Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 301734

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1234 PM CDT TUE AUG 30 2016


1142 AM CDT

Main concern this afternoon is the potential for more heavy rain
and flash flooding potential. We have been throwing around the
idea of issuing a flash flood watch for this afternoon,
especially given the antecedent very wet conditions. However,
after discussing with neighboring offices and internally we have
opted to hold off and watch convective trends into the early
afternoon. The main issues are who to include in the watch, given
that the flash flood threat will likely remain confined to small
areas, we feel just issuing one for the whole area may be
overdoing it.

There is a lot of cloud cover around the area this morning, and
this has kept temperatures in the 70s for the most part. However,
some clearing has occurred recently over western sections of the
area. This appears to be in the wake of an MCV currently moving
over far northeastern IL. This could help conditions warm some in
the coming hours. Additionally, with the presence of this
tropical air mass, Amdar soundings out of KMDW indicate that
convective inhibition is already very weak, so any farther
warming will only help eliminate the inhibition.

The main area we are currently watching is the outflow boundary
currently shifting southward into southern Wisconsin. This
boundary has recently fired off some scattered convection. It is
quite possible that this boundary could continue southward into
northern Illinois this afternoon, and if it does so, thunderstorms
would become likely over northern portions of the area. However,
additional storms will also be possible over my southern areas
this afternoon. It appears that another mid-level disturbance,
now over NE MO, will work its way eastward this afternoon,
possibly driving additional areas of convection. Unfortunately,
however, confidence in convective trends and areas to see the most
is still to low to go with a watch, but one may be needed in the



147 AM CDT

Through Wednesday...

Concerns in the short term center around what has become a bit of a
theme during this latter half of summer with the potential for
slow moving heavy rainfall producing showers and thunderstorms.

Before getting to storms, like in past nights dewpoint depressions
are very low following a period of rainfall today, and with a
broad surface high stretched across the Great Lakes and Midwest
resulting weak to no wind, fog is occurring with visibility down
to as low as 1 mile in spots, locally to 1/4 mile at KJOT. High
clouds from low pressure to west continue has kept the fog from
getting dense in many areas, but expect that it could be in spots.
There will likely be some fog through at least daybreak.

The upper pattern features a broad upper level ridge centered
across the Ohio/Tennessee Valleys and extending north to the lower
Great Lakes. Southwest flow continues to steer moisture around the
western and northern fringes of the ridge which happens to be laid
out across Northern Illinois. Meanwhile, a weak surface stationary
front is also stretched east-west across northern Iowa and into
central Illinois.

0z soundings at DVN/ILX still depict around 1.75 of precipitable
water with SPC mesoanalysis and model guidance depicting values
near 2.00 once again today. RAP soundings depict modest
instability develops as well with around 1000 J/KG of MLCAPE
(slightly less than yesterday) in a weakly capped low shear
environment. Weak shortwaves will continue to slide eastward near
surface boundary today, and this combined with subtle southwest
warm/moist advection will lead to additional showers and
thunderstorms today capable of mainly heavy rainfall. We are
already seeing some increase at the moment as weak warm advection
ahead of the leading shortwave increases. The HRRR suggests this
will continue into the morning hours and it is possible that we
just see continued waves of rain showers today, beginning this
morning with decent coverage, and then becoming more convective in
nature as the day continues. Guidance does depict widely varying
solutions with regards to the extent of coverage today, but do
generally agree on some favoring to coverage on the I-88 southward
corridor given some nod to the current location of the stationary
front. The thermodynamic and kinematic environment again will
suggest these will continue to be slow movers with heavy rain.

Signs of change are evident from a surface analysis where a
seasonally strong cold front continues to drive a cluster of
thunderstorms across central Minnesota and northwest Wisconsin. It
is this front that will move through in the coming days that will
bring in a considerably drier and cooler airmass to wrap up
meteorological summer. The leading edge of the cold front, still
considerably removed from the core of colder air near an upper low
in Ontario, will arrive tonight. Continued vorticity advection
around the upper high and on the southern end of Canadian upper
low, combined with some surface convergence and upper height falls
should keep a threat of showers and storms late this afternoon,
tonight and into early Wednesday. The cold front will continue
southeast on Wednesday. Upper 60s dewpoints hang on long enough in
NW Indiana to keep chances for the day, with many areas seeing a
much drier day.



147 AM CDT

Wednesday night through Labor Day...

By Wednesday night it appears the front is through with 50s dewpoints
entrenched across northeast Illinois and northwest Indiana. The
core of the upper low will head east into Quebec, but a sharp back
edge to the trough axis will drive south across Michigan and
graze the local area with a more significant push of seasonally
colder air on Thursday. Significant thermal differences between
this air mass and the warm lake could lead to some lake induced
clouds, but the moisture appears shallow enough preclude much more
than that in spite of some hints of some weak qpf and the GFS
depicting lake induced CAPE of about 200-300 j/kg. Lake
equilibrium levels are also quite shallow at around 6-7k ft. Highs
hold in the 70s.

High pressure takes over for Friday for dry and sunny conditions
with highs into the mid to maybe upper 70s. The high retreats
east Saturday and southeast flow will allow some temperature
recoveries to near 80. Sunday into early next week will feature a
return to warmer conditions, possibly quite warm for early
September standards as strong southwest flow sets up ahead of deep
low pressure that will move through the northern lakes. Labor Day,
which some consider the unofficial end of summer, could indeed be
quite summer-like with mid 80s for highs and dewpoints back near
70. A cold front will be approaching from the northwest which
could bring some thunderstorms into north Central Illinois Labor
Day afternoon, with better chances area-wide in the evening into



For the 18Z TAFs...

The main weather concern/challenge will be timing the threat for
another period of thunderstorms with heavy rainfall this

We are currently watching the behavior of an outflow boundary
over southern Wisconsin, which has been spawning showers and
thunderstorms over the past couple hours. This boundary is
expected to continue to shift southward into northern Illinois by
mid afternoon. Given skies are beginning to clear over northern
Illinois, it is expected that instability will build over the next
couple hours to support thunderstorm develop over the area this
afternoon. With this in mind, we have included a tempo for thunder
at the terminals. It looks like the best timing will be after 20Z
for the Chicago area terminals, and after 19Z at KRFD. A period of
very heavy rain with tempo visibilities below 2SM are possible
with this activity. The light and variable winds could also trend
more northeasterly with the storms and outflow later this
afternoon, so I also included this in the tempo group as well.

A cold front will shift over the area by daybreak Wednesday, with
winds becoming north-northeasterly in its wake for Wednesday. It
appears that a period of lower end MVFR CIGS could overspread the
area in the morning before improving by the afternoon.



254 AM CDT

High pressure will be in place across southern Lake Michigan today
with fresh southwesterly flow in place across the north end of
the lake ahead of an approaching cold front. The front will pass
south across the lake tonight into early Wednesday with a strong
north to northeast breeze developing behind the front and
continuing through Thursday evening. These winds will allow waves
to build into the 4 to 6 ft range across southern portions of Lake
Michigan which means a small craft advisory may be needed for the
Indiana and Illinois nearshore waters starting Wednesday and
possibly going through early Friday. Ridge of high pressure will
build across the region Friday with southerly flow spreading back
across the lake Friday night into Saturday and continuing into
early next week.






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