Area Forecast Discussion
Issued by NWS Chicago, IL

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847
FXUS63 KLOT 170839
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
339 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...
209 PM CDT

Thursday night through Wednesday...

We dry out Thursday night as the upper level trough axis rotates
through.  Slightly cooler and drier air moves in leading to a dry
day Friday. A secondary trough axis rotates through Friday night
through Saturday, and showers and storms may form in response to the
trough axis.  Models disagree on how widespread precip will be
Friday night. The ECMWF is the most bullish while the GFS and NAM
have precip focused south of I-80. Decided to keep a chance of
showers and storms across the forecast area.

The upper level pattern becomes more zonal over the weekend and heat
builds back into the area. Dry conditions are expected for the
beginning of next week, and we may see 90s Monday.

The next significant chance of precip arrives Tuesday with an upper
level wave and surface cold front. Cooler air and an expansive
region of high pressure move in behind the front for mid week.

JEE

&&

.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...
209 PM CDT

High pressure over the eastern Great Lakes will reach New England
this evening. Meanwhile a low pressure system over Iowa will reach
the upper Midwest by Thursday morning and then move over the western
Great Lakes Thursday night. Winds will become southeast and increase
to 30 kt especially over the northern half of the lake tonight as
the low approaches the region.  Gusts to 30 kt are possible over the
northern half of the lake Thursday morning.  The low`s cold front
will swing across the lake Thursday evening/night, and winds will
become west to 30 kt behind it.

Winds will diminish and become northwest Friday, and high pressure
builds over the Great Lakes Saturday.  Winds will back to southwest
Saturday night into Sunday. Guidance then suggests a cold front will
slide down the lake Tuesday or Wednesday next week.

JEE

&&

.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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