Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 302334
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
634 PM CDT TUE AUG 30 2016
241 PM CDT
Cloud cover has broken up nicely across much of the area this
afternoon, which has allowed for some INSOLATION and warming
temperatures into the lower 80s. This will likely allow the line
of thunderstorms ongoing across southern WI to build southward
into northern Illinois through the afternoon. While heavy rain
will be likely with this activity, it appears the heaviest rain
producing storms will move quick enough to limit overall rainfall
amounts. For this reason, we have opted to hold off on a flash
flood watch. However, we certainly cannot rule out some isolated
areas of renewed flash flooding over northern IL, especially
given the recent heavy rains. One area of possible concern could
be over Lake and Cook counties in IL, where the storms could
interact with a lake breeze boundary to result in more substantial
localized rain rates. This recently happened in the MKE area
resulting in the issuance of a FFW. This potential will have to be
monitored in the coming hours.
The storms from the disturbance to our southwest really have not
done much recently. However, visible satellite imagery does
indicate that CU have began popping over the southern CWA, so some
additional scattered thunderstorm development is certainly
possible ahead of the main line of storms currently moving towards
far northern IL.
Any storms across the area should dissipate by early
this evening with the loss of INSOLATION. A cold front will move
over the area late tonight into Wednesday. While it is possible
that some scattered showers/storms could develop along this front,
the poor diurnal timing we felt it prudent to limit the chances
for storms tonight. Low clouds do look like a good bet in the wake
of the cold front into Wednesday morning. However, there is also
another chance of fog later tonight, especially as the front is
approaching the area overnight.
237 PM CDT
Wednesday through Tuesday...
Quite the change in air mass is set to unfold for the latter half
of the week. The transition day will be Wednesday as the cold
front scoots southward. Low chances for showers and some storms
will be found ahead of this front south of I-80 in the morning.
Given the non-favorable time of day, do not expect that great of
coverage of morning rain, so at this time think the flood threat
will remain low, especially with increased cell motions. The
higher dew points should lag the true wind shift, so a secondary
dew point discontinuity might spark a couple showers behind the
front in the early-mid afternoon, again south of I-80.
The cooler air mass will spread in on breezy north-northeast
winds with Wednesday afternoon temperatures peaking in the mid to
upper 70s for many and only lower to mid 70s on Thursday. Wind
gusts of 20 mph are expected during the daytime Wednesday and
Thursday, with 25+ mph more likely closer to the lakefront. A
unidirectional wind component to the lake fetch will quickly build
waves with 4 to 6 ft expected for a good part of the Illinois and
Indiana nearshore by Wednesday afternoon and persisting through
Thursday. The orthogonal wind direction especially along the
Indiana shore supports high confidence in rip currents, with
possibly a bit more marginal conditions up in Lake County,
Lake-induced instability will kick off Meteorological Autumn on
Thursday morning, with likely lake effect clouds into northeast
Illinois and northwest Indiana. These clouds may reach a ways
inland. The projected depth is forecast a tad shallow for
showers, but cannot rule out sprinkles in these setups.
The Rex block in the middle of the CONUS and central Canada on
Friday will be slow to ease east, and maybe even slower than
guidance forecasts given the blocked pattern. This allows for a
nice tranquil weekend, with the next chance for storms arriving on
the holiday. Temperatures will gradually moderate with 80s
returning for Saturday.
For the 00Z TAFs...
SCTD SHRA/ISOLD TSRA already decreasing in coverage and intensity
and potential for any direct impacts on terminals is now very low
before all activity dissipates early this evening. Light/variable
or light northwest winds expected through the night before winds
veer to a more north or north-northeasterly direction early
Wednesday morning behind a cold front. There is a threat of some
MVFR, possibly low end MVFR CIGS post frontal, especially near the
lake. Due to low confidence, opted to back off to just a SCT MVFR
deck for now, but is a threat that will need watching. could also
be some patchy ground fog overnight before winds veer more
northerly and pick up pre-dawn. NNE to NE winds likely downwind of
Lake Michigan in the Chicago terminals Wednesday with some oncl
gusts into the mid-upper teens possible.
300 PM CDT
A cold front will move down the lake tonight, reaching the
southern tip by early Wednesday morning. Northerly winds behind
this feature will initially gust to 15 to 25 kt, but a
reinforcement in the northerly push should occur Wednesday
afternoon through Thursday. Some gusts to 30 kt could occur in
that time frame. Waves will quickly build behind the frontal
passage. Small craft advisory winds and waves are expected along
the Indiana and a portion of the nearshore areas beginning
Wednesday afternoon. Further north of Wilmette Harbor, small craft
advisory conditions are likely to start there Wednesday night.
Choppy conditions will persist through Thursday followed by high
pressure for the weekend.
IL...Beach Hazards Statement...ILZ006-ILZ014...1 PM Wednesday TO 10
IN...Beach Hazards Statement...INZ001-INZ002...1 PM Wednesday TO 10
LM...Small Craft Advisory...LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...3 PM
Wednesday TO 10 PM Thursday.
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