Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191757

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1257 PM CDT Wed Jul 19 2017

335 AM CDT

Through Thursday Morning...

Forecast concerns in the short term center on potential storm
complex moving into the area tonight, and with it an attendant
wind and heavy rain threat. Have expanded the likely thunderstorm
mention in the forecast for overnight in northern Illinois north
of I-80/88 and boosted rainfall amounts some. Still modest to high
uncertainty for 24 hours out in the more precise path of this
storm complex from the Upper Mississippi Valley toward our region,
as well as the rate of any weakening overnight/early Thursday
morning. Also, while this would produce a period of very heavy
rainfall, confidence in this propagating quick is higher, so that
will limit duration of such rain.

The humid air has invaded the area early this morning with dew
points generally in the upper 60s/lower 70s. A surface trough/weak
cool front is draped across southern Wisconsin and will move
slowly southward this morning, and become stationary this
afternoon as it re-orients as a warm front. With very rich
moisture in the low-levels (850mb-surface), spotty storms are
possible this morning and afternoon. The focus this afternoon will
be primarily along this boundary. Convergence looks to be quite
weak during peak heating, however given the moisture and mixed
layer CAPEs (1500 J/kg), it may not be too difficult to trigger
a couple storms.

Temperatures today will depend on that boundary location, with
northeast winds expected northeast of this. The boundary is
presently forecast to be near a Rockford-to-Valparaiso line, with
mid 80s northeast (upper 70s lakeside), and around 90 south. With
that forecast, heat index readings in the south/southwest will
dabble around 100.

With regard to storms overnight, initiation of storms is expected
in the eastern Dakotas/southern Minnesota region this afternoon
ahead of a potent short wave on the northern periphery of the
strong, summer ridge. While the upper jet of 115+ kt moves nearly
due east and the mean layer winds are closer to due east, expect
that the forecast organized storm complex will ride the
instability gradient, taking it more southeast. This means into
or clipping northern Illinois overnight. In addition, Corfidi
vectors which take into account replenishing inflow, take the
complex more southeast on all guidance. So have followed closer to
the 19.00 GFS and NAM solutions, with highest PoPs in northeast
Illinois (east of I-39 and north of I-80). While the HRRR
solutions are bringing the storm complex into northern Illinois by
late evening, feel they are too quick, and favor more so after

The maintenance parameters for this mesoscale convective system
(deep layer shear, shear balance, instability, low-level jet) all
favor the MCS to hold together fairly well to at least near the
northern CWA edge. A damaging wind threat does exist with this as
far southeast as our area especially if the system accelerates.
The plethora of CAMs generally indicate a well-defined both at
least to our northwest, and some into our area. For rainfall, see
the hydrology discussion for more.



346 AM CDT

Thursday Afternoon Through Tuesday...

Certainly for Thursday afternoon and night, and likely even into
Friday, will depend on how the overnight MCS pans out, as its
associated outflow/modulated frontal boundary will be key and have
a domino effect lasting 24-48 hours. Not only will this impact
storm chances and focus areas but also temperatures, especially

Assuming the MCS has some in-roads into the CWA late tonight, this
lowers confidence quite a bit in upper 80s to lower 90s on
Thursday. It will not take much recovery time though if we do
have morning clouds/cooler air, as the thermal ridge of 850mb
temperatures above 20C will be directly over the area. If the
southern CWA is not affected at all, there could be mid 90s and
near heat advisory.

As the boundary begins to move back northward, most likely on
Thursday night, the baroclinic zone will serve as a focus for
isolated/scattered storms. These would have a heavy rain/training
concern, though very difficult to say where in the region that
would be. For Friday, a stronger short wave and associated speed
max in the westerlies is forecast and that would likely yield more
storm coverage and robustness into Friday night. Presently this is
in southern Wisconsin/far northern Illinois, and if that were to
pan out verbatim there would be heightened flooding concerns, but
confidence remains lower. The boundary is forecast to shift into
the area during the start of the weekend and clear the CWA by the
middle to end of the weekend. A lot of uncertainty in this period,
but needless to say, thunderstorm chances maintained.

