Thunderstorm Wind Event   (Gustnado)
Southeast Wisconsin
October 4, 2002


Photo of damage in Hartland Photo of damage in Hartland Photo of damage in Hartland

Example photo of a Gustnado During the mid-morning hours of October 4, 2002, a line of convective showers, orientated north to south moved east through south-central and southeast Wisconsin, and produced strong, gusty west winds generally in the 40 to 55 mph range with scattered reports of 60 to 70 mph, an isolated "gustnado", and heavy rains. Interestingly, no thunder or lightning was reported by spotters or the general public, nor did any cloud-to-ground lightning strikes register on the national lightning detection network. However, off-duty National Weather Service (NWS) employees in Oconomowoc observed some cloud-to-cloud lightning. Technically, the NWS documents any convective cloud event that results in deaths, injuries, or property damage as a "Thunderstorm Wind" event, regardless of the amount of lightning. The line of convective showers was moving through an atmosphere that already had winds of 40 to 70 mph between 2 and 10 thousand feet above the ground. Mixing of air inside and near the downdraft/rain showers allowed for the transfer of these stronger winds down to the surface.

WSR-88D Doppler weather radar indicated that the winds behind the line of showers were weakening as the line moved east across Jefferson County. The NWS Forecast Office southeast of Sullivan had a peak wind gust of 47 mph (41 knots) out of the west with poor visibilities due to heavy rains moving sideways. For the day, 0.91 inches of rain fell at the Weather Office. There were reports from across Jefferson County of small branches knocked out of trees.


Photo of damage in Hartland Photo of damage in Hartland Photo of damage in Hartland

As the line of showers moved through the remainder of southeast Wisconsin, it briefly intensified in scattered locations such that winds gusts over 55 mph were noted. A wind gust of 65 mph occurred at an Oconomowoc around 10 am, a wind gust of 62 mph was measured in Germantown by a storm spotter around 1025 am, a 74 mph gust from the west-southwest was measured at the home of an Amateur Radio operator about 1 mile south of Hartland at about 1015-1020 am, a 56 mph gust was measured by a storm spotter in West Bend, and a 67 mph gust was reported from Sheboygan school/TV network site.

Example photo of a Gustnado The 74 mph wind gust was associated with a “gustnado” that tore the roof off a home just 2 doors away on Manchester Lane. Roof debris was carried northwest to the other side of the street near an apartment building. The apartment building sustained roof damage, and two support columns for the front door overhang were toppled. A sliding glass door on the 2nd floor of the apartment building was sucked out of the wall. In addition, a large 60-foot tree on an adjacent street was pushed over onto the roof a home, resulting in damage. Several other large trees were uprooted, and roof shingles were lifted off on a couple homes. Another apartment building sustained damage to a wooden deck. Residents remarked that they saw something that resembled a “funnel” going through. It is possible that the winds in the gustnado reached 80 mph. Remember, hurricane-force winds start at 74 mph. Just west of Hartland, the high winds knocked over a flat-bed farm trailer was over-turned.

Elsewhere in southeast Wisconsin, the Oconomowoc area had uprooted trees and toppled power lines, with one home sustaining damage due to a toppled tree. At least 4 trees were uprooted in the backyard of a Thiensville (Ozaukee Co.) home, and in Racine and Kenosha counties the high winds knocked tree branches loose which hit some power lines. Small tree branches were also knocked loose in all of the other southeastern Wisconsin counties.

At Milwaukee Mitchell Field, a wind gust from the west peaked at 44 mph, and 0.37 inches of rain fell. At the Dane County Regional Airport, a gust from the southwest peaked at 38 mph, and 0.94 inches of rain fell.


Definition of a gustnado - A slang term for a short-lived, ground-based, shallow, vortex that develops on a gust front associated with either thunderstorms or showers. They may only extend to 30 to 300 feet above the ground with no apparent connection to the convective cloud above. They may be accompanied by rain, but usually are “wispy,” or only visible as a debris cloud or dust whirl at or near the ground. Wind speeds can reach 60 to 80 mph, resulting in significant damage, similar to that of a F0 or F1 tornado. However, gustnadoes are not considered to be a tornado, and some cases, it may be difficult to distinguish a gustnado from a tornado. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones) that is involved with true tornadoes; they are more likely to be associated visually with a shelf cloud that is found on the forward side of a thunderstorm.

Definition of a tornado - A violently rotating column of air (vortex) extending from the ground to the base of a thunderstorm, that is intense enough at the surface to do damage. A condensation “funnel” does not need to be visible from the ground to cloud base for a tornado to be present; a debris cloud at the ground beneath a thunderstorm is all that is needed to confirm the presence of a tornado (even in the total absence of a condensation funnel. Wind speeds in tornadoes vary from 40 mph to 318 mph (F0 to F5 on the Fujita scale).

