Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270707

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
207 AM CDT Thu Jul 27 2017

854 PM CDT

Monitoring a line of showers and storms marching east through
northern IL. My overall impression is the line is falling apart,
but slower than I originally thought it would. The latest RAP
analysis has a ridge of 1000+ J/kg through north central IL, but
the 0-6km shear axis has shifted further east. Surface CIN is also
increasing over the Chicago metro area. IR imagery shows cloud top
warming over northern IL and cloud top cooling over central IL.
Therefore, expecting the storms over northern IL to continue to
weaken and remain unorganized as they continue east. However,
cannot rule out an isolated cell or two pulsing up and producing
sub-severe wind gusts.

Also closely monitoring areal and flash flooding as PWAT values
are 1.5-2 inches. Storm motion is the key. If storms begin to
train or slow, flooding is much more likely. Upstream reports from
the Davenport office indicated as much as 2 inches of rain falling
in an hour. Not expecting rates that high since storms are falling
apart, but heavy rainfall still may lead to localized ponding or



229 PM CDT

Through Tonight...

For the remainder of the afternoon conditions should remain
relatively quiet with increasing mid and high cloudiness in advance
of an area of pcpn extending from srn WI, through ern Iowa and nrn
Missouri.  This activity is largely in response to a modest
shortwave tracking across the middle Mississippi Valley.  Warm,
moist, unstable conditions with MU CAPE of 2000-3000 J/kg,
temperatures climbing into middle to upper 80s and sfc dewpoints in
the middle to upper 70s.  Latest radar trends are for more sct
shra/isold tsra and in spite of a well defined MCV generated by
earlier convective activity over the cntrl plains, Extensive cloud
cover is likely helping to inhibit stronger, more widespread
thunderstorms this afternoon.

Looking into the evening hours, focus will shift to timing and
location of a swatch of heavy rainfall.  The 12Z DVN sounding
indicated pwats of 1.3 inches and the short range guidance indicates
that pwats should climb to slightly in excess of 2 inches as the
shortwave tracks east across the region.  Latest guidance is
suggesting that the shortwave should take a more sely track across
the region, which would suggest that the heaviest pcpn should be
suppressed a little farther south than previously anticipated.  While
locally heavy rainfall is still likely with some of the stronger
storms, the areal average heavier rainfall should be confined to
areas generally south of the I-80 corridor.  The overall severe
threat is also diminishing, but not totally out of the question,
with marginal mid level lapse rates and extensive cloud cover
somewhat limiting sfc based destabilization.  Also, with relatively
weak mid level flow, there primary weather threat will be more for
locally heavy rainfall, with an isolated wind threat more related to
wet microbursts than any kind of upscale thunderstorm development.


222 PM CDT

Thursday through Wednesday...

Main forecast concerns/challenges are with lingering chances for
showers and thunderstorms on Thursday, with a low chance for an
isolated strong storm mainly along/southeast of I-57.

Any lingering showers and thunderstorms Thursday morning will be
confined to areas south of I-80, but will be steadily southeast
out of the CWA through mid morning. Mainly dry conditions expected
for much of the cwa then anticipated through midday. Anticipate
cloudier conditions to continue throughout the day and with these
clouds likely to remain, have maintained similar high temp
forecast of highs around 80 with slightly cooler temps near the

There remains a chance for additional showers and thunderstorms
by early afternoon mainly for areas along and southeast of I-57.
Higher dewpoints pooling along surface trough still in place
across the southern CWA could serve as focus for additional
development. Coverage should remain widely scattered with any
development, with some isolated development possible further to
the north in northern Illinois. Most development likely to remain
subsevere, however, there is some potential for an isolated
stronger storm. This will be conditional on any clearing
occurring, and if that occurs, isolated areas of higher
instability are not out of the question. Storm chances will
quickly diminish by late afternoon and early evening, as focus
quickly shifts south of the area. High pressure then expected to
build across the region late in the work week into the weekend and
early next week, with no significant weather expected.



For the 06Z TAFs...

The concerns for the Chicago area TAFs for today are:

- Isolated showers through the morning

- MVFR ceilings and visibility at times through the morning, with
  patchy IFR conditions possible especially around and shortly
  after daybreak

- North-northeast winds steadily increasing in speed, reaching 10
  kt around 18Z.

Broad low pressure will scoot eastward south of the TAF sites this
morning. Its influence will be light, at times variable winds
through daybreak, eventually turning north near sunrise and
shortly after north-northeast as the low pulls away. In addition,
the ample moisture associated with the system will lead to widely
scattered showers overnight and variability in the ceilings, but
a decent possibility of MVFR at times. There has been some IFR
ceilings observed in the region and cannot rule out pockets of
that, and if it were to occur likely will be 600-900 ft. IFR
visibility in fog has been seen in Wisconsin, but cannot see that
low of visibility occurring in our area unless we scatter our
clouds briefly (low chance of such).

The north-northeast winds will steadily increase into this
afternoon, with gusts probable by mid-afternoon. The north-
northeast flow should support clouds, though with gradually
raising bases, through much of the afternoon before scattering or
clearing into this evening.



205 AM CDT

An active couple days to end the workweek are in store for Lake
Michigan. A somewhat broad surface low south of the southern part
of lake by about 150 miles this morning, will move east-southeast
today. Behind this, the pressure gradient will strengthen between
this low and an incoming high to the northwest. This will drive
increasing north-northeast winds that will spread southward this
afternoon, with gusty winds reaching the Illinois nearshore early-
mid afternoon and the Indiana nearshore shortly after. A few
gusts exceeding 25 kt are possible late day into the evening
across the south. In response to the wind increase, waves will
build quickly. So have a Small Craft Advisory beginning near the
wind increase forecast time for both the Illinois and Indiana

The high gradually spreads south on Friday, while the low deepens
in the Mid-Atlantic. In response, gusty north-northeast winds
will continue, with 25+ kt gusts across the south Friday afternoon
and evening. Waves will accordingly remain high and should remain
4+ feet into early Saturday morning. There is potential for these
waves to linger even through midday Saturday, so the Small Craft
Advisory may need to be extended.






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