging with the northern cell into one supercell that continued to produce very large hail over Lake and far northern Cook counties before exiting over Lake Michigan around noon CDT. Hail was as large as 2” in diameter portions of Boone, McHenry and Cook counties. The most significant wind damage occurred to a farmstead near the McHenry-Boone county border west-northwest of Marengo Illinois. Given that the near surface environment was still fairly dry when the storms moved across (shown by the Rockford 9am observation) this likely enhanced the wind potential from these storms.
Radar image at 1003 am on April 12, 2014
Radar image at 1139 am on April 12, 2014
The intense radar reflectivities of 65 to 75 dBZ (purple to blue colors) indicative of large hail at the lowest radar level.
Photos
 Photo by Caleb Johnson, Rockford, IL |
 Photo by Issac Reiter, Huntley, IL |
 Photo by Nathan Monney, 1.5 miles north of Belvidere, IL |
 Photo by Teri Jones Kolls, Harvard, IL |
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Local Storm Report Summary
NWUS53 KLOT 121921
LSRLOT
PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
221 PM CDT SAT APR 12 2014
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1152 AM HAIL NORTHBROOK 42.13N 87.83W
04/12/2014 E1.00 INCH COOK IL TRAINED SPOTTER
UP TO QUARTER SIZED.
1147 AM HAIL NORTHBROOK 42.13N 87.83W
04/12/2014 M1.00 INCH COOK IL TRAINED SPOTTER
1144 AM HAIL DEERFIELD 42.17N 87.85W
04/12/2014 E1.00 INCH LAKE IL TRAINED SPOTTER
1144 AM HAIL DEERFIELD 42.17N 87.85W
04/12/2014 E1.00 INCH LAKE IL TRAINED SPOTTER
1144 AM HAIL DEERFIELD 42.17N 87.85W
04/12/2014 E1.00 INCH LAKE IL TRAINED SPOTTER
1130 AM HAIL PALATINE 42.11N 88.04W
04/12/2014 E1.75 INCH COOK IL TRAINED SPOTTER
AT PALATINE HILLS GOLF COURSE
1130 AM HAIL BUFFALO GROVE 42.17N 87.96W
04/12/2014 E1.00 INCH LAKE IL TRAINED SPOTTER
1130 AM HAIL PALATINE 42.11N 88.04W
04/12/2014 M1.50 INCH COOK IL TRAINED SPOTTER
1130 AM HAIL ARLINGTON HEIGHTS 42.09N 87.98W
04/12/2014 M1.00 INCH COOK IL TRAINED SPOTTER
DUNDEE RD AND ROUTE 53.
1130 AM HAIL WHEELING 42.13N 87.92W
04/12/2014 E1.00 INCH COOK IL TRAINED SPOTTER
1130 AM HAIL LAKE ZURICH 42.19N 88.09W
04/12/2014 E1.00 INCH LAKE IL TRAINED SPOTTER
1130 AM HAIL MCHENRY 42.34N 88.29W
04/12/2014 E0.75 INCH MCHENRY IL TRAINED SPOTTER
1130 AM HAIL LONG GROVE 42.20N 88.00W
04/12/2014 E1.25 INCH LAKE IL TRAINED SPOTTER
HAIL BEGAN AT 1125 AM.
1130 AM HAIL N BUFFALO GROVE 42.17N 87.96W
04/12/2014 E1.50 INCH LAKE IL PUBLIC
LARGEST HAIL WAS JUST UNDER GOLF BALL SIZE. VIA FACEBOOK.
1130 AM HAIL N PALATINE 42.11N 88.04W
04/12/2014 E1.50 INCH COOK IL TRAINED SPOTTER
REPORT VIA FACEBOOK. TIME ESTIMATED BY RADAR.
1130 AM HAIL HOFFMAN ESTATES 42.06N 88.14W
04/12/2014 E1.00 INCH COOK IL TRAINED SPOTTER
1128 AM HAIL 1 N BARRINGTON 42.17N 88.13W
04/12/2014 E1.00 INCH LAKE IL TRAINED SPOTTER
VIA LAKE COUNTY EMA.
1125 AM HAIL PALATINE 42.11N 88.04W
04/12/2014 E1.75 INCH COOK IL TRAINED SPOTTER
1125 AM HAIL PALATINE 42.11N 88.04W
04/12/2014 M1.50 INCH COOK IL TRAINED SPOTTER
1124 AM HAIL BARRINGTON 42.15N 88.13W
04/12/2014 M2.00 INCH COOK IL TRAINED SPOTTER
1124 AM HAIL LAKE ZURICH 42.19N 88.09W
04/12/2014 E1.00 INCH LAKE IL TRAINED SPOTTER
1122 AM HAIL HOFFMAN ESTATES 42.06N 88.14W
04/12/2014 E0.75 INCH COOK IL TRAINED SPOTTER
1122 AM HAIL HAINESVILLE 42.34N 88.06W
04/12/2014 E0.50 INCH LAKE IL TRAINED SPOTTER
1121 AM HAIL LAKE ZURICH 42.19N 88.09W
04/12/2014 E1.75 INCH LAKE IL TRAINED SPOTTER
UP TO GOLF BALL SIZED.
