Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170846
AFDLOT

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

.SHORT TERM...
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.

KJB

&&

.LONG TERM...
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.

Lenning

&&

.AVIATION...
For the 06Z TAFs...

Surface low pressure in southern MN early this morning will shift
northeastward over the Upper Great Lakes by this evening. As it
does so, an associated cold front will shift across northern IL
around midday, with a gusty westerly wind expected in its wake.
Expect gusts up around 26 KT this afternoon following the frontal
passage.

Another shot at some rain showers looks to return early this
morning (9-13z) as the main upper-level disturbance approaches
the area from the west. It does appear that thunder may be few
and far between with this batch of precip, so we have continued to
leave the mention for such out of the forecast at this time. A
short period of some MVFR CIGs could also develop over the area
overnight due to high lower-level moisture, but it does appear
likely that any lower CIGS would quickly improve after daybreak.
The main weather story for the rest of the day will then be the
breezy conditions expected.

KJB

&&

.MARINE...
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
Friday.

Lenning

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...11 AM Thursday to 3 AM Friday.

&&

$$

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