Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 301524

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1024 AM CDT Fri Sep 30 2016

1021 AM CDT

The theme of cloudy, breezy, and showery continues. Have added
areas of drizzle into the forecast as well through the next few
hours and may need to into early afternoon. Continued low-level
isentropic ascent within a saturated layer looks like it could
persist on upstream observations and the 12Z NAM has trended more
pessimistic. The true showers are more isolated and believe that
will be the case at least through early afternoon before another
spoke of vorticity and associated ascent wraps westward into the
region. Otherwise have inched down highs a degree or so.



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.




Updated for the 15Z TAFs...

Low-level clouds right around 800 ft are likely at ORD through 17Z
with some temporary lower cloud bases observed at nearby locations
of PWK and UGN. Also area webcams have indicated scattered to
broken ragged lower cloud bases. Think prevailing of 800 ft will
be dominant, but cannot rule out temporary BKN of 600 or 700,
especially prior to 17Z when drizzle is most likely. After 17Z,
have trended more pessimistically and may need to further (i.e.
there could be IFR bases at times early this afternoon).

For the 12Z TAFs...

Light rain showers will continue at times through the day as a multitude
of shortwaves on the north end of deep low pressure to our
southeast shift through the area. It appears a better chance for
more organized rain will come this afternoon and this evening as
the low continues northward. Guidance hints at more organized
waves during this period, but timing in this pattern is certainly
a challenge and light rain/drizzle is possible at any point.
Satellite and observations do suggest that the morning period
could be a lull and cigs could also briefly lift but think that
MVFR will be around in spots all day as pressure falls continue.
Gusty NE winds continue as well. Guidance is hitting IFR and even
LIFR again tonight. Will leave the return of IFR Friday night in
place, though confidence is low on this time as to how long it
last. Certainly it is possible in the showers, with visby
reductions to IFR as well. The surface low will inch northward
tonight also, and under the low itself precip coverage is low but
cigs are IFR to even LIFR that is hinted at in guidance overnight.
This will bear watching.



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.



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




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