Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210554

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1254 AM CDT Mon Aug 21 2017

1005 PM CDT

Scattered storms have developed since 9 pm across northern
Illinois, immediately ahead of a mid-level short wave disturbance
per satellite water vapor imagery. Moist advection in the lower-
mid levels appears to be gradually increasing in general across
the region, and a modified 00Z ILX soundings providing CAPEs above
850mb of 2000+ J/kg. While that instability is fairly robust, the
strength of the low-level jet/moisture transport is still
minimal, with only 15-20 kt per ILX VWP data. So the severe threat
still remains low, but do expect some storm
organization/longevity with this current activity given the
instability and deep layer shear around 30 kt. In addition, the
ILX sounding indicated 1.89 inches of precipitable water, so heavy
downpours will be favored under any of these storms.

Immediately behind this wave, which should clear the CWA around 1
a.m. or so, there should be a break in convective coverage and
maybe in showers and storms altogether through the rest of the
overnight. Convection should start developing and organizing
across Iowa overnight at the nose of the analyzed low-level jet
and underneath strengthening upper level flow. Organization of
these storms into an MCS would favor a progressively south of
due east motion by the time the activity were to progress into
Illinois. However, confidence is low on how that activity will
evolve in Iowa. In addition, scattered storms further northeast
into parts of southern Minnesota and Wisconsin are likely into the
Monday morning hours. In general, some of this upstream activity
is likely to move into at least north central Illinois during the
morning hours, though how widespread again is highly uncertain.
The severe threat that exists over Iowa overnight would likely be
diminishing with any morning storms into the CWA.



219 PM CDT

Through Monday...

Forecast details are complex through tomorrow with several features
of concern resulting in potential for showers/storms as well as
complicating the sky cover forecast for the eclipse. Water vapor
imagery shows a fairly zonal west to east flow with several
shortwave troughs embedded. One trough is approaching the local
area with an attendant low level circulation remnant from early
morning convection that had occurred over Iowa. This low level
feature is currently passing the Quad Cities and interacting with
a NW-SE arching warm frontal boundary. A band of rain has
persisted and arcs from west of Dubuque to near Peru. Dewpoints
have increased across the area with temps in the mid to upper 80s
resulting in an unstable airmass under a mix of sun and clouds.
Satellite shows scattered cu development across the Chicago metro
with a look out the window showing little vertical extent to
these. As ascent increases through the afternoon and daytime
heating/instability peaks, would not be surprised to see some
spotty shower/thunderstorm development. A wind gust threat may
accompany any organized cell thanks to an axis of high downdraft
cape which aligns with SPC Day 1 outlook thinking. With the
remnant circulation looking to fester into the evening,
shower/thunder chances will linger as well and have reflected this
in the hourly grids across NE IL and NW IN.

Attention turns to the overnight hours into Monday. Surface low
pressure across the central Plains this afternoon will develop east-
northeastward. Low-mid level warm advection will maximize into
western/northern Iowa on an increasing low level flow in conjunction
with the development of another upper shortwave. Though guidance is
mixed on the details, expect thunderstorms to develop generally
across northwest Iowa and expand eastward overnight. Steering flow
would take this activity eastward and it may turn somewhat
southeastward with time. Will continue with thunderstorm chances
entering the northwest part of the forecast area closer to
daybreak, though the timing may need better refinement. Guidance
is pretty consistent with activity decaying through late morning
which means it may struggle to reach the Chicago area and IL/IN
border region. Have trended pops down as you go east. Cloud cover
does look extensive through the morning and into the afternoon
but there are caveats that may allow for at least a window of
reduced cloud cover. However, these primarily depend on the
thunderstorm trends and timing. Should a more persistent area of
showers/storms move through then associated subsidence behind it
could yield a break in the clouds during the early afternoon.
Lesser activity may result in lingering high level cloudiness.
Temp- wise, expect highs similar to today.



219 PM CDT

Monday night through Sunday...

Highlights of the longer term portion of the forecast include
warm and humid weather into Tuesday with shower and thunderstorm
potential, then cooler and drier conditions mid-late week as a
deep upper trough develops across the eastern CONUS.

Fast, relatively zonal flow is progged to be in place across the
Upper Midwest Monday evening, with guidance in good agreement in
depicting a strong short-wave trough digging southeast into the
northern Great Lakes region overnight. In the process, surface low
pressure develops across the northern Lakes, along a cold front
stretching southwest into the Central Plains, and this front then
pushes slowly southeast across the forecast area Monday night into
Tuesday afternoon. While convection timing and coverage will
likely be modulated by MCS remnants which will likely affect the
region during the day Monday, strengthening large scale forcing
and approaching surface cold front will likely support potentially
strong convection across much of the area Monday night. High-res
convection allowing guidance suggests the potential for MCS
development to our west during the afternoon/evening, which would
then spread east into the forecast area overnight.

