Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180651

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
151 AM CDT Fri Aug 18 2017

224 PM CDT

Tonight and Friday...

Gusty west-southwest winds and isolated showers across far
northern IL are the main forecast concerns through the remainder
of the afternoon and very early evening hours. Breezy and drier
conditions expected overnight and much of Friday.

Stacked surface and upper level low pressure noted across
northern WI this afternoon. Tight pressure gradient associated
with the surface low was producing gusty west-southwest winds near
30 mph, and these winds will continue through the afternoon
before gradually diminishing this evening. Regional radar mosaic
also depicts scattered showers wrapping across northeast IA/far
northern IL and much of WI, though these are expected to mainly
affect areas of far northern IL along the WI border area through
the remainder of the afternoon. Loss of weak diurnal instability
with sunset, and gradual drying of the low levels behind the cold
front, should allow these to dissipate by early this evening.

The upper low and occluded surface system lift northeast away
from the area later tonight and Friday, with modest height rises
and subsidence developing across the forecast area during the day.
Weak surface high pressure ridge passes downstate, with enough
gradient lingering farther north for breezy conditions across the
area especially during the morning and early afternoon hours
before the gradient relaxes further. Subsidence and slightly drier
low level flow should allow for more sunshine than today, though
mid-level cloud cover will likely increase from the west late in
the day as the next upstream short-wave approaches. Overnight
temps tonight should dip to the low-mid 60`s across the region,
with highs on Friday generally between near 80 far north and the
low-mid 80`s central and southern parts of the cwa.



224 PM CDT

Friday night through Thursday...

Showers and storms are possible Friday night as an upper level
trough axis rotates through the region.  Showers may linger east of
I-55 Monday morning, but dry conditions are then expected for the
rest of the weekend.  Cooler air moves overhead with the trough
leading to highs in the low 80s away from the lake Saturday.

High pressure spreads over the region Saturday afternoon bringing a
warming trend with it.  Highs will be in the upper 80s to near 90 by
Monday.  I have low confidence in precip chances Monday.  Guidance
indicates a short wave trough will approach the region and my kick
off showers and storms. The ECMWF has the trough much further north
than the GFS, and storms should be closely tied to the location of
the shortwave. Kept a slight chance to chance of showers and storms
mainly due to the GFS` solution.

For those hoping to view the eclipse, it looks like cloud cover will
be increasing through the day. I kept partly clouding wording

A cold front pushes down the lake and through the region Tuesday
evening/night, and we could see additional showers and storms ahead
of the front. The 12Z ECMWF is much more aggressive with vorticity
and precip Tuesday than the 00Z run was. The 12Z ECMWF and GFS also
have much more consistent front timing. While the 12Z ECMWF gives me
higher confidence, I still kept precip values in the chance
category. Quieter weather returns Wednesday. High temps are forecast
to be around 80 Wednesday and then in the upper 70s Thursday.



For the 06Z TAFs...

Concerns with the Chicago area TAFs today are:

- Small chance of MVFR ceilings near through 15Z
- Chance for SHRA, slight chance of TSRA this evening into

Lingering isolated light showers are departing northeast Illinois
very early this morning, and may graze ORD through 07Z and cannot
totally rule out MDW and GYY through 08Z. Otherwise, scattered to
broken mainly VFR ceilings are expected. Patchy MVFR is possible
especially near the IL/WI state line and northward. Confidence in
this remaining north of the Chicago area airports is medium-high.
Clouds should become more of a cumulus by late morning.

Westerly winds will turn a little north of west after daybreak.
Occasional gusts are likely. A clipper-like system will move
eastward in the region between mid-evening into overnight. This
will likely bring scattered showers and possibly a few storms,
though the most favored area is presently south of all the TAF
sites. The envelope of uncertainty certainly includes the
airports though, and felt it prudent to note a PROB30 at southern
sites (MDW and GYY).

By daybreak Saturday light winds will likely be trying to turn
north-northeast at Chicago area airports, but confidence on when
they do specifically shift in direction is low at this distance.
They may be light and variable much of tonight into early Saturday



151 AM CDT

It will be a gusty Friday over the lake with an anomalously deep
low for summer over far eastern Lake Superior early this morning
slowly moving northeast. The strongest west-northwest winds to 30
kt will be primarily over both the open waters and the eastern
shores of the lake. For the Illinois and Indiana nearshore areas,
breezy west-northwest winds will prevail through midday before
gradually easing. Near Small Craft Advisory conditions will be
seen this morning along the Illinois shore while likely reaching
it along much of the Indiana shore. Waves will be around 4 ft
along the Porter County Indiana shore, with lower wave heights
west along the Illinois shore as wind direction will be offshore.

A weak high pressure ridge will move over the lake this weekend.
For Saturday in the Illinois and Indiana nearshores, winds will
turn onshore by late morning or so, mainly through a lake breeze.
For Sunday, a lake breeze cannot be ruled out, though direction
is likely to remain south around 10 kt.

After south-southwest winds early next week, a turn to north is
presently forecast during midweek. It is possible when this
occurs, that there is a period of Small Craft Advisory along the
southern shores.



LM...Small Craft Advisory...nearshore waters
     until 3 AM Friday.




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