Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210827

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
327 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.



326 AM CDT

Tuesday through Sunday...

Tuesday will be a transition day as the pattern shifts from zonal
flow aloft to more highly amplified as a deep upper low settles into
sern Canada, broad long wave troughing develops over the ern CONUS
while upper ridging builds over the Rockies.  This will set up cool
nwly flow aloft over the midwest with the cold front pushing quickly
to the south and east of the local area and high pressure builds out
of south-central Canada and spreads across the northern plains.  By
Wednesday, sfc high pressure will spread across the region while
nwly flow persists aloft.  This will set up an extended period of
dry weather with below normal temperatures.  For Tuesday through the
weekend, high temps will generally be in the 70s across the region.
A prevailing north component  to the wind field will keep temps
along the lakefront in the lower 70s while inland locations should
climb into the middle to upper 70s.  The coolest day of the period
should be Thursday as north to northeast winds bring the coolest air
into the area and lakefront locations may see temps only in the
upper 60s while inland locations only see lower to middle 70s, which
would be about 10 degrees below normal.  As the surface high drifts
east we should see a return to south-southeast surface winds and a
gradual moderation in temps late in the week and into the weekend,
with temperatures returning back closer normal for late August.


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.



326 AM CDT

A weak trough of low pressure extends across the upper Great Lakes
with an associated frontal boundary extending southwest to low
pressure over the western plains. The front will remain nearly
stationary through Tuesday evening. High pressure building out of
central Canada will force the front south down Lake Michigan Monday
night and pushing south of the lake by Tuesday afternoon.
Winds will turn northerly and strengthen following the frontal
passage, with winds and waves reaching Small Craft Advisory levels
tuesday and Tuesday night, especially for the Indiana nearshore
waters. at this point, it looks like the Illinois nearshore waters
may remain just under Small Craft Advisory levels.  High pressure
will then park over the upper Midwest through midweek and gradually
spread east through the rest of the week and into the weekend.  As
the high spreads across the lake, winds should diminish, but a
generally northerly component to the wind field should persist into
the weekend.





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