In many cases, a tornado is associated with a wall cloud that is usually found on the south, southwest, or west side of the thunderstorm (the rear or backside of the storm, relative to storm motion). The wall cloud, a visual indication of where the most intense portion of the updraft, is usually tucked up under the rain-free base. Above the rain-free base is the updraft tower that may have a mesocyclone 2 to 6 miles wide, and extend from cloud base to 30,000 ft. The “supercell” thunderstorm has a mesocyclone. It is possible for rain to be present as the tornado spins up, but generally a tornado spins up in the rain-free, or nearly rain-free part of a thunderstorm. In some tornado cases, rain-wrapping can occur as the tornado matures, thereby reducing visibilities near the tornado.

It is the wind associated with the rotating column of air (vortex) that does the damage, and not the visible condensation funnel that sits inside the vortex. Air and wind are invisible until one sees a debris cloud or the condensation funnel form. The condensation funnel appears to be descending only because the air pressure inside the vortex column doesn’t have to fall as far for the air inside to reach condensation as it does further down near the ground level.

Many people refer to “touchdowns” when referring to tornadoes. This is incorrect. Tornadoes do not “touch down,” or develop downward in the sense that something is falling out of the sky. Instead, the vortex is already at the surface as a pre-existing circulation (because of some other physical process), with weak wind speeds, but may ultimately increase it’s rotational speed while decreasing it’s diameter (because of some other physical process) until it is strong enough to inflict surface damage (vegetative or structural); at which point it becomes a tornado.


Map of damage area

TIME(CDT)  .....CITY LOCATION..... STATE  ...EVENT/REMARKS...
           ....COUNTY LOCATION....

1003 AM    OCONOMOWOC                WI   WIND DAMAGE
10/04/02   WAUKESHA                       TREES AND LIMBS DOWN.
                                          REPORTED BY STORM
                                          SPOTTER.

1008 AM    OCONOMOWOC                WI   WIND DAMAGE
10/04/02   WAUKESHA                       NUMEROUS LARGE TREES
                                          AND BRANCHES DOWN.
                                          HOUSE DAMAGE DUE TO  
                                          FALLEN TREE. 
                                          REPORTED BY PUBLIC.  

1010 AM    OCONOMOWOC                WI   WIND DAMAGE
10/04/02   WAUKESHA                       TREES, LIMBS AND POWER
                                          LINES DOWN. REPORTED 
                                          BY PUBLIC.

1015 AM    2 S HARTLAND              WI   WIND DAMAGE
10/04/02   WAUKESHA                       60 MPH ESTIMATED
                                          WIND GUST. REPORTED
                                          BY STORM SPOTTER.

1020 AM    1 W HARTLAND              WI   WIND DAMAGE
10/04/02   WAUKESHA                       FLATBED FARM TRAILER
                                          OVERTURNED. REPORTED
                                          BY OFF-DUTY NWS
                                          EMPLOYEE.
										  
1020 AM    1 S HARTLAND               WI   WIND DAMAGE
10/04/02   WAUKESHA                       GUSTNADO DAMAGED HOMES
                                          AND TREES. ROOF LIFTED
                                          OFF ONE HOME AND AT
                                          LEAST 4 OTHER
                                          STRUCTURES  HAD SOME
                                          DAMAGE. MEASURED WIND
                                          GUST OF 74 MPH AT HOME
                                          OF AMATEUR RADIO
                                          OPERATOR 2 DOORS AWAY
                                          FROM HOME WITH LIFTED
                                          ROOF.  GARBAGE BIN WAS
                                          FLIPPED OVER AND A
                                          BACKYARD PLAYSET WAS
                                          PUSHED OVER.  RESIDENTS
                                          SAID THEY SAW SOMTHING
                                          THAT RESEMBLED A
                                          FUNNEL.  NWS AND
                                          EMERGENCY MANAGEMENT
                                          CONDUCTED DAMAGE
                                          SURVEY. REPORTED BY LAW
                                          ENFORCEMENT OFFICIAL.

1025 AM    GERMANTOWN                WI   62 MPH TSTM GUST
10/04/02   WASHINGTON                     MEASURED WIND
                                          GUST. REPORTED
                                          BY STORM SPOTTER.

1033 AM    WEST BEND                 WI   WIND DAMAGE
10/04/02   WASHINGTON                     56 MPH MEASURED WIND
                                          GUST. 6-7 " TREES
                                          DOWN. REPORTED BY
                                          STORM SPOTTER.

1035 AM    THIENSVILLE               WI   WIND DAMAGE
10/04/02   OZAUKEE                        TREE DAMAGE IN ONE 
                                          BACKYARD.
                                          REPORTED BY PUBLIC.

1040 AM    HARTLAND                  WI   WIND DAMAGE
10/04/02   WAUKESHA                       ROOF DAMAGE TO HOMES.
                                          REPORTED BY PUBLIC.

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