NWS Milwaukee/Sullivan, WI

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

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.

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. 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.

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.

National Weather Service Weather Forecast Office