Aviation Forecasts Discussions (AFD) INFO

(EXTRACTED FROM FXUS63 KLOT 111820)


    For the 18Z TAFs...
    
    Sort of complicated forecast this afternoon and evening as low
    pressure tracks through the area. The first issue is wind
    directions, followed by precipitation type, timing, and duration,
    and then focus shifts back to gusty nw winds.
    
    The core of low pressure will pass just to the south of the
    terminals, and then spread east of the area tonight. This will
    cause SE winds to hold in the 8-11 kt period for a few hours in
    the better pressure falls ahead of the low, then as winds shift
    further to the E, expect some decrease in speeds. Warm advective
    precipitation is attempting to occur farther upstream of the upper
    level low across NE IA/SE MN. Expect during the afternoon some
    attempt to precipitate over the coming hours, but with drier low
    levels expect this would be some light snow or sleet, non
    accumulating. As the better forcing approaches RFD around 21-22z,
    there could be a brief burst of snow which could put down a quick
    coating of snow, possibly even a few tenths of an inch in a short
    period. Cigs will fall to MVFR and vsby too, possibly briefly
    IFR. As this snow axis shifts closer to the Chicago terminals,
    there appears to be some weakening to the stronger forcing, but
    there is still a small window for ORD/MDW in the 0z-02z time
    frame for a quick coating of snow, medium confidence on timing.
    
    Later this evening there could be a few lingering snow showers,
    more so wind driven flurries. CIGS will eventually head back to
    VFR. Expect NW winds to exceeds 20-25 kt from a 330-340 direction
    after 3z or so at least for a few hours if not longer.
    
    KMD