Wake Low Produces Strong Winds Tuesday Evening

...Rare Meteorological Phenomena Produced Strong Winds Tuesday Evening...

A period of strong...gusty...and shifting winds occurred along the back edge of a dissipating area of showers and thunderstorms Tuesday evening across Northern Illinois. Traces of pressure and wind from various reporting stations indicate that the strong, shifting winds were likely the result of a meteorological phenomena known as a wake low, which is simply a small scale area of low pressure.

The wake low that moved across northern Illinois produced rapid pressure fluctuations, which is what ultimately caused the strong and quickly shifting winds. Pressure changes as large as 2-4 milibars in less than 30 minutes were observed. To put that into perspective, a pressure change of 3 milibars over a 3 hour period is considered meteorologically significant, so for that large of a change to occur in less than 30 minutes is quite noteworthy. While impressive, much stronger wake lows have occurred in the past producing much stronger and more damaging winds with much larger pressure fluctuations of up to 10 milibars in an hour.

The rapidly fluctuating pressure also resulted in several wind shifts. Winds most of today were from the south...however as the wake low approached, winds backed to southeast or even east-southeast, followed by an abrupt shift to to the southwest as the wake low passed by. The southwest winds then became more southerly again and diminished some as the wake low moved away and it’s influence waned.

Wind gusts of 40 to well over 50 mph were common along the path of the this wake low. Based on radar and surface observations it appears that the wake low moved from Lee and Ogle counties eastward across DeKalb, Kane, DuPage, southern McHenry, and northern Cook counties. Many areas along the path experienced winds strong enough to down trees and powerlines, resulting in many power outages. Here are some of the peak wind gusts as measured by  automated weather stations from across the region...

               Location              

Peak Wind Gust (MPH)

Hoffman Estates*

59

South Elgin*

58

Elgin*

55

Schaumburg*

55

Hampshire*

51

Wheeling Airport

51

Sugar Grove/Aurora Airport

50

West Chicago Airport

44

DeKalb Airport

44

Aurora*

41

Geneva*

41

Rochelle Airport

39

O’Hare Internation Airport

38


                            * = station not quality controlled by the National Weather Service

Here are plots of sea level pressure and wind direction every 5 minutes from 4 of the ASOS (automated surface weather observation) sites from across the Chicago Metropolitan Area affected by the wake low. Notice the abrupt drop in pressure at each location, such an abrupt drop at Aurora/Sugar Grove that the pressure sensor actually failed! Also, notice how the wind direction changes, first backing (changing from south to southeast) then (qucikly veering) as the wake low approaches the moves past the site.



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