Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 291956

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
256 PM CDT MON AUG 29 2016

234 PM CDT

Through Tonight...

Very slow moving thunderstorms continue to develop over portions
of the area this afternoon. As a this writing, the most pronounced
heavy rain producer is the cluster of storms over Lee county.
These storms have been dropping up to 2" per hour over a small
portion of the county south of the Dixon area. While the
individual storm cells are moving slowly eastward, the cluster of
storms is propagating slowly north-northwestward into the Dixon
area as new development occurs along the outflow. If this heavy
rain continues into the Dixon area a flash flood warning may be
needed in the near future. Otherwise, expect SCT thunderstorms to
continue to develop and impact portions of the area for the next
few hours before dissipating by early this evening. The slow
eastward movement of these storms could result in some isolated
areas of flash flooding due to very heavy rainfall.

Some isolated storms are possible over the area tonight, but most
areas will likely remain dry. The high dew point air mass and the
expectation of light winds tonight could allow for some areas of
dense fog to develop. As a result, I have added patchy fog to the
forecast for tonight.



250 PM CDT

Tuesday through Monday...

Tuesday will again be like the past few with a humid, deep
moisture air mass in place. The stout upper high with 593dm
heights over central Illinois today will begin to be shunted
slowly southward as a northern stream trough begins to dig into
Ontario. Minor mid-level ripples on water vapor satellite imagery
over Iowa today will likely be moving over northern Illinois
Tuesday morning and could support isolated/scattered storms in the
a.m. hours. In addition, at the low-levels there will be a tad
more southwesterly to westerly flow ensuring dew points remain
high and once again pooled in the mid 70s by midday. So would
expect at least scattered storms, potentially with more coverage
than today. Guidance is indicating a decent amount of morning
cloud cover which may slow destabilization, though it still should
be still achieved given the moisture pooling. Deep layer shear is
likely to be 15-20 kt keeping the concerns mainly tied to flooding
with efficient rates and slow storm motions. The occasional wet
microburst, especially any congealed deep cells, will be possible.

As a cold front associated with the aforementioned trough moves
southward over Lake Michigan on Tuesday night, the chance for
scattered storms should continue into the night. In addition,
upper forcing within a right entrance region of an upper jet will
also support at least broad lift. Besides the rain chances, Tuesday
night looks to remain humid and mild.

On Wednesday morning the cold front should be shifting southward
into the area and by Wednesday afternoon clearing the south. The
front will continue to serve as a focus for storms, though it is
looking more likely Wednesday afternoon much of that may be in
central Illinois and central Indiana. Reinforcement of drier air
will occur later in the day into the night. Winds along the lake
front will be the strongest, with gusts of 25 to possibly 30 mph
by Wednesday evening. Waves will build to levels conducive for rip
currents by mid-late afternoon Wednesday based on the current
forecast, with the threat continuing through Thursday.

Thursday will see highs in the lower to mid 70s as high pressure
advances in. There actually could be some lake effect clouds into
northeast Illinois Thursday morning. Pleasant conditions look to
persist through the weekend with a rex block in the middle of the
continent likely to keep the next precipitation chances not until



For the 18Z TAFs...

The main weather concerns/challenges will be the extent of
thunderstorms over or near the terminals this afternoon and again
on Tuesday afternoon.

A warm and humid air mass will remain in place over the area
through Tuesday. Recent AMDAR soundings out of MDW indicate that
thunderstorm development could begin initiation at any time. The
main question remains where will thunderstorms focus this
afternoon. At this time, it appears that low-level convergence
along a developing lake breeze boundary may help act as a focus
for SCT storms over or near some of the Chicago area terminals
after 19 Z. As a result, I have opted to add a VCTS mention from
19 to 22 Z this afternoon. However, overall confidence on coverage
remains a bit on the low side.

The next main threat for storms looks to arrive by Tuesday
afternoon as a cold front approaches the area. Given the range,
it is a bit difficult to pinpoint the best timing at ORD for
thunder. So, at this time I have just gone with a long (6hr)
PROB30 mention for such until a shorter time range can be

In addition to the threat for storms, some fog will be possible
later tonight, especially outside the Chicago area. The extent of
the fog will likely depend on the amount of cloud cover around
tonight, but given the high dew point air, some MVFR to IFR
visibilities are certainly possible late tonight.



256 PM CDT

Quiet winds and waves will occur over the lake tonight into
Tuesday, outside of possibly patchy fog late tonight and Tuesday
morning. A cold front will shift southward onto the northern part
of the lake by Tuesday evening and then down the lake through the
remainder of the night. This will abruptly turn winds northerly
with an increase in speed. Gusts to 30 kt are possible by
Wednesday afternoon through Thursday night, especially across the
central and south. Small craft advisory conditions are presently
forecast Wednesday evening through Thursday night across the
Illinois and Indiana nearshore zones.






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