Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200745

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
245 AM CDT Wed Sep 20 2017

242 AM CDT

Through tonight...

Main forecast challenge this morning is dealing with ongoing fog
in the area, dense in spots. Later today, focus will shift off
to the west as a line of thunderstorms approaches the area.

Early this morning, dense fog has developed over portions of
northwest Indiana and far northeast Illinois, primarily outside
the heart of the Chicago Metro. A dense fog advisory has been
issued for these areas through 9 AM CDT. Over northwest Indiana,
split channel fog imagery from GOES 16 shows fog developing
westward along the Iroquois River and lifting north. It`s possible
this fog expands farther west into eastern Kankakee and Will
counties through the morning in which case the Dense Fog Advisory
may need to be expanded. Otherwise, gradually increasing gradient
from the west should help keep fog from getting too out of hand
for the remaining counties.

By this afternoon, a strong shortwave currently over the Dakotas
will lift across Minnesota and Manitoba while a surface cold front
pushes across the Upper Mississippi Valley. Out ahead of the
front, temperatures warming into the upper 80s and dewpoints
nearing 70F will contribute to strong instability. GFS indicates
MLCAPE in excess of 3000 J/kg will be in place over portions of
northern Illinois and northwest Indiana, but capped by 850mb temps
warming to around 18-20C. Showers and thunderstorms are expected
to redevelop along the front over eastern IA and western WI and
track east to portions of north central Illinois this evening.
There isn`t a really clear signal between the global models and
convective allowing models with respect to intensity and coverage
of the storms this evening, and the capping and lack of upper
level support will be a concern. Continue to favor lower
coverage/intensity solutions, especially east of the I-39 corridor
as the boundary layer starts to stabilize after sunset and
because of the separation of the upper wave over Manitoba/Ontario
with upper ridging in place locally. Bottom line, whatever forms
to our west will at some point begin to weaken late this evening
as it crosses the local area and the eastern half to two thirds or
so of the CWA may see little if any precipitation. Deep layer
shear appears to be sub-marginal to support any widespread or
organized severe threat over the area although an isolated pulse
threat given the strong instability primarily in the western
counties is not out of the question.



242 AM CDT

Thursday through Tuesday...

A closed upper low is expected to dig from the Pacific NW into the
Intermountain West Thursday and Friday with downstream
amplification of the upper ridge occurring over the eastern half
of the U.S. The pattern becomes very blocky late in the week
through the weekend which will result in fairly stagnant
conditions allowing a prolonged period of above normal heat and
humidity along with primarily dry conditions. For now, both the
ECMWF and GFS are in reasonable agreement that this pattern will
begin to break down around midweek next week allowing cooler air
back into the region moderating temperatures closer to seasonal
norms. In the meantime, expect afternoon highs area-wide in the
mid to upper 80s and a few spots may tag the 90 mark Thursday
through Saturday. In addition, dew points in the mid to upper 60s
will be in place through this time frame.



For the 06Z TAFs...

GYY will be the most likely spot for dense fog tonight. GYY
briefly dropped to 1/4SM, and expecting dense fog to return to GYY
early this morning. Other sites will likely report MVFR vsby
early this morning, and I have low confidence in lower vsby. Since
ORD and MDW briefly dropped to MVFR, decided to keep IFR in the
TAFs and adjust at 09Z if needed. However, I`m thinking all three
sites will stay above IFR.

VFR conditions are expected after the fog dissipates this morning,
and gusty southerly winds develop in the early aftn. Max gusts
will be arnd 20 kt.

Convection fires along a cold front across WI and IA this
afternoon. RFD has the best chance in reporting SHRA and TSRA. I
think the line of storms will quickly dissipate as they encounter
more stable air across northern IL this evening. Therefore, the
only TAF with precip is RFD with SHRA and a TEMPO for TSRA. The
window of TSRA should be short, but I have low timing in exact
precip time of arrival. Guidance members very with how quickly the
convection will advance.



242 AM CDT

Southerly winds will gust to 25 kt this afternoon. High pressure
passes over Lake Superior tonight, and winds become north to
northeast over the northern half of the lake. Southerly flow
returns across the lake Thursday night due to a blocky pattern. A
low pressure trough will develop from the northern Mississippi
Valley through the southern Plains Friday while a high pressure
ridge remains parked over the eastern U.S. This pattern will hold
and result in southerly winds through early next week.



IL...Dense Fog Advisory...ILZ005-ILZ006 until 9 AM Wednesday.

IN...Dense Fog Advisory...INZ001-INZ002-INZ011 until 9 AM Wednesday.




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