Aug 27, 2007 Strong Winds - Wake Low

Strong winds were observed across parts of south-central and southeast Wisconsin during the late morning and early afternoon hours of Monday, August 27th.  Below is a Public Information Statement that was issued by the Milwaukee/Sullivan NWS office, and then you’ll find more images and expanations. 

...STRONG WINDS REPORTED ACROSS SOUTH-CENTRAL AND SOUTHEAST
WISCONSIN...

STRONG SOUTHEAST WINDS GUSTING TO 40 TO 49 MPH AT THE GROUND LEVEL
RESULTED IN SCATTERED REPORTS OF TOPPLED TREES OR BROKEN TREE
BRANCHES...AND DAMAGED POWER LINES AFTER A DECAYING THUNDERSTORM
COMPLEX MOVED SOUTHEAST THROUGH SOUTH-CENTRAL AND SOUTHEAST
WISCONSIN.

THE STRONG WINDS OCCURRED BETWEEN 1130 AM AND 200 PM. WET SOILS FROM
RECENT RAINS MAY HAVE MADE IT EASIER FOR SOME OF THE TREES TO BE
TOPPLED BY THE STRONG WINDS.

SCIENTIFICALLY...THE WINDS WERE GENERATED BY A WAKE LOW PRESSURE
FOUND ON THE BACK SIDE OF A THUNDERSTORM COMPLEX. A LARGE SURFACE
PRESSURE GRADIENT SOMETIMES DEVELOPS ON THE BACKSIDE OF A DEPARTING
THUNDERSTORM COMPLEX.  THIS LARGE PRESSURE GRADIENT RESULTS IN
STRONG WINDS...AND IS VERY DIFFICULT TO ANTICIPATE.

CONCEPTUALLY...SLIGHTLY HIGHER ATMOSPHERIC PRESSURE IS FOUND IN THE
MIDDLE OF A THUNDERSTORM COMPLEX DUE TO THE RELEASE OF RAIN-COOLED
AIR WHICH IS HEAVY AND DENSE. BY DEFAULT...THE ATMOSPHERIC PRESSURE
WILL BE LOWER OUTSIDE OR AWAY FROM THE THUNDERSTORM COMPLEX.  IN
TODAY/S SITUATION THE AIR PRESSURE ON THE NORTHWEST SIDE OF THE
THUNDERSTORM COMPLEX JUST HAPPENED TO BE LOWER THAN NORMAL. THIS LED
TO THE STRONG WINDS.  THE LARGER THE AIR PRESSURE GRADIENT...THE
STRONGER THE WINDS.

SOME OF THE HIGHEST PEAK WIND GUSTS REPORTED...

58 MPH  TOP OF 15-STORY AOS BUILDING ON UW-MADISON CAMPUS 1147 AM
49 MPH  LODI M.S. /COLUMBIA CO./ TV-15 SCHOOL NETWORK 1210 PM
47 MPH  JACKSON /WASHINGTON CO./ TRAINED SPOTTER   1247 PM
46 MPH  SLINGER M.S. /WASHINGTON CO./ TV-6 NETWORK E1245 PM
46 MPH  GERMANTOWN KENNEDY M.S. /WAUKESHA CO./ TV-6 NEWTWORK E1245 PM
45 MPH  DOUSMAN KETTLE MORAINE M.S. /WAUKESHA CO./ TV-6 NET 1250 PM
44 MPH  DANE COUNTY REGIONAL AIRPORT /TRUAX FILED/  1136 AM

A BIG THANKS IS EXTENDED TO THE TELEVISION STATIONS IN THE MILWAUKEE
AND MADISON MARKETS FOR THEIR WEATHER REPORTS ON THIS STRONG WIND
SITUATION. ADDITIONALLY...A THANKS IS EXTENDED TO LAW ENFORCEMENT
OFFICIALS AND TRAINED SPOTTERS FOR THEIR REPORTS.

BELOW IS THE DEFINITION OF A WAKE LOW AS IT APPEARS IN THE AMS GLOSSARY
OF METEOROLOGY...

WAKE LOW - 1. /OR WAKE DEPRESSION/ IN METEOROLOGY...A SURFACE LOW
PRESSURE AREA...OR MESOLOW TO THE REAR OF A SQUALL LINE...MOST
COMMONLY FOUND IN SQUALL LINES WITH TRAILING STRATIFORM
PRECIPITATION REGIONS...IN WHICH CASE THE AXIS OF THE LOW IS
POSITIONED NEAR THE BACK EDGE OF THE STRATIFORM RAIN AREA. 2. /OR
WAKE DEPRESSION/ IN FLUID DYNAMICS...A LOW PRESSURE AREA ON THE
DOWNSTREAM SIDE OF AN OBJECT EMBEDDED IN A FLOW.

Below is an image showing the surface pressure pattern at 12 Noon August 27, 2007.  The blue "H" signifies the center of the so called "bubble high" or "meso-high" pressure due to the rain-cooled air that has come out of the bottom of the thunderstorm complex mentioned in the statement above. To the norrthwest of the meso-high is found the "wake-low" pressure, which is signified by the red "L" - with a low pressure trough indicated by the dashed red line.  The tight packing of the lines of equal atmospheric pressure  (white lines) found over southeastern Wisconsin indicates a strong pressure gradient - the change from high pressure to lower pressure is greatest in this region.  In the weather world, when strong pressure gradients exist, the surface winds become stronger (more forcing) and generally blow from high pressure to low pressure.  Now you know why the strong winds were observed on Monday August 27th!

Surface Weather Map

Below is a radar image of the situation at 10:00 A.M this morning, August 27, 2007. 

Radar Image 10 am

Below is the radar image at 12 Noon today, August 27, 2007.  Note how the thunderstorm complex fell apart as it moved east into more stable air.   However, the leftover rain-cooled air was enough to aid in the development of a strong surface pressure gradient over southern Wisconsin today.

Radar Image Noon

Below is an digital image taken by Jeff Trickel, from the Mineral Point area in south-central Iowa County this morning as the thunderstorm complex approached.  Note the horizontal-orientated cloud feature that resembles a rope.  In the weather world we identify this cloud feature as a "shelf cloud."  Shelf clouds are usually found on the front side of a line of storms, and signify the approach of possibly heavy rains, gusty winds, hail, and rain-cooled air that comes out of the base of the thunderstorms (where the thunderstorm is exhaling).

Sometimes the shelf cloud may have several layers - like a stack of pancakes.  The shelf cloud may extend for tens of miles and you may not see its ends.  Sometimes they can look very scary, and some severe weather spotters can mistake them as being a rotating wall cloud (a cloud feature that usually precedes tornado development).  Typically, shelf clouds do not show signs of organized, persisten, vertical rotation that a rotating wall cloud would possess.  Therefore, tornadoes usually do not form in conjunction with shelf clouds. 

You should also note the low-hanging fragments of cloud material below the shelf cloud.  These cloud fragments are called "scud."  Scud clouds can be attached to the underside of the shelf cloud or form very close to the ground and can temporarily exhibit swirling motions due to wind shear near, below, or in front of the shelf cloud.  In other words, scud clouds can look very scary!   We call these "scary-looking clouds."  When this happens, people may call their County 911 Communications Center and tell the dispatcher that they see a funnel cloud or even a tornado!  Make sure this doesn’t happen to you!  This is why the National Weather Service conducts free, 2-hour severe weather spotter classes each spring in your county - so that spotters can tell the difference between scary-looking scud clouds and the real rotating wall clouds, funnel clouds, and tornadoes.   The best thing about these classes is that you don’t have to be a severe weather spotter to attend!

Shelf cloud picture

End



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