Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221906

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
206 PM CDT Tue Aug 22 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 18Z TAFs...

West-northwest winds gusting in the 20-25 kt range will trend
northwesterly early this afternoon with gusts gradually
diminishing later this afternoon. MVFR ceilings are lifting to
lower end VFR and decreasing in coverage, though variable
scattered to broken coverage will persist through early afternoon.
Winds will ease this evening then increase modestly Wednesday
morning. Wind direction at ORD/MDW will be tricky Wednesday
afternoon as it may turn north-northwest or possibly north-
northeast, while winds further inland across Illinois remain
west-northwest to northwest. GYY should see more of a north
component by afternoon.



241 PM CDT

Main marine forecast concerns continue to be on a period of
northerly winds down the length of the lake Tuesday night into
Wednesday. While not overly strong, the long fetch will likely
result in small craft advisory conditions for the shores of
southern Lake Michigan.

In the near term, a cold front has become nearly stationary near
the north shore of the Lake in the Upper Peninsula. Low pressure
is expected to develop along this front over Wisconsin tonight,
moving northeast over far northern Lake Michigan early Tuesday
morning and trailing a cold front which will push down the lake
during the day. The low is expected to deepen later in the day as
it pulls away into Quebec, setting the stage for breezy northwest
winds across the lake. These north-northwest winds will continue
into Wednesday before weakening as high pressure spreads into the
region. The high eventually late Thursday, allowing winds to
diminish and waves to subside. The strongest winds, 20-25 kts, are
expected late Tuesday and Tuesday night, with small craft
advisory conditions expected along the southern shores of the lake
from Tuesday afternoon through Wednesday.



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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