Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 252351

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
651 PM CDT WED MAY 25 2016


625 PM CDT

A semi-organized cold pool associated with a broken line of
thunderstorms that supported near severe to marginally severe
winds has shifted east over Lake Michigan early this evening and
remaining severe weather threat should be greatly diminish in the
worked over airmass of northern Illinois north of I-80. Outflow
boundary is evident on radar stretching from northern LaSalle
County east to southern Cook County. While moderately strong
instability persists south of the outflow boundary...deep layer
shear is not supportive of severe thunderstorms across that area.
Immediate concern will shift to very modest flooding potential
with slowly moving thunderstorms currently near and south of the
I-88 corridor. Persistent isentropic ascent for the next few
hours and flow over-running the remnant cold pool should continue
to support additional development into the evening...though
convective allowing models do show a downward trend in activity in
the 2-4Z time-frame as the focus begins to shift east. For now it
appears any flooding would be nuisance/low impact type in poor
drainage/urban areas.



232 PM CDT

Through Tonight...

No real significant changes with thoughts for thunderstorms this
afternoon into the evening, with scattered development likely
impact much of the area.

Latest radar imagery depicting isolated to widely scattered
storms developing just west of I39, focused along weak convergence
over northwest Indiana. Increasing instability supported by
steepening lapse rates and with diminishing CIN will continue over
the next couple of hours as large scale ascent further increases
with the approach of mid level energy. Most short term guidance
still indicating further expansion of what is occurring at this
time over the next 2-3, while likely shifting east with time and
feel fairly confident with this solution. Also feel fairly
confident with timing, once again, highlighting the 20/22z time
frame over north central Illinois and then the 21/23z period for
northeast Illinois and northwest Indiana for increasing coverage
and intensity of thunderstorm development. This also includes the
possibility for isolated strong to maybe severe storms across the
entire area, with hail/wind still the likely hazards. Overall
development should likely diminish by mid evening, however, some
guidance continues to develop additional thunderstorms beyond this
period likely owing to an increasing LLJ with large scale ascent
still overhead. Do think the guidance that is showing this
development is a bit overdone with coverage at that time, which
could be associated to boundaries that more than likely wont be in
place. Nonetheless, still maintained low chance to slight chance
pops wording overnight, but focus it more south with time as the
focus shifts south tonight.



239 PM CDT

Thursday night through Wednesday...

Thursday through Saturday...
The pattern continues to be very summer like over the next several
days...heading into the holiday weekend. Deep trough continues to
rotate across the Southwest CONUS Thursday but will begin to drift
East Friday and weaken. However as the trough begins to approach the
Western Great Lakes/Ohio Valley Friday...the added low level
moisture and vorticity throughout the lowest layers of the
atmosphere...and a weak sfc shortwave could translate into increased
chances for precip/thunderstorms heading into Friday. The challenge
for Friday could be that several guidance members indicate some mid-
lvl height rises through the afternoon...which could hold onto the
diffluent flow in the mid-lvls longer and delay precip chances until
late afternoon/evening. Low level moisture will be present
however...with dew points pushing well into the 60s and a few near
70 degree dew points is conceivable as precipitable water values
approach 1.5 inches Friday. So the humidity will be felt throughout
the region Friday with highs warming into the lower to middle 80s
once again.

Saturday could be very similar to the overall pattern
remains relatively un-changed. The better precip chances will be
shifting North throughout the day...with temps generally in the
lower 80s. Cloud cover and perhaps some morning convection could
linger and keep a thicker cloud shield in place which may hold temps
down a few degrees.

Sunday through middle of next week... For the second half of the
holiday weekend through the middle of next week the pattern
continues to feature weak ridging across the Great Lakes region.
There is some weakening in the longwave pattern...however the spread
amongst ensemble members in the longer term does not grow much. Thus
the confidence does remain higher that the continued warm/humid
pattern will likely persist with occasional showers/thunderstorms
throughout the extended periods.




Main concern is with thunderstorm and wind trends this evening.
An upper level disturbance kicked off areas of TSRA late this
afternoon, with associated stronger winds in the first wave.
Concern has diminished for strong to severe gusts at the
terminals. However, until the disturbance departs to the east
later this evening, expecting TSRA to hang around the northeast IL
terminals, with occasional IFR to LIFR vsby in heavy rain and
ocnl cloud to ground lightning. Pushed back end time to 03z. The
wind field is disturbed by the thunderstorms and various outflow
boundaries generated by them. Confidence is low with trends.
Including temporary east winds in the ORD TAF and started MDW with
current north winds, with the potential for a temporary shift to
east winds.

After the thunderstorms this evening, cannot completely rule out
isolated SHRA or TSRA overnight, but there is no good source of
lift noted on satellite imagery. Next disturbance is expected to
move across the area toward daybreak. Confidence is low with
trends with this wave overall. Per timing of disturbance and model
guidance, pushed back timing of PROB30 for TSRA. After any
potential TSRA ends by mid morning, atmosphere will become
unstable, so isolated SHRA/TSRA are possible. However, confidence
is low and it appears more favored timing will be associated with
a stronger disturbance moving in beyond the ORD 30-hour TAF. Kept
all TAFs dry after the morning PROB30 given the low confidence.
Winds will be south-southwest on Thursday with gusts of 15-20 kt
and otherwise VFR conditions.



306 PM CDT

Have continued the dense fog advisory for the Northern half of
Lake Michigan through this evening. With increasing low level
moisture flowing North/Northeast over the lake...dense fog will
likely persist and could result in the advisory needing to be
extended. Winds will continue to be Southerly with speeds between
10 to 20 kts...then slowly diminish overnight to around 10 to 15
kts. A frontal boundary has become stationary over the Southern
half of the lake...but will lift back North Thursday with another
frontal boundary remaining well West of the Great Lakes through
the next few days. A large area of low pressure will remain
positioned West of the lakes with continued warm Southerly flow
and the potential for areas of fog through Friday and perhaps






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