Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251838

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
138 PM CDT SUN SEP 25 2016

136 PM CDT

Through Monday...

Warm and humid conditions are in place early this afternoon ahead of
an approaching cold front. Temperatures have warmed into the mid to
upper 80s with dewpoints in the upper 60s across the area
contributing to moderate instability. Modified sounding from ILX
indicates just over 1000 J/kg SBCAPE in place across portions of the
area which initialized well on latest RAP guidance. With minimal CIN
in place, cu field has become increasingly agitated across much of
central IL and far northwest IL with scattered showers and
thunderstorms developing across those areas. Hi-res convective
allowing models continue to show widely scattered to scattered
showers and thunderstorms shifting across the remainder of the
forecast area through the late afternoon and evening hours. Deep
layer shear will likely be a limiting factor for a more organized
severe thunderstorm threat. Values of 15-20kt are in place south of
I-80 though do increase to 25-30 kt as you head northwest corner of
the CWA. With steep low/mid level lapse rates, cannot rule out an
isolated pulse severe wind threat, but overall am not impressed with
the severe setup.

Cold front will sweep across the region this evening and tonight
with winds shifting to the west/northwest and remaining breezy
overnight. Guidance suggests some precip possible immediately behind
the front as the upper wave moves across the region, but soundings
show the low levels quickly drying out behind the front so not sure
it will amount to much.

Cooler and blustery conditions will be in place on Monday. Should
see plenty of sunshine in the dry airmass outside of some scattered
afternoon cold-air stratocu development. Cold airmass will promote
deep mixing with top-of-the-channel winds of 30-35kt resulting in
strong wind gusts at the surface, especially during the afternoon
hours. Expect widespread wind gusts of 30-35 mph during the
afternoon, and cannot rule out a few sporadic gusts pushing 40 mph.
Afternoon highs will only be in the mid to upper 60s.



322 AM CDT

Monday through Saturday...

Monday through Tuesday: 500mb trough axis will be over the Great
Lakes region Mon, with continued cold air advection over Northern
Illinois. Thermal trough aloft will remain in the single digits
through much of Mon, with the cloud shield clipping far Northeast IL
late morning through early afternoon, before lifting northeast late
mon eve. Highs Mon will struggle to warm from the mid/upr 60s, and
could see a few locations in far Northern IL remaining around 60 for
highs. Tue will only be slightly better as the trough axis begins to
drift east and mid-lvl heights rise as high pressure builds in from
the west. But the overall trend continues to be dry air returning
Mon ngt through Tue.

Wednesday through Monday: Oper EC continues to differ with GFS on
how the 500mb vort will meander in the extended periods. The
downstream ridging is progged to linger, which would have a lagging
effect on the Great Lakes cutoff shortwave, which will likely
translate into a prolonged period of a dry airmass persisting for
the Great Lakes, especially Northern IL. Then as the weekend
approaches ensembles prog the trough finally drifting east of the
Great Lakes as a trough develops across the Southwest CONUS
meanwhile ridging returns for the Great Lakes/Upper Midwest this
weekend. Temps will be steadily rebound to seasonal conditions, with
highs in the 70s and overnight lows in the upr 40s to mid 50s. Broad
surface ridging will maintain a dry extended period.



For the 18Z TAFs...

A slow moving cold front will push across the region this evening
and tonight with showers and thunderstorms expected out ahead,
and strong winds behind. Mother nature starting to show her hand
early this afternoon with prefrontal showers developing from
central WI into far northern IL while the cumulus field is
starting to become more agitated across portions of central IL.
Hi-res convective allowing models have been fairly consistent
showing some scattered development mid-afternoon affecting the
terminals, though timing and coverage details have varied.
Additional convective development this afternoon could be very
near or over the terminals so will not have much time to further
hone in on timing details. Additional showers are possible with
the front this evening and possibly even some rain for a couple
hours behind the front, though drier air is eventually expected to
arrive behind the front.

Outside of the influence of precip, SSE winds late this
morning/early afternoon are expected to veer to the SSW by mid
afternoon with gusts to around 20 kt possible. Expect SW flow to
continue into the evening hours until the cold front sweeps
through with winds turning W/WNW behind the front. Even stronger
winds are expected tomorrow with mid to high 20 kt gusts possible
and may even tag 30 kt at times.




210 am...High pressure over the eastern lakes will slowly move
across Quebec and New England today and tonight and then off the
East coast Monday. Low pressure over southern Manitoba will move
into western Ontario today and then slowly move to Lake Superior
Monday night. A strong cold front will move across Lake Michigan
this evening shifting south/southeast winds to the west. As much
cooler air spreads across the lake and the gradient tightens...
westerly winds will increase to 30 kt by Monday morning and likely
remain at 30 kts through Monday night...possibly into Tuesday
morning. There remains some potential for gale force winds...
especially Monday afternoon into Monday evening...but confidence
remains low.

The low pressure will weaken as it slowly drifts southeast across
the Great Lakes region Tuesday into Wednesday. As it does...winds
will turn more northerly across Lake Michigan as high pressure
builds across Ontario and the upper midwest. cms





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