Area Forecast Discussion
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FXUS63 KLOT 221744

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1244 PM CDT Tue Aug 22 2017

246 AM CDT

Through Tonight...

A change in air mass will gradually occur through today with the
tropical air mass ousted for one more seasonal of late August.

Early morning satellite and radar imagery indicate a broad area of
convection, with a couple embedded MCVs, from northern Indiana
to western Illinois, back through the Kansas/Missouri border
region. One MCV will track across the southern CWA edge through
daybreak, keeping thunderstorms confined to that area. Some parts
of far southern Ford/Iroquois/Benton Counties will likely finish
by mid-morning with 2-3.50 inches of rain, but spread out over
many hours which is limiting flash flooding concerns. To the
north of this MCV, stratiform rain will occur for the early
morning commute up to near the I-88 corridor.

Most of this activity, assisted by the long wave through
progression and embedded cold pools, will shift southeast of the
area by mid-morning. Isolated showers behind this may develop on
the true cold front, which will clear the area by early afternoon.
The morning clouds will be scoured out shortly after the frontal
passage as well. Northwest winds will gust to around 20 mph
behind the front, gradually advecting in drier air. Upstream
temperatures yesterday peaked in the upper 70s and would expect
similar for local highs today, with dependence on specific time
of clearing.

The orientation and speed of the winds along the northwest Indiana
shore will cause concerns for rip currents, particularly the
Porter county shore. A Beach Hazards Statement for this goes into
effect this afternoon through midday Wednesday.



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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