Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201142

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
642 AM CDT SAT AUG 20 2016

254 AM CDT

Through tonight...

The main forecast concerns continue to focus on the timing of
shower and thunderstorm activity today as a very dynamic storm
system impacts the Great Lakes region.

A broad and loosely organized frontal trough of low pressure is
in place over the Mississippi Valley, northeastward across the
upper Great Lakes. This will change today, however, as a rather
stout mid-level PV anomaly, currently over the Dakotas shifts
east-northeastward towards the upper Great Lakes later today.
This will lead to rapid intensification to the surface frontal
trough, ultimately resulting in a sub 998 MB low tonight over far
southern Ontario. As this surface low organizes today, expect an
attendant cold front to be driven eastward over northern Illinois
and northwestern Indiana into this afternoon.

Thus far overnight the storms have remained to our north and west.
The storms currently moving into western Illinois are being
driven by a second mid-level disturbance, which is currently
pushing over northern Missouri, per the latest water vapor
imagery. This disturbance is expected to shift northeastward
across northern Illinois through the morning, and this will allow
these storms to continue eastward over the area through mid to
late morning.

The question then becomes how much thunderstorm activity will
develop in the wake of this first period of storms? The cold front
is expected to shift into northeastern Illinois by early to mid
afternoon, so it appears the best window for the second round of
SCT storms will be from around noon through 3 PM. While not much
isolation is expected following this mornings activity, the
decent dynamics with the developing synoptic system may end up
compensating for the modest instability and allowing for at least
some SCT storms developing ahead of the front. This second area of
storms will be most favored over eastern Illinois and Northwest
Indiana. While some strong storms cannot be ruled out, it appears
that deep layer shear of 20-25 KT will result in less organized
storms and hence less of an overall severe threat. However, a few
isolated strong storms could produce some marginally severe wind
gusts. Heavy rainfall will can also be expected with these storms,
though the fast movement to the storms should limit overall
rainfall amounts.

The storms should end area-wide by late afternoon or early this



344 AM CDT

Sunday through Friday...

Surface high pressure will set up over the area for late Sunday
and Monday. Sunday will be a breezy but much cooler day across the
region in the wake of the strong system impacting the region
during the short range period. Temperatures will likely only top
out in the mid 70s on Sunday, and the mid to upper 70s on Monday.
Also, with the surface high setting up over the area for Sunday
night, light winds will set up under mainly clear skies. This
should allow for temperatures to drop into the low 50s outside of

Temperatures will warm back into the low 80s on Tuesday as the
low level flow turns back out of the south. Another potent weather
system is expected to spawn another eastward moving surface low
across the Dakotas and into south central Canada on Tuesday. This
storm system will bring another period of showers and storms to
the mid section of the country through Thursday. Our best chances
of storms locally will be Wednesday into Thursday as a another
cold front shifts over the area. This could also end up being
another period favorable for heavy rainfall given the likelihood
of a return of deep Gulf moisture.

Following this good chance of storms, high pressure and cooler
weather looks to make a return to the area for Friday and into
next weekend.



For the 12Z TAFs...

Ragged line of showers and thunderstorms are approaching TAF sites
at 13z and may see some lingering convection through 17-18z;
however, the progressive nature of the system appears to push the
chances for convection through TAF sites earlier. The frontal
boundary is expected to arrive around 18z with increased gradient
and speeds nearing 10-15kt and gusts to 25kt and then turning
westerly by midday. This morning low cigs will persist with bases
around 1kft agl and a few sites may dip further to around
600-800ft agl through mid-morning. Gusty west winds will continue
through early this afternoon with cigs slowly lifting back to mvfr
conds, and winds turning northwesterly. Precipitation is expected
to end at TAF sites by 20z and drier air will be arriving.



346 AM CDT

Main concerns for today will be on the strengthening low pressure
system that will arrive over the northern portions of the lake
this afternoon. Winds will steadily increase ahead of the low
pressure from the south to southwest with winds approaching 25 kt
and likely nearing 30 kt for the northern half of the lake. This
will create hazardous conditions for small craft this afternoon
through Sun morning. Furthern north tonight the gradient will
tighten further and likely produce a period of gale force gusts to
35kt and possibly 40 kt over the far northern Lake Michigan zones.
Gales will continue through Sun morning, with only occasional gale
force gusts further south tonight early Sun.

Winds will be turning west/northwest with much cooler air flowing
over Lake Michigan Sun, which will allow waves to remain elevated
through Sun ngt before subsiding. Then high pressure will briefly
calm winds then turn southwest and increase Mon in advance of
another system towards the middle of next week.


IN...Beach Hazards Statement...INZ002...10 PM Saturday TO 7 PM Sunday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744...1 PM
     Saturday TO 10 AM Sunday.

     Small Craft Advisory...LMZ745...1 PM Saturday TO 4 PM Sunday.




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