1121 AM HAIL BARRINGTON 42.15N 88.13W
04/12/2014 E1.75 INCH COOK IL TRAINED SPOTTER
1120 AM HAIL HOFFMAN ESTATES 42.06N 88.14W
04/12/2014 E1.00 INCH COOK IL TRAINED SPOTTER
VIA FACEBOOK. QUARTER SIZED AND SLIGHTLY LARGER. TIME
ESTIMATED BY RADAR.
1120 AM HAIL LAKE ZURICH 42.19N 88.09W
04/12/2014 M1.25 INCH LAKE IL TRAINED SPOTTER
1116 AM HAIL ROUND LAKE PARK 42.32N 88.06W
04/12/2014 E1.00 INCH LAKE IL TRAINED SPOTTER
1115 AM HAIL CARPENTERSVILLE 42.12N 88.27W
04/12/2014 M1.50 INCH KANE IL TRAINED SPOTTER
1115 AM HAIL ALGONQUIN 42.16N 88.30W
04/12/2014 M2.00 INCH MCHENRY IL PUBLIC
HAIL AS LARGE AS 2 INCH DIAMETER BEGAN JUST AFTER 11 AM
AND ENDED AT ABOUT 1115 AM. REPORT AND PHOTO VIA TWITTER.
1115 AM HAIL ISLAND LAKE 42.28N 88.20W
04/12/2014 E0.75 INCH MCHENRY IL TRAINED SPOTTER
1113 AM HAIL CARY 42.21N 88.25W
04/12/2014 E1.00 INCH MCHENRY IL TRAINED SPOTTER
1112 AM HAIL ALGONQUIN 42.16N 88.30W
04/12/2014 M1.00 INCH MCHENRY IL TRAINED SPOTTER
1112 AM HAIL HAINESVILLE 42.34N 88.06W
04/12/2014 E1.00 INCH LAKE IL EMERGENCY MNGR
1110 AM HAIL 1 SW PISTAKEE HIGHLANDS 42.39N 88.23W
04/12/2014 E0.88 INCH MCHENRY IL TRAINED SPOTTER
IN JOHNSBURG
1108 AM HAIL CRYSTAL LAKE 42.23N 88.33W
04/12/2014 E1.00 INCH MCHENRY IL TRAINED SPOTTER
1107 AM HAIL LAKE IN THE HILLS 42.19N 88.32W
04/12/2014 E1.00 INCH MCHENRY IL TRAINED SPOTTER
1105 AM HAIL ANTIOCH 42.47N 88.08W
04/12/2014 M0.75 INCH LAKE IL TRAINED SPOTTER
1105 AM HAIL N ALGONQUIN 42.16N 88.30W
04/12/2014 E1.00 INCH MCHENRY IL TRAINED SPOTTER
MAINLY QUARTER SIZED HAIL WITH A FEW PIECES OF GOLF BALL
SIZED.
1105 AM HAIL CRYSTAL LAKE 42.23N 88.33W
04/12/2014 E1.75 INCH MCHENRY IL TRAINED SPOTTER
1104 AM HAIL HUNTLEY 42.17N 88.42W
04/12/2014 E1.25 INCH MCHENRY IL PUBLIC
1100 AM HAIL LAKE IN THE HILLS 42.19N 88.32W
04/12/2014 E1.25 INCH MCHENRY IL TRAINED SPOTTER
1100 AM HAIL LAKE IN THE HILLS 42.19N 88.32W
04/12/2014 E1.50 INCH MCHENRY IL TRAINED SPOTTER
QUARTER TO PING PONG BALL SIZED HAIL.
1100 AM HAIL SPRING GROVE 42.44N 88.24W
04/12/2014 E0.50 INCH MCHENRY IL CO-OP OBSERVER
COVERING THE GROUND.
1059 AM HAIL 4 N HUNTLEY 42.23N 88.42W
04/12/2014 E1.00 INCH MCHENRY IL TRAINED SPOTTER
PEA TO QUARTER SIZED HAIL COVERING THE GROUND LASTED 5
MINUTES.
1058 AM HAIL HUNTLEY 42.17N 88.42W
04/12/2014 E1.25 INCH MCHENRY IL TRAINED SPOTTER
1054 AM HAIL MCHENRY 42.34N 88.29W
04/12/2014 E1.50 INCH MCHENRY IL TRAINED SPOTTER
ROUTE 120 AND RIDGE ROAD. HAIL LASTED 5 MINUTES.
1054 AM HAIL CRYSTAL LAKE 42.23N 88.33W
04/12/2014 E0.75 INCH MCHENRY IL CO-OP OBSERVER
COVERING THE GROUND.
1047 AM HAIL UNION 42.23N 88.54W
04/12/2014 M1.00 INCH MCHENRY IL TRAINED SPOTTER
HAIL COVERING GROUND. 1 INCH LARGEST STONES. AT ILLINOIS
RAILWAY MUSEUM.
1045 AM HAIL WOODSTOCK 42.31N 88.44W
04/12/2014 E1.00 INCH MCHENRY IL TRAINED SPOTTER
DIME TO QUARTER SIZED HAIL QUEEN ANNE STREET AND TODD
ROAD COVERING THE GROUND.