Moderately strong instability indicated by forecast MUCAPES near
2000 J/kg, and increasing deep layer bulk shear of 30-40 kts in
strengthening southwest flow would support the potential for
strong to severe storms, particularly with respect to damaging
winds. A Locally heavy rainfall potential will also likely exist,
given a southwesterly low level jet parallel to the approaching
cold front and high Theta-E air mass characterized by surface dew
point temps of 70F or higher and p-wats around 2 inches. WFO LOT
cwa is included in both the SPC day 2 severe outlook and WPC day 2
excessive rainfall outlook.

Current guidance trends depict weakening convection continuing to
push southeast across the area Tuesday morning, with chances
primarily confined to the southeast half or so of the cwa by
Tuesday afternoon. Despite the presence of the cold frontal
boundary, the greatest potential for additional strong-severe
storms on Tuesday looks to be east/southeast of the forecast area,
where outflow boundary from overnight MCS will provide better
instability and convergence.

Breezy, cooler and drier conditions are then expected to develop
from the north and northwest late Tuesday, as a sprawling area of
surface high pressure builds southeast from Canada and the
Northern Plains in the wake of the deepening eastern CONUS upper
trough. GFS and ECMWF do depict a secondary cold frontal boundary
pushing into the region early Thursday and a hint of some isolated
shower potential along that especially near the lake as cool (+8C
at 850 mb) spreads over the area, but otherwise the period from
Tuesday night into next weekend appears dry. Below normal temps
are anticipated (coolest Thursday), with daytime highs in the low-
mid 70`s and overnight lows in the 50`s, and probably a few upper
40`s in cooler spots. As the surface high drifts east we should
see a return to south-southeast surface winds and a gradual
moderation in temps next weekend, back closer normal for late



For the 06Z TAFs...

Aviation forecast concerns for Chicago area airports today and
tonight are:

- Scattered showers with a couple storms nearby through 07Z

- Returning shower and storm chances by late morning, though the
  highest storm chances by far look to be overnight Monday night

- Winds varying between southeast and southwest through the
  morning, though light in speed.

Scattered showers and storms forced by an upper level disturbance
will continue to shift east away from the TAF sites by 07Z or so.
Under these are brief heavy downpours with IFR visibility. In the
wake of these, rain-cooled air coupled with the humid conditions
could allow for fog to develop, most likely across north central
Illinois (RFD and DPA areas). Confidence in this is low, as
cirrus should be spreading overhead by 09Z, which may limit how
low visibility can get, though some IFR fog is certainly

Storms across western/central Iowa early this morning will track
east-southeast with other scattered development across that
state. These storm will be driven on a strong low-level jet, and
as such expect the nose of that to be the primary focus. This will
weaken some after daybreak and storms should show a similar
trend though it may be very gradual. There is low confidence in
the forward speed of any of these storms and just what coverage
activity will be as it moves into/over northern Illinois during
the morning/early afternoon. At least showers and possibly will
likely be near RFD during the mid-late morning.

There should be a gap from roughly mid-afternoon into the evening
for storm coverage across the local area, with storms developing
from Minnesota through Missouri/Kansas ahead of the system cold
front. This front will track into the area and high shower and
storm coverage is likely ahead of the front. This is most favored
overnight for the TAF sites. Storms could be near ORD and MDW as
early as 03Z this evening, but have favored after 06Z.



219 PM CDT

Main marine forecast concern is with a period of stronger
northerly winds late Tuesday through Wednesday, as high pressure
builds in behind a cold front which will push south of the lake

South-southwest winds will start the week, in advance of an area
of low pressure system which will pass across the northern Lakes
Tuesday morning. Thunderstorms, a few strong to severe, will
develop across Lake Michigan Monday night, and will continue into
early Tuesday as the low passes and trails a cold front down the
lake. The front will clear the lake later in the day, while the
low deepens while pulling away into southern Quebec. The gradient
will tighten between the deepening low and an expansive area of
high pressure which will spread southeast from the Canadian
prairies and Northern Plains, and this is expected to produce a
somewhat prolonged period of northwest-north winds in the 20-25 kt
range, and possibly some higher gusts, from late Tuesday through
the day Wednesday. The persistent northerly wind and building
waves may result in small craft advisory conditions on southern
Lake Michigan during this mid- week period.

The surface high will eventually spread across the western Lakes
Thursday through Friday, allowing winds to diminish and waves to






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