Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201140

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
640 AM CDT Sun Aug 20 2017

414 AM CDT

Through tonight...

Quiet weather is on tap for the short term forecast period, with the
primary concern being timing of lake breeze development and the
impact on temperatures along the lakefront.

The center of a large area of sfc high pressure is slowly sliding to
the east though weak ridging will still extend west across the
region.  Also, low pressure moving across central Canada is dragging
a cold front across the upper Mississippi while another low is
developing over the central plains.  The high will limit the
eastward progression of the frontal system, with the northern
portion of the front pushing east across the upper Great Lakes, and
then extending southwest across northern Wisconsin and then into the
Upper Missouri Valley.  More broad scale warm advection will set up
across the midwest, with 925mb temps of 24-25C, supporting temps
approaching 90 F.  A couple factors may limit warming a little
today, cirrus blow-off from the MCS over Iowa and nrn MO, and a mid
level shortwave crossing nrn/cntrl IL this afternoon may generate
some cu or a few showers.  However, have maintained a dry forecast
since the warm advection aloft should keep the area capped and there
is no sfc forcing to help break the cap.  So, have gone with temps
in the upper 80s over much of the CWA, through an isolated 90 F is
not out of the question if the cirrus is thin enough.  The exception
will be the Illinois lakefront.  A lake breeze should develop, but
the orientation of the boundary should be oriented more northwest to
southeast, reflective of the southerly synoptic flow and with wind
speeds arnd 10 mph this afternoon, the boundary should not penetrate
inland very far.  Locations north of downtown Chicago to the
Wisconsin border should see highs only in the middle 70s.  The cold
front should see some sewd progression overnight tonight as the sfc
high center moves to the mid Atlantic coast.  Cloud cover and pcpn
chances will be in the increase overnight as the front progresses to
the southeast.


414 AM CDT

Monday through Saturday...

Multiple concerns are shaping up for the longer term forecast
period, most of which will be confined to a very unsettled period
from Monday through Tuesday afternoon.  Upper level flow is trending
to more zonal aloft with fast westerly mid and upper level flow
setting up across the region.  As this happens, the sfc cold front
will begin to lay out into a more west-east orientation.  A series
of shortwaves rippling through the fast wly flow aloft will help
keep tsra/shra chances going through the period.  While the daytime
hours on Monday look to be somewhat unsettled with some chances for
shra/tsra, most guidance suggests that activity should be scattered,
and periods of broken sky cover may be limited.  Sky cover will
likely not be very conducive to direct observation of the much
anticipated eclipse.  With little progression to the larger scale
pattern, temperatures on Monday should be very similar to Sunday,
with highs in the upper 80s likely for much of the area.  Increasing
cloud cover and slightly higher pcpn chances could keep ncntrl IL a
little cooler.  Some isolated 90 F reading may be possible again in
areas that experience some breaks in the cloud cover in the

Forecast concerns will shift to more significant weather Monday
night into Tuesday morning. There should be significant moisture
pooling along the front, with pwats reaching or slightly exceeding 2
inches.  With flow aloft still zonal into Monday night, the swd
progression of the front should be relatively slow.  The combination
of high levels of deep layer moisture, slow progression of the front
and upper level flow generally parallel to the front all point to
the potential for widespread heavy rain across the area.  Latest
guidance suggest that the highest rainfall amounts should be over
portions of the CWA north of the I-80 corridor where areal average
QPF in excess of 1 inch is likely and locally higher amounts
possible.  There may be flooding potential, especially for
ncntrl/nwrn IL where the heaviest rainfall is expected.  There will
also be a severe thunderstorm threat Monday night.  The latest
guidance remains consistent on indicating that the environment will
be especially moist and unstable, with steep mid level lapse rates,
CAPE values arnd 2000 J/kg and mid-level flow of 30-40 kt.  So,
thunderstorms will not only have the potential to produce very heavy
rainfall, strong, damaging winds may be possible as well.  Given
these factors, SPC has highlighted nrn IL in a Slight Risk Severe Wx
area and WPC has highlighted nwrn IL with a Slight Risk for
Excessive Rainfall.  The remainder of the CWA are highlighted with
Marginal Risk for Severe and Excessive Rainfall.

By Tuesday morning, high pressure building out of Canada and
spreading across the upper Mississippi Valley along with increasing
amplitude to the upper level pattern will help accelerate the front,
pushing it more quickly south and east of the CWA by mid-day
Tuesday, bringing an end to both pcpn and the warm/humid conditions.
Much quieter weather is expected for Tuesday afternoon through the
end of the period as the upper level pattern transitions to high
amplitude, slow progression with a deep upper low setting up over
sern Canada and upper ridging building over the Rockies.  Northwest
flow aloft and sfc high pressure will cover the area through Friday,
with the longer range guidance suggesting that the pattern will
break down and become more progressive by Friday or Saturday.  Until
then, dry weather and below normal temperatures will prevail, with
highs in the upper 60s to low 70s along the lakefront and middle to
upper 70s inland.  The next chance for any pcpn will likely not be
until next weekend.


For the 12Z TAFs...

For the Chicago TAF sites today and tonight, concerns are south-
southwest winds approaching 10 kt during the afternoon, a chance
for isolated showers late this afternoon and evening, and then a
chance of storms late tonight into Monday.

Light south-southeast winds will become south-southwest by mid to
late morning. Speeds will increase with 9-10 kt expected at ORD
and MDW by 19Z-20Z. A few gusts in the mid teens are possible
mid- late afternoon. Winds will drop in speed near sunset and back
slightly to due south or a little east of south. Confidence in
winds for this TAF is medium-high.

The convective complex across central Iowa early this morning will
continue to move east-southeast with a gradual weakening likely
to continue. The upper level disturbance supporting the activity
will continue to move east and will drift over the area this
afternoon/early evening. Isolated showers are possible with this,
and cannot 100% rule out a storm or two. This window of
opportunity for showers will be near RFD from 18Z-21Z and ORD/MDW
from 21Z-02Z. Confidence on the showers occurring is low.

For overnight into early Monday morning, the next upper
disturbance will likely trigger storms across Iowa and possibly
further north/northeast. This convection should slowly progress
southeast and may impact RFD as early as overnight and its
possible the Chicago TAF sites see this during Monday morning.
However, confidence on evolution of storms tonight into Monday
morning is very low. It is possible the most robust Monday
morning convective activity passes to the southwest of local area



159 AM CDT

As high pressure moves east today, winds will turn southerly
across the lake. The exception will be in the Illinois nearshore,
where southeast to possibly due east onshore winds will be seen
in the afternoon. The highest speeds over the lake today will be
across the northern part, with the far north seeing gusts of
20-25 kt late today into this evening.

Low pressure will take shape across northern Lake Michigan by
daybreak Tuesday, and accelerate and deepen as it tracks eastward
during the day Tuesday. Given the pressure gradient in its wake,
northwest winds will increase with 20-25 kt gusts likely. The
northwest direction on Tuesday would favor Small Craft Advisory
winds and waves across the Indiana nearshore and possibly the
Illinois one due to winds.






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