Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171719

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1219 PM CDT Thu Aug 17 2017

1109 AM CDT

Minor updates to near term of going forecast, mainly to lower
temps a bit across the north/northwest counties this afternoon.

Fairly deep low pressure system centered over western WI late this
morning, with a cold front moving east across the Mississippi
River. Deeper moisture was primarily off to the east of the cwa as
the low is occluding, though surface dew points in the upper 60s
to lower 70s persist along/ahead of the front. Water vapor imagery
depicts drying in the mid-low levels across central/western IA
indicative of a mid-level short wave wrapping around the southern
periphery of the upper low across the Upper Mississippi Valley,
and this should allow for isolated-scattered shower development to
continue through mid-afternoon with forcing associated with the
front and weak low-level instability, especially across far
northern parts of the cwa.

Extensive cloud cover was keeping temps down a bit this morning,
and low level thermal fields suggest afternoon highs may struggle
to reach 80 degrees across the far north/northwest IL counties.
Thus lowered temps a bit there for this afternoon.



339 AM CDT

Through tonight...

A potent area of low pressure (for August standards) over
southern MN, will shift northeastward over the Upper Great Lakes
by tonight. As it does so, an associated cold front will shift
eastward across the area later this morning and into the
afternoon. Once this front passes, expect some gusty westerly
winds (up around 30 MPH) to set up across the area this afternoon
and into the evening.

It appears that the main threat of any strong thunderstorm
development this afternoon ahead of the cold front may occur east
of the area in Indiana and lower Michigan. We are not out of the
woods yet for precipitation, however. There is still likely to be
another period of showers, and possibly a few thunderstorms early
this morning. While there is an area of widely scattered
showers/thunderstorms over north central IL at this time, there is
also another area of mainly showers shifting northeastward over
west central Illinois. This activity appears to be associated with
a surge in the low-level jet being induced just ahead of the
approaching main upper trough. As this activity continues to shift
northeastward, it will overspread much of eastern IL and
northwestern Indiana early this morning. Additional widely
scattered activity is also likely to continue over far northern
Illinois as well, at least through daybreak. All of this activity
should end by mid morning as it shifts out of the area. A few
scattered showers may redevelop over the area this afternoon,
especially over northern IL, where cyclonic flow aloft will drive
another weak disturbance across southern WI.



345 AM CDT

Friday through Wednesday...

Only two periods look favorable for any sort of active weather from
Friday through the middle of next week.  The first would be Friday
night in association with a weak low center dropping out of the
northern Plains, visible early this morning as a disturbance on IR
and WV satellite imagery moving from Alberta into Montana.  The
likely timing of this appears to be overnight Friday into the
predawn hours of Saturday which hopefully should limit its impacts
on outdoor activities Friday evening and Saturday morning.

The second period of active weather appears to be in the Tuesday
timeframe as a cold front moves across the area.  The ECMWF is
faster than the GFS with this feature by about 12 hours which
actually is not a vast disagreement five or six days out.  To
address the differences in timing we will maintain a mention of
precip chances from Monday evening through Tuesday evening, though
the actual event likely would not be of that duration.

Beyond these two periods, the big focus for the extended period is
on the eclipse Monday.  Assuming there is not a big acceleration in
the activity mentioned for Monday night or Tuesday, daytime Monday
appears to be dry and warm under the influence of southwesterly
flow. There are no strong signals yet to nudge sky cover forecasts
higher or lower, so will maintain fair weather partly cloudy
conditions for now.



For the 18Z TAFs...

Gusty west-southwest winds 25-30 kts are the main aviation
forecast concern this afternoon, with MVFR cigs continuing to
rise. A few isolated showers possible, though these are expected
to primarily be north of the terminals closer to the IL/WI border.

Surface low pressure over western WI will continue to lift slowly
northeast across the northern Lakes through this evening, with a
trailing cold front moving east across the area this afternoon.
With the low unseasonably deep for mid-August, the tight pressure
gradient will support winds gusting 25-30 kts during the afternoon
during best diurnal mixing. Gusts will tail off this evening with
the approach of sunset, though breezy west winds will continue
into the night. MVFR cigs starting to rise fairly quickly with
more breaks in the cloud cover, and should generally be VFR by
early-mid afternoon. Scattered showers over northeast IA and
southwest WI will spread east along the IL/WI border as an upper
level trough moves across the region through the afternoon.

VFR conditions and breezy but less windy conditions expected
later tonight and Friday.



345 AM CDT

The next two days will be fairly breezy over the lake with the
approach and passage of a cold front.  Southerly flow will increase
today and prompt headlines for small craft but fortunately waves in
the IL/IN nearshore will not grow too large with the offshore winds.
Passage of a cold front toward evening will turn winds westerly but
maintain a fairly tight pressure gradient, which with the cooler air
over warm water will sustain higher speeds into at least the late
evening to overnight hours.  Weak high pressure moving over the lake
for the weekend will gradually reduce wind speeds through the day on



LM...Small Craft Advisory...nearshore waters
     until 3 AM Friday.




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