Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230702

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
202 AM CDT Wed Aug 23 2017

137 PM CDT

Through Tonight...

Drier air continues to spread southeastward into the forecast area
on breezy northwest winds as a cold front continues southeast. There
is enough convergence near the boundary to trigger a few isolated
showers across north-central Indiana and cannot rule out an isolated
shower developing further west across Newton, Jasper, Benton or
Iroquois Counties over the next few hours. Otherwise clouds will
scatter into the late afternoon and dissipate this evening. Clear
skies are expected overnight and temps will cool into the 50s away
from the heart of the Chicago metro.



206 PM CDT

Wednesday through Tuesday...

A seasonably strong upper trough currently setting up over
Ontario and much of the Great Lakes will allow a strong northwest
flow pattern aloft to persist across much of the area through the
end of the week. Within this flow pattern, a shortwave disturbance
over northern portions of Alberta and Saskatchewan is expected to
dig southeastward across southern WI and northern IL Wednesday
night into Thursday morning. While precipitation potential with
this feature will be low, it will help drive another surface cold
front with a reinforcing shot of a cool autumn-like airmass
southward across the area by early Thursday.

Prior to the arrival of the cold front, it looks like Wednesday
will be great day across the area as temperatures warm into the
mid to upper 70s under partly cloudy skies. However, by Thursday,
the lower-level airmass looks to cool by around 3 degrees C as
cooler air filters southward over the area. This in combination
with the prospects of some lake effect cloudiness will likely
yield high temperatures only in the lower to mid 70s for most
areas (a good 5 to 8 degrees below average). Also I cannot rule
out a few isolated light showers or sprinkles Thursday morning,
especially near the lake.

A surface ridge of high pressure then looks to set up over WI and
into northern IL Thursday night into Friday. Light winds and
clearing skies look to set up a rather chilly night across the
area. Many of the outlying areas of northern IL could drop into
the mid 40s into early Friday morning. Also, we could see some
spotty fog develop as the temperatures radiate into the 40s.

Temperatures are only expected to slowly moderate back through the
70s over the weekend as the region remains under the influence of
high pressure over the Great Lakes region. Otherwise, the next
potential for precipitation will not be until early next week.
Model guidance suggests that another upper disturbance will
set up over the mid section of the country during this period.
However, at the same time a strong tropical system could
impact portions of the TX or LA Gulf Coastal areas, and this
could play havoc with any local chances of precipitation early
next week. For this reason, I will only continue to mention low
end chances at the end of the period.



For the 06Z TAFs...

Only forecast concern is a possible lake breeze Wednesday

Under clear skies and with high pressure spreading across the
middle Mississippi Valley, the lower levels are decoupling from
winds aloft and sfc winds have diminished to nwly at 5-10kt for
the Chicago area terminals and lgt/vrbl for ncntrl IL arnd RFD.
Northwesterly winds will increase through the morning and into
early tomorrow afternoon, with some gusts into the 15-20kt range
likely from mid morning to early afternoon. Wind speeds/gusts
will begin to diminish during the mid/late afternoon as high
pressure continues to spread east, weakening the pressure gradient
over nern IL/nrn IN. A lake breeze is expected to develop...moving
likely moving through GYY and MDW by mid/late afternoon. Have
maintained the wind shift to nely at 00z at ORD, but confidence
remains low on the chances for the lake breeze to move through
ORD. In any case, winds will likely be less than 10 kt regardless
of direction. High pressure will continue to spread east tomorrow
night, keeping cigs/vis at vfr through the period, with light
winds tomorrow night.


216 PM CDT

North-northwesterly winds of 15 to 20 KT will continue down Lake
Michigan into Wednesday as a stout area of low pressure continues
to shift over northern Quebec and a surface ridge of high pressure
builds over the Upper Midwest. With these winds, expect waves of 4
to 7 feet over southern Lake Michigan with hazardous conditions
for small craft into Wednesday for the Indiana near shore waters.
Expect the winds over the lake to gradually abate for a short
period on Wednesday before another cold front shifts southward
down the lake Wednesday night night into Thursday morning. As this
front passes, expect a bit of resurgence in some of the northerly
winds over the lake for a period. A few showers, and possibly an
isolated water spout may also be possible over Lake Michigan
early Thursday morning with the frontal passage. However, with
time expect the surface ridge of high pressure to build eastward
over the lake late in the week and into the weekend. This will
result in a period of lighter winds, which should gradually become
southeasterly late in the period as another area of low pressure
takes shape over the Upper Midwest.



IN...Beach Hazards Statement...INZ001-INZ002 until 4 PM Wednesday.

LM...Small Craft Advisory...IN nearshore waters until 4 PM Wednesday.




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