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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
647 PM CDT sun Aug 20 2017

Short term...
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 northwest-southeast 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 Storm Prediction Center 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 Illinois and northwest 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 Illinois/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.

Mdb

&&

Long term...
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 Continental U.S..

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 mesoscale convective system 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 mesoscale convective system
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-east air mass characterized by surface dew
point temps of 70f or higher and p-wats around 2 inches. Weather forecast office lot
cwa is included in both the Storm Prediction Center 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 mesoscale convective system 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 Continental U.S. Upper
trough. GFS and European model (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
August.

Ratzer

&&

Aviation...
for the 00z tafs...

647 PM...main forecast concern is thunderstorm potential.

Light showers/sprinkles continue to spread across northern Illinois with
some stronger showers developing south of rfd recently. Short term
guidance suggests this activity may continue to expand and spread
east/southeast through the evening. Included shower mention at rfd
and dpa but just vicinity further east with low confidence. Trends
will need to be monitored this evening. It is possible that this
activity does increase to produce isolated/scattered thunder.

Additional thunderstorm development is expected over western Iowa
tonight which may spread east/southeast into western Illinois Monday
morning...likely in a weakening phase. No changes to previous
forecast for Monday morning but confidence is low for this time
period and its possible all of the terminals remain dry Monday
morning.

A cold front will move across the area Monday night with a period
of showers and thunderstorms expected...likely after 00z at rfd
and perhaps around 06z at Ord so no mention with this forecast
but confidence of thunderstorms during this period is medium to
high.

A very weak area of low pressure is moving across parts of north
central Illinois with light east/southeast winds. Ord/mdw still south/
southwest at this time but winds should turn more southerly or
southeasterly this evening...though speeds will be under 10kts.
Light/southerly winds overnight will turn back south/southwest on
Monday with speeds approaching 10kts. Cms

&&

Marine...
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
Tuesday.

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
subside.

Ratzer

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&

$$

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