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    For the 18Z TAFs...
    Isolated/scattered late afternoon/evening TSRA potential is the
    primary aviation forecast concern.
    Weak low pressure wave was crossing the Mississippi River into
    west central IL at midday. Isolated to scattered TSRA were
    expanding in coverage west and southwest of Bradford IL, and would
    expect this activity to continue to spread east-northeastward this
    afternoon as atmosphere continues to destabilize in very warm air
    mass. Expect isolated activity near RFD by 21-22Z, several hours
    later into Chicago. Current extrapolation would suggest 02-03Z for
    ORD and MDW, although some eastward expansion would likely bring
    activity into the vicinity earlier. Have indicated VCTS for DPA by
    23Z, ORD/MDW 00Z. Should eventually be a decrease in intensity and
    coverage this evening with approach of sunset, so confidence in
    coverage is medium at this point with respect to Chicago
    Potential also exists for showers/storms mainly over northwest IL
    after midnight tonight, though this activity should be dissipating
    and confidence is low in it making it into IL. Have maintained
    prob30 for RFD.
    Southwest winds 10-15 kts will persist through the afternoon. Lake
    breeze may form along the shore, but is not expected to make it to
    MDW/ORD. GYY could see a brief period of variable or light
    northeast winds, though winds should shift back to southwest by
    evening. Southwest or west-southwest winds around 10 kts expected
    during the day Saturday.