Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 300830

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
330 AM CDT Fri Sep 30 2016

310 AM CDT

Through tonight...

The main weather story through at least Saturday night will be the
continued impacts of the gradually northward shifting upper low
back over the Lower Great Lakes region. As a result, little
change is expected in the sensible weather over the area though
this period.

Water vapor imagery this morning indicates that the center of the
upper low is still over KY, with numerous disturbances shifting
westward along its northern periphery (Ohio Valley into the Lower
Great Lakes). These disturbances will likely spread more scattered
showers over the area today. There also appears to be a possibly
of some thunderstorms this afternoon over my southeastern areas,
namely east central Illinois and into portions of northwestern
Indiana. These areas will be in closer proximately to the cold
pool aloft (-17C 500 MB temperatures), in association with the
upper low. More showers are expected to push westward into
portions of NE IL by this evening as the upper low begins to
shift northward over IN.



329 AM CDT

Saturday through Thursday...

The aforementioned upper low, and its associated surface low,
will shift northward towards southern Lake Michigan by midday
Saturday. This will continue to allow for periods of rain showers
to continue to shift westward across northern Illinois during the
day. There will also be better chances for some afternoon
thunderstorms across the eastern half of the CWA (including the
Chicago area), as the upper level cold pool (-17 to -19C 500 MB
temperatures) moves over this area area. With better thermodyamics
also setting up over southern Lake Michigan, thunderstorms could
move westward off of the Lake during the afternoon and evening.
Can also not rule out the possibly of a few more water spouts just
off shore on Saturday.

This rather stagnate upper low will finally begin to be pushed to
the east by early next week as another upper level trough digs
over the western CONUS. As this occurs, it appears that we will be
in for some dry and a return to seasonal temperatures through mid
week. Thereafter, it appears that the western CONUS system will
shift eastward over the Plains, with surface low pressure likely
to shift over the Upper Midwest and into Ontario during the second
half of the work week. This looks to send a decent cold front over
the area, possibility by Thursday. Showers and some storms may be
possible with the frontal passage, but some cooler weather looks
to shift over the area by Friday.



For the 06Z TAFs...

Not much change in guidance for the 6z set of TAFs. A batch of
rain showers will continue to drift westward as a multitude of
shortwaves on the north end of deep low pressure to our southeast
shift through the area. Cigs continue to fall early this morning,
and the axis of IFR cigs getting closer, and expect at least
occasional IFR cigs and at minimum low MVFR cigs overnight into
early Friday. It is possible to have a mix of MVFR and IFR during
the day but the propensity is to remain MVFR as there will be
break in the shower activity. Several compact shortwaves look to
increase shower coverage in the afternoon time frame, though
showers can really occur at any time in this pattern. Gusty NE
winds continue. Guidance is hitting IFR and even LIFR hard again
tonight. Will leave the return of IFR Friday night in place,
though confidence is low on this time frame to see if we keep the
IFR cigs around today or not.



255 AM CDT

Strong northeast winds will continue across the lake through
today before easing. Waves have eased a little but still hold near 5 ft at the
south buoy and around 7 ft at the Wilmette and Michigan City
buoys. Would expect these to hold steady or undulate slightly.
Winds to 30 kt remain a good bet today, especially for the south
half, and occasional gale force gusts also cannot be ruled out
this morning but 25-30 should generally rule. The threat for
waterspouts appears slightly lower today, but we may have a better
chance on Saturday. This is due to colder air aloft arriving as
the main low pressure system moves northwestward and we get some
additional wind convergence across the southern tier of Lake
Michigan from a surface low currently across the Ohio Valley that
will drift northwest as well. Waves may be slow to subside into
Saturday in spite of the wind field weakening, and it is possible
the current Small Craft Advisory for Illinois and Indiana needs to
be extended through Saturday morning. Much quieter conditions
arrive Sunday into early next week as the low will finally kick
eastward and high pressure will approach from the northwest.



IL...Lakeshore Flood Advisory...ILZ006-ILZ014 UNTIL NOON Friday.

IN...Lakeshore Flood Advisory...INZ001 UNTIL NOON Friday.

LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 4 AM Saturday.




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