For late week heat, the northern extent of the highest
temperatures will depend heavily on convective influence. The
high dew points will be found areawide, and may even be a little
low especially Friday into Saturday, as moisture pooling ahead of
the boundary could occur. Also with crops now fully mature,
evapotranspiration could boost surface moisture a bit further than
a guidance blend indicates. The southern/southwestern forecast
area may need a heat advisory for Friday and possibly Saturday
based on the current forecast (heat index readings 103-107). As
for Rockford to Chicago, less confidence exists in uninhibited
heating for temperatures to reach 90 and heat index readings to
exceed 100.



405 AM CDT

The Hydrologic Outlook (ESF) for along and north of I-88 has been
continued for concerns of multiple rounds of storms through early
this weekend. Observational trends are supporting the forecast in
terms of a very moist atmosphere over the area from now through
Saturday. The confidence on receiving a burst of heavy rain with
a quick-moving storm complex in far northern Illinois overnight
has increased, uncertainty still resides in exact track of this
system. Where the center of it passes certainly will have
potential for 1"-2" in around 1-2 hours. Regularly with such
quick movement, even if intense, these tend to produce widespread
averages more of 0.50"-1.00". If there were to be some slower
movement or backbulding, concerns would raise for flooding.

Wet antecedent conditions particularly over the Des Plaines and
Fox River basins provide concerns through the rest of the week as
storm chances inch back up Thursday night and into Friday.
Location of this rainfall is even more uncertain, however there
may be higher concern of flooding with that given it is a regime
that could favor training storms.



For the 18Z TAFs...

Main forecast concern is thunderstorm potential/timing tonight
into Thursday morning. Another concern will be timing of a lake
breeze/wind shift to northeast this afternoon and wind speed
following the lake breeze passage.

For this afternoon, the main concern will focus on the timing of a
wind shift and wind speeds following the passage of a weak cold
front/lake breeze boundary. Latest radar imagery shows the
boundary steadily pushing inland. Winds have been lgt/vrbl this
morning, but should shift to nely following the passage of the
boundary. There is not much push behind the boundary, so expect
that winds will remain under 10 kt following the passage of the
boundary, with prevailing 7-8 kt speeds likely, though an isold
gust up to 10-12 kt is possible for an hour or so following the
lake breeze passage.

The next concern will be thunderstorm trends tonight and tomorrow.
A complex of thunderstorms has developed over ern SD/swrn MN
this afternoon. This cluster of storms is expected to track
generally esely along a corridor of enhanced instability. The
various models, including the high res convection allowing models
and the larger scale models have a general trend of tracking the
thunderstorm complex into nrn IL late tonight, but are trending a
bit slower than previous runs. While there is a relatively decent
consensus on the general trends of the convective complex, there
are significant differences on the exact track as well as the ts
coverage of the system by the time it is close enough to impact
the terminals. So, the latest update has mainly to push back the
prob30 ts groups, but have maintained the prob30 mention due to
the continued uncertainty in the timing and coverage. some
guidance is also indicating the potential for secondary
development following the passage of the initial round of ts, so
have included a vcsh mention for the morning hours tomorrow,
though some isold ts is not out of the question. Thunderstorms
late tonight into early tomorrow morning will only be the first
round of thunderstorms to impact the area as a pattern continues
to evolve there multiple rounds of thunderstorms may impact the
region. Latest guidance suggests a second round of organized
convection moving into the area tomorrow afternoon as the weak
cold front/lake breeze boundary that pushed through the area this
afternoon lifts back north as a warm front with weak frontal
convergence lining up with the persistent corridor of instability.
So, have added a prob30 mention to the 24-30hr forecast period of
the ORD TAF.



224 am...A weak cold front will move south across Lake Michigan
this morning shifting winds northerly. But this shift will be
brief as winds turn back southerly over northern and central
portions of the lake this evening. Exact wind directions become
problematic late this evening and overnight with the expectation
that thunderstorms will be moving across southern portions of the
lake and will likely cause erratic winds with outflows. Thus...
confidence is low for wind directions...particularly on the
southern part of the lake. A weak area of high pressure is
expected to move across the northern and central Great Lakes
region Thursday night and Friday as a broad area of low pressure
develops over the northern and central plains. This low will move
across the Great Lakes region on Sunday as high pressure builds
across the upper midwest Sunday night into Monday.

Areas of fog have developed over northern portions of Lake
Michigan early this morning and these appear like they will
continue and possibly move south as the front moves south this
morning. Confidence is low...especially regarding how dense this
fog may become so trends will need to be monitored. cms





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