Gusty Winds Thursday April 29th

Gusty Winds Thursday April 29th, 2010

A deep low pressure system moved slowly across the Plains on Thursday, resulting in a broad area of very gusty south winds across much of the Midwest.   During the period of deepest mixing of the low levels in the mid and late afternoon hours, sustained wind speeds ranged from 20 to 35 mph, with gusts of 40 to 50 mph measured at many locations across northern Illinois and northwest Indiana.  A few reports of minor damage accompanied the gusty winds.

Here are some of the peak measured wind gusts at various NWS and FAA locations across northern Illinois and northwest Indiana Thursday afternoon:


VALPARAISO      39 MPH           332  PM
WAUKEGAN        46 MPH           522  PM
ROCKFORD        52 MPH          1259  PM
WEST CHICAGO    47 MPH           309  PM
OHARE           49 MPH           338  PM
MIDWAY          53 MPH           320  PM
DUPAGE          48 MPH           327  PM
DEKALB          49 MPH           303  PM
AURORA          51 MPH           212  PM
KANKAKEE        48 MPH           404  PM
ROCHELLE        47 MPH           445  PM
PERU            47 MPH           125  PM
JOLIET          41 MPH           425  PM
GARY            46 MPH           445  PM
PONTIAC         48 MPH           505  PM
LANSING         42 MPH           522  PM
PORTAGE         52 MPH           528  PM

In addition, the annemometer 80 feet above the water at the Harrison Crib off Chicago measured a peak gust of 59 mph. 

Why was it so windy?

A few factors combined to produce these strong, gusty winds across the area Thursday.   FIrst, deep low pressure was moving across northwest Iowa into southern Minnesota, and surface pressure was falling rapidly across the upper Midwest in response to a strong mid level disturbance lifting across the region.  Second, a strong mid and upper level jet streak (speed maxima) was spreading northeast across the region producing 50 knot (58 mph) winds in the mid levels of the atmosphere.  Third, sunshine, combined with a very warm and relatively dry airmass resulted in steep low level lapse rates which helped to mix these stronger mid level winds down to the surface.  

Fig 1:  Surface Map at 2 pm CDT

The shaded colors are the 3 hour pressure fall values between 11 am and 2 pm CDT.  Note that pressures were falling the greatest over northern IL, nearly 5 milibars in the preceeding 3 hours.  Localized areas of strong pressure falls such as these tend to increase the wind speed significantly.   This area of strong pressure falls continued to move slowly east across the area during the afternoon.   The actual center of the surface low is out of the image to the upper left, across Minnesota.


Fig 2: AMDAR  Aircraft Sounding from O'Hare, 709 pm CDT

This is temperature and wind data from a commercial airliner descending into O'Hare International Airport in Chicago.  Note the straight line of the lower portion of the temperature profile, and the 50 knot (approximately  58 mph) wind speeds at the top of this layer (flag on red wind barbs at right).  Although the air right near the surface was already starting to cool slightly by this time, allowing winds to diminish a bit at the ground, this image still displays the deep well-mixed layer of air which help to transport the momentum of the mid level 50 knot winds to the surface during the afternoon.

Surface winds began to diminish by sunset, as the surface pressure fall maxima moved off to the east and weakened, and the temperatures near the ground began to cool with the approach of sunset.

M. Ratzer  4/29/10


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