Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270118

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
818 PM CDT FRI AUG 26 2016

817 PM CDT

Made some minor changes in orientation and magnitude of pops
across southern and western counties of the cwa this evening
based on observed trends, otherwise going forecast appears very
reasonable and blended changes prior to midnight into the previous

Radar mosaic continues to depict an area of rain/isolated thunder
lifting slowly northeast across east central IL and northwest IN
this evening. This area of precip appears to be associated with a
weak mid-level disturbance wrapping around the western periphery
of the mid-Atlantic region upper ridge, impinging upon a
moderately unstable air mass along/south of the baroclinic zone
associated with the stationary front positioned just south of the
forecast area. While lightning has decreased considerably over the
past our or so (with decreasing cape north of the surface front
and cooling of the boundary layer with nightfall), did need to
boost pops across the southern counties of the cwa given some
moderate rainfall moving across that area. Stronger large scale
forcing is observed off to the west and southwest of the cwa,
where development of a southwesterly low level jet is expected to
lead to an expansion of rain and thunderstorms with the north-
northeastward moving warm front overnight. Going forecast has this
well in hand, with pops ramping up to likelies from the
west/southwest from midnight onward and continuing into Saturday

Otherwise, going forecast in very good shape and no other changes



300 PM CDT

Through Saturday...

An air mass we have become quite familiar with this mid-late of high moisture and modest to high
instability...will return late tonight into Saturday bringing with
it thunderstorm and heavy rainfall potential. This pattern is
unfortunately somewhat messy, which makes it challenging for
specific storm timing/coverage, especially into the day Saturday.

A surface warm front is draped from west to east across the mid-
section of Illinois this afternoon and will begin to accelerate
northward through tonight in response to a short wave lifting
northeast from the Central Plains. Gradual moist advection amid
broad scale ascent north of the warm front should result in
isolated showers developing this evening over northern Illinois.
As a 30 kt low- level jet develops into eastern Iowa, convection
should blossom to the immediate west of the CWA. While some of the
moisture transport nose will be pointed into northern Illinois, it
is somewhat uncertain how quickly storms will move into the area.
The greater deep layer moisture convergence and short wave-induced
lift is late tonight into the early morning hours Saturday.
Continue the highest PoPs during that time frame. Convective
allowing models show a plethora of solutions, not surprising given
this type of air mass, but an ensemble approach such as from the
SSEO favors late tonight especially in north central and far
northeast Illinois.

Increasing shear tonight will support some storm organization, but
the threat for any severe storms remains small. Precipitable
waters near two inches and deep warm cloud depths as observed on
the morning ILX sounding will expand over the area and once
again support very efficient rainfall rates in any of the
overnight and Saturday activity. The collaborative thoughts are
the highest QPF will be over north central Illinois tonight with
some locations possibly realizing around two inches or higher
depending on duration. The greater moisture convergence is just to
our west, but will be something to monitor, and confidence is not
enough at this time for any type of flood watch.

For Saturday, much will depend on influence of early morning
activity. Do expect there to be a high amount of morning cloud
cover along/behind the warm front lifting northward. As the warm
sector expands in there should be a decent amount of afternoon
destabilization. Some confluence/focus is noted in the model low-
level wind fields, but again tough to say where exactly that may
end up due to influence of morning rain. Maintain a chance of
afternoon storms, with a little higher noted central/eastern CWA.
The deep layer shear/instability would certainly be enough to
support some severe storms with any afternoon development, as well
as locally very heavy rainfall.



330 PM CDT

Saturday night through Friday...

Chances for showers and thunderstorms continue from late in the
weekend into early next week, while a moist and unstable airmass
remains in place. Potential exits for thunderstorm development to
continue Saturday night, however, not overly confident of this
potential. Guidance continues to be all over the place with QPF
development, and am not completely sold on anyone solution.
Although I do think there is a chance, think it will be tied with
whatever occurs in the afternoon. With this lingering potential,
did leave chance pops in the evening but lower after midnight with
the general focus remaining away from the area. Low confidence
with intensity of any thunderstorm development as well, but do
think some more robust development will be possible if anything
does develop.

A period of height rises aloft will occur on Sunday, while a
fairly warm, moist, and unstable airmass remains in place. At this
time, there does not appear to be any real focus for thunderstorm
development but guidance continues to advertise this possibility.
Can`t really argue with at least scattered thunderstorms by midday
into Sunday afternoon as instability increases in this moist
environment. Flow aloft will be rather weak and am not expecting
any widespread severe threat. However, can`t rule out an isolated
strong to severe storm. Heavy rainfall threat will continue with
high PWATs expected to still be in place and with the potential
for slow movement of any development, some risk of flooding will
also be present. Do think temps will have a chance to rise before
these scattered storms and so did increase temps by a degree or
two in most locations. With high dewpoints still in place,
humid/muggy conditions will continue. Similar conditions/setup
appears likely on Monday, with almost mirroring high temps and
pops/weather. Although with a better potential for some weak
energy to traverse the region, did increase pops slightly more on



For the 00Z TAFs...

Primary concern for this TAF cycle is with timing of SHRA/TSRA.
Warm front over downstate IL will lift north tonight and combined
with an upper level disturbance should provide for an increase in
SHRA/TSRA over northern IL tonight, especially late tonight. Hard
to pinpoint timing of best TSRA chances, but previous TAFs seemed
to have a very reasonable approximation, so made no changes in the
00Z update. More widespread SHRA/TSRA activity looks to exit the
terminals Saturday morning, however cannot rule out some isold
SHRA into the late morning and perhaps even redevelopment of
scattered SHRA/TSRA Saturday afternoon.

Other than SHRA/TSRA, weather shouldn`t be too impactful of
aviation operations. Could see some lower cigs, probably MVFR for
a time Saturday morning as warm front lifts north. Guidance is
hitting RFD and points north and west the hardest, so kept lowest
conditions there with just a period of higher MVFR for Chicago
area terminals.



333 PM CDT

High pressure overhead this afternoon allowing for some lighter
winds over the lake, but expect the center of this high to shift
to the east tonight into Saturday. System moving through the
region will allow the winds to become more organized out of the
east tonight, and then southeast to south Saturday. Winds will
then continue to shift more southwest Saturday night, before
becoming light/variable once again on Sunday/early next week.






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