Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 292339

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
639 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 00Z TAFs...

Main concerns this evening are lingering light rain/diminishing
thunder trends, and convectively messy wind directions (though
with light speeds). Little drying prior to sunset also raises
potential for at least some mvfr fog late tonight, though initial
cloud cover this evening will limit development for a while.

Radar trends show significant decrease in convective reflectivity
over the past hour or so, and with little large scale support for
continued thunderstorm development am expecting continued
dissipation of thunderstorms early this evening. Stratiform
light-moderate rain will linger a little longer, with an eastward
drift, so a 1-2 hour period of some light rain with generally vfr
cigs is expected at the start of the TAF period. VFR cloud cover
will linger through the evening, so fog development not expected
to be an issue through at least midnight/05z. MVFR fog possible,
with locally IFR/LIFR if skies clear out significantly overnight.
Will watch trends and may need to add this especially to more
fog-prone RFD/DPA for early Tuesday morning. Weak E-SE wind field
has been disturbed by convective outflow (and outflow/clouds have
disrupted lake breeze circulation as well). An E-SE wind 5 kts or
so may settle in this evening, though will likely see periods of
variable/calm winds.

Low confidence on TSRA timing and coverage at this point Tuesday,
with similar unstable but very weakly forced atmospheric
conditions. Relatively high likelihood of development, with some
models showing initiation across the area as early as mid-late
morning. Current forecast has a large prob30 window in the
afternoon and have not changed that for now, but will likely be
able to narrow things down a bit as time approaches.



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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