Isolated thunderstorms caused extensive wind damage across portions of south central and southeast Wisconsin during the evening hours of Tuesday, May 14th. The thunderstorms were weakening as they moved across southern Wisconsin. However, rain falling into dry air near the ground evaporated completely in some areas and only partially evaporated in others. This explains the light returns on the radar reflectivity as the decaying storms moved through the region. But, it was this evaporative process that generated the strong winds. Below is a loop of radar reflectivity from 730 to 930 pm. The circled areas highlight where the worst wind damage was occuring. Note how weak the radar returns look for storms that caused so much damage.
The evaporating rain caused the air to cool, become more dense than the air around it, and sink rapidly to the ground. The event last night showed signs of being a heat burst event. In a heat burst, all of the water in the sinking air is evaporated before it reaches the ground, and the air begins to warm before it hits the ground. The air rushes to the ground as a sudden burst of hot, gusty wind. For more information about heat bursts, visit the NWS Norman, OK website at http://www.crh.noaa.gov/oun/?n=heatburst_info .
The weather instruments at our office near Sullivan reported nearly a 10-degree temperature rise coincident with sustained winds of 20 mph and gusts up to 39 mph between 8 and 9 pm. See the image below.
The seemingly "weak" thunderstorms produced damaging straight-line winds from northern Dane county all the way east to Lake Michigan and also in portions of Rock and Walworth counties. Strong winds can be detected by Doppler radar. Below is a loop of radar velocity from 730 pm to 930 pm. The green colors indicate that air is moving toward the radar (the black dot near the middle of the radar image), and the red colors show the air moving away from the radar. The brighter green/blue and red/pink/orange colors correlate with the strongest winds. The general path of the wind damage is annotated at the beginning of the animation.
There were numerous reports of downed trees and power lines across Dane, Jefferson, Waukesha, Milwaukee, Rock and Walworth counties Tuesday evening. The Local Storm Report (LSR) graphic and text summary are posted below:
PRELIMINARY LOCAL STORM REPORT...SUMMARY NATIONAL WEATHER SERVICE MILWAUKEE/SULLIVAN WI 1105 PM CDT TUE MAY 14 2013 ..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON... ..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE.... ..REMARKS.. 0800 PM TSTM WND GST SAUK CITY 43.27N 89.73W 05/14/2013 E50.00 MPH SAUK WI TRAINED SPOTTER 3 INCH DIAMETER TREE BRANCHES BLOWN DOWN. CONSTRUCTION BARRELS BLOWN OVER. REPORT RELAYED FROM NWS-LA CROSSE. 0800 PM TSTM WND GST SUN PRAIRIE 43.18N 89.23W 05/14/2013 E60.00 MPH DANE WI TRAINED SPOTTER MULTIPLE POWER LINES DOWN AND/OR SNAPPED OFF, LIGHT POLE DOWN, WINDSPREAD 2 FT DIAMETER TREES DOWN 0815 PM TSTM WND GST ASHIPPUN 43.21N 88.52W 05/14/2013 E50.00 MPH DODGE WI PUBLIC HWY 67 AND 0. 0815 PM TSTM WND DMG MARSHALL 43.17N 89.06W 05/14/2013 DANE WI TRAINED SPOTTER 6 INCH TREE LIMBS DOWN, SIDING, GUTTERS, DOWNSPOUTS TORN OFF HOUSE 0820 PM TSTM WND DMG WATERLOO 43.18N 88.99W 05/14/2013 JEFFERSON WI PUBLIC LARGE BARN BLOWN DOWN AND 4 X 4 POSTS RIPPED OUT OF THE GROUND AT W7769 HIGHLAND ROAD. REPORT RELAYED BY JEFFERSON COUNTY SHERIFF AT 1040 PM. 0835 PM TSTM WND GST 2 SE OCONOMOWOC 43.09N 88.47W 05/14/2013 E50.00 MPH WAUKESHA WI TRAINED SPOTTER SUSTAINED 50 MPH WINDS WITH GUSTS UP TO 65 MPH 0840 PM TSTM WND GST OCONOMOWOC 43.11N 88.50W 05/14/2013 M68.00 MPH WAUKESHA WI TRAINED SPOTTER TIME ESTIMATED. SOCIAL MEDIA REPORT. 0855 PM TSTM WND DMG HARTLAND 43.10N 88.34W 05/14/2013 WAUKESHA WI BROADCAST MEDIA WAUKESHA COUNTY SHERIFF REPORTED TREES AND POWER LINES DOWN IN HARTLAND...MERTON AND DELAFIELD AREAS. RELAYED BY TV MEDIA VIA SOCIAL MEDIA. 0857 PM TSTM WND DMG 3 E PEWAUKEE 43.09N 88.19W 05/14/2013 WAUKESHA WI PUBLIC 60 FOOT PINE TREE DOWN. TIME ESTIMATED. RELAYED VIA SOCIAL MEDIA. 0900 PM TSTM WND DMG DELAFIELD 43.07N 88.39W 05/14/2013 WAUKESHA WI LAW ENFORCEMENT WIRES AND TREES DOWN IN THE DELAFIELD...MERTON...AND HARTLAND AREAS. 0905 PM TSTM WND GST WAUWATOSA 43.06N 88.03W 05/14/2013 E50.00 MPH MILWAUKEE WI TRAINED SPOTTER ESTIMATED SUSTAINED WINDS OF 30 MPH WITH GUSTS TO 50 MPH ON I-94/MILWAUKEE COUNTY ZOO. CONSTRUCTION BARRELS BLOWN AROUND AND LOOSE CARDBOARD DEBRIS FLYING IN THE AIR. 0920 PM TSTM WND DMG 3 SW MILWAUKEE 43.00N 87.96W 05/14/2013 MILWAUKEE WI BROADCAST MEDIA MILWAUKEE FIRE DEPARTMENT REPORTED SEVERAL TREE FIRES...LIVE WIRES DOWN AND TREES DOWN. TIME ESTIMATED. HOUSE FIRE DUE TO LIVE WIRES ON HOUSE. RELAYED BY TV MEDIA VIA SOCIAL MEDIA. 0930 PM TSTM WND DMG WEST ALLIS 43.01N 88.03W 05/14/2013 MILWAUKEE WI PUBLIC LARGE BRANCHES DOWN NEAR 71ST AND BECHER STREET. SOCIAL MEDIA REPORT. 0941 PM TSTM WND DMG 3 N MILWAUKEE INTERNATI 42.99N 87.90W 05/14/2013 MILWAUKEE WI AMATEUR RADIO AT INTERSECTION OF HOWELL AND MORGAN AVENUES...6 INCH TO 8 INCH DIAMETER TREE ACROSS THE ROAD. 0945 PM TSTM WND GST MILWAUKEE C-MAN 43.05N 87.88W 05/14/2013 M39.00 MPH MILWAUKEE WI C-MAN STATION 34 KNOT WIND GUST AT MILWAUKEE LAKESHORE SITE...MLWW3. 0957 PM TSTM WND DMG 2 NW CLINTON 42.58N 88.89W 05/14/2013 ROCK WI LAW ENFORCEMENT TREE DOWN.
National Weather Service - Milwaukee/Sullivan