1042 AM HAIL MARENGO 42.25N 88.60W
04/12/2014 E1.00 INCH MCHENRY IL TRAINED SPOTTER
QUARTER SIZED HAIL COVERING THE GROUND BEGAN AT REPORT
TIME AND LASTED 2 TO 3 MINUTES.
1030 AM TSTM WND GST HARVARD 42.42N 88.62W
04/12/2014 M55.00 MPH MCHENRY IL TRAINED SPOTTER
ALSO HALF INCH DIAMETER/DIME SIZED HAIL.
1025 AM TSTM WND DMG 5 WNW MARENGO 42.28N 88.69W
04/12/2014 MCHENRY IL PUBLIC
LARGE GRAIN AUGERS RIPPED FROM TOP OF GRAIN
BINS...SHINGLES PEELED OFF ROOFS...IRRIGATION SYSTEMS
BLOWN OVER...BARNS BLOWN DOWN AND A BRANCH STUCK INTO THE
SIDE OF A HOUSE. REPORT VIA FACEBOOK.
1015 AM TSTM WND DMG CHERRY VALLEY 42.24N 88.96W
04/12/2014 WINNEBAGO IL LAW ENFORCEMENT
LARGE TREE BRANCHES DOWN NEAR CHERRY VALLEY.
1015 AM HAIL POPLAR GROVE 42.37N 88.82W
04/12/2014 M2.00 INCH BOONE IL PUBLIC
ROUTE 76 NEAR POPLAR GROVE AIRPORT. TIME ESTIMATED BY
RADAR. REPORT VIA FACEBOOK.
1014 AM HAIL ROCKFORD 42.27N 89.06W
04/12/2014 E1.75 INCH WINNEBAGO IL TRAINED SPOTTER
UP TO PING PONG BALL TO GOLF BALL SIZED.
1013 AM HAIL SE ROCKFORD 42.27N 89.06W
04/12/2014 E1.25 INCH WINNEBAGO IL TRAINED SPOTTER
1012 AM HAIL ROCKFORD 42.27N 89.06W
04/12/2014 E1.50 INCH WINNEBAGO IL TRAINED SPOTTER
UPDATED REPORT FROM 1 MILE NORTHWEST OF I-39 AND US 20
BYPASS.
1011 AM TSTM WND DMG S ROCKFORD 42.27N 89.06W
04/12/2014 WINNEBAGO IL LAW ENFORCEMENT
JUST SOUTH OF THE CITY IN UNINCORPORATED ROCKFORD...TREES
DOWN ACROSS ROADWAYS.
1010 AM HAIL LOVES PARK 42.34N 89.01W
04/12/2014 E1.00 INCH WINNEBAGO IL TRAINED SPOTTER
1010 AM HAIL ROCKFORD 42.27N 89.06W
04/12/2014 M1.75 INCH WINNEBAGO IL TRAINED SPOTTER
GOLF BALL HAIL ON WYOMING STREET IN ROCKFORD.
1008 AM HAIL N BELVIDERE 42.25N 88.85W
04/12/2014 E1.00 INCH BOONE IL TRAINED SPOTTER
NORTH OF BELVIDERE.
1007 AM HAIL ROCKFORD 42.27N 89.06W
04/12/2014 E1.00 INCH WINNEBAGO IL LAW ENFORCEMENT
1001 AM HAIL ROCKFORD 42.27N 89.06W
04/12/2014 M1.00 INCH WINNEBAGO IL BROADCAST MEDIA
SOUTHEAST SIDE OF ROCKFORD.
0958 AM HAIL ROCKFORD 42.27N 89.06W
04/12/2014 M1.25 INCH WINNEBAGO IL TRAINED SPOTTER
1 INCH TO 1 AND A QUARTER SIZED HAIL. HAIL BROKE GLASS IN
AN OUTSIDE LIGHT FIXTURE. LOCATION 1 MILE NORTHWEST OF
I-39 AND US 20 BYPASS.
0958 AM HAIL SE ROCKFORD 42.27N 89.06W
04/12/2014 E1.00 INCH WINNEBAGO IL TRAINED SPOTTER
LASTED ABOUT 2 MINUTES. SOUTHEAST PORTION OF ROCKFORD
NEAR PERRYVILLE ROAD.
0954 AM HAIL ROCKFORD AIRPORT 42.20N 89.10W
04/12/2014 M1.00 INCH WINNEBAGO IL OFFICIAL NWS OBS
REPORTED ON 954 AM OBSERVATION.
0945 AM HAIL 4 SSE ROCKFORD 42.22N 89.03W
04/12/2014 E1.00 INCH WINNEBAGO IL TRAINED SPOTTER
QUARTER SIZED HAIL ON THE SOUTHEAST SIDE OF ROCKFORD.
0944 AM HAIL 1 E ROCKFORD AIRPORT 42.20N 89.08W
04/12/2014 M1.00 INCH WINNEBAGO IL TRAINED SPOTTER
HAIL LASTED ABOUT 3 MINUTES.
&&
$$
RC
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