Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221838

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
138 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.



246 AM CDT

Wednesday through Monday...

By Wednesday morning, a strong upper long wave trough for late
summer will have taken shape over eastern Canada into the Great
Lakes region. After a nice day Wednesday, the northwest flow aloft
will drive a short wave impulse down across the western Great
Lakes into early Thursday. This clipper feature has been indicated
on several runs of global guidance, and will likely be enough to
boost clouds Thursday morning with possibly some sprinkles or
light showers. This wave also influences a secondary cold front
southward and a cool enough air mass (850mb temperatures <8C) over
the lake to support some lake effect clouds. The moisture depth
is pretty marginal for any lake effect showers, but could see
lakeside areas especially in northwest Indiana more cloudy on

Highs look to be in the lower 70s Thursday leaning toward raw
model guidance. With a 1025 mb high forecast to be centered over
Wisconsin Thursday night after the cooler day, lows into Friday
morning look cool. Some patchy mid 40s in outlying locations
seem reasonable, with 50s in the metro area. For the start to
high school football for many communities on Friday night,
temperatures will likely still have that early autumn taste as the
air mass only gradually modifies.

While some significant differences in global guidance exist
heading into the latter half of the weekend into early next week,
the chances for convection presently look limited, and only have
small chances for showers mentioned at this time. With such a
stout high over the region heading into the weekend, it may take a
while to usher it out, and do not see any significant systems
forecast from the west/northwest in the GFS/EC/GEM.



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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