Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 260820
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
320 AM CDT MON SEP 26 2016

.SHORT TERM...
319 AM CDT

Through tonight...

Early this morning the front was reaching the eastern portions of
the forecast area, with a narrow channel of showers extending along
the boundary from Central Illinois northeast through Gary and
Southern Lake Michigan. The drier air was evident behind the
boundary with dewpoints falling into the lower 50s and upper 40s
across Eastern Iowa. IR imagery shows the clouds clearing across far
Western Wisc/Central Iowa. Latest guidance indicates this clearing
will arrive after daybreak, and likely linger through this afternoon
with mostly sunny skies. It is possible that with the cold air
advection aloft, enough of a thermal differential and lingering
moisture could develop some shallow stratus but likely not beyond a
few/sct coverage. The bigger concern may end up being increased fire
concerns by early afternoon, as deep mixing will help to produce
afternoon Min RH to around 30-35% with gusty winds drying the
surface considerably. Highs today will remain in the 60s, with the
exception being the far southern CWA that could touch 70.

Surface ridging will remain to the west of the region, with a
continued west/northwest wind across Northern IL. With minimal cloud
cover tonight, temps should radiate into the mid/upr 40s to low 50s
closer to Lake Michigan.

Beachler

&&

.LONG TERM...
319 AM CDT

Tuesday through Sunday...

Tuesday through Wednesday: Main focus for Tue/Wed will be on the
placement of the 500mb vort and surface low. Current guidance is
indicating the shortwave will be stalled over Lake Superior early
Tue, then begins to drift south Tue aftn/eve. This will likely
spread precip chances further south towards Northern IL late in the
day Tue through Wed as the shortwave drifts south towards Southern
Lake Michigan Wed morning. Guidance continues to trend cooler for
Wed as cloud cover will be considerable and limiting much direct
heating. Have nudged temps cooler towards the lower 60s, but could
see this not being cool enough with the possibility of several
locations remaining in the upper 50s.

Thursday through Monday: In the extended periods there remains
considerable uncertainty with the shortwave that will start the week
across the Northern Great Lakes and likely trend towards the Ohio
Valley/Middle Altantic region Thursday. Downstream 500mb ridging
over the Northern Atlantic will act to block or stall the shortwave,
which may result in the shortwave retrograding back towards Lower
Michigan late in the week. Ensembles continue to struggle with how
to resolve the downstream ridge, which is resulting in several
solutions diverging with differing forecasts. Have opted to not
stray too far from a blend of EC/GFS in the extended periods, with
temps slowly expected to moderate towards the low 70s. Towards the
second half of the weekend ensembles are returning to greater
confidence in the 500mb wave pattern, with weak troughing over the
Southwest CONUS and a downstream ridge for the Great Lakes/Upper
Midwest. This should help to lift the lingering shortwave towards
the Northeast CONUS.

Beachler

&&

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

1222 am...Westerly winds 10-15 kts will continue through sunrise
with some higher gusts at times. Speeds/gusts will steadily
increase after sunrise with prevailing speeds 16-20kts and gusts
into the 25-30kt range from mid/late morning through sunset...
when speeds/gusts will diminish.

A few light showers will be possible for the next few hours as a
weak disturbance moves northeast across the area but cigs/vis
should remain vfr. cms

&&

.MARINE...

220 am...Low pressure over western Ontario will slowly move
southeast to Lake Superior Tuesday afternoon and then move south
across western lower Michigan Tuesday night...as it slowly
weakens. Westerly winds will persist across the lake through
Tuesday with speeds to 30 kts. There remains the potential for
some gale gusts this afternoon into tonight but confidence on how
widespread these will become remains low and will maintain gale
gust mention. Winds will turn northerly Tuesday night as the low
moves south with speeds diminishing. As the low moves to the mid
Atlantic Wednesday...high pressure will build across Ontario and
the upper midwest...tightening the gradient again across the
western Great Lakes. North to northeast winds may increase back to
30 kt Wednesday night before diminishing on Thursday.

With this low slowly moving across the lakes region and associated
cold air aloft...there will be a chance of waterspouts Tuesday
into Wednesday. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 7 PM
     Tuesday.

     Small Craft Advisory...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 10 PM Wednesday.

&&

$$

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