Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 291122

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

314 AM CDT

Through Tuesday Night...

Warm and humid conditions will continue to be in place today and
tomorrow across northern Illinois and northwest Indiana, with
continued chances for thunderstorms, diurnally favored, across the
region. High pressure is in place across the central Great Lakes
region early this morning and will shift east today with subtle
mid-level return flow overspreading the area allowing PWats of
1.7-1.9 inches to inch northward back toward the WI state line
through the day today. Main challenge today will be pinpointing
favored areas for thunderstorms which appear likely again today
despite lack of any significant forcing over the region.
Confidence remains low on location details, but similar to
yesterday, very weak low/mid level flow and high PWat air will
result in slow moving and very efficient rain producers so
flooding will be a concern again this afternoon. Deep layer shear
of only 15-20kt will greatly limit the severe threat, but cannot
rule out an isolated wet microburst or two with some of the
stronger storms. Coverage should diminish after sunset, but as has
been the case the past couple nights, there will probably be a few
very isolated storms festering through the overnight hours.

Expect similar conditions to be in place through the day Tuesday
with scattered showers and thunderstorms possible across the
forecast area. Meanwhile, a shortwave is progged to dig across
northern Ontario and across James Bay with an attendant surface
low pushing a cold front across the region Tuesday evening and
overnight. The frontal boundary will likely be the focus for
convection Tuesday night as it drops across northern Illinois,
with showers and thunderstorms shifting south of I-80 on



314 AM CDT

Wednesday through Monday...

Cooler weather still on track to begin trickling into the region
starting midweek as unseasonably cool post frontal airmass pushes
into the Great Lakes Region along with a strong area of high
pressure. Continue to trend the forecast for Thursday in
particular slightly cooler with low 70s north and mid 70s south.
Still feel the current forecast is on the warm end of the spectrum
with some models showing highs in the 60s across much of the CWA
on Thursday. Either way, cooler and drier air will be in place
(dew points in the 50s) for the latter half of the week with no
notable chances for precip under the influence of the high.
Meanwhile, strong upper ridge will build across the mid section of
the country by midweek and gradually inch east across the Plains
allowing a gradual warming trend as we head into the weekend.
Appears southwest return flow will move back overhead by Sunday
allowing warmth and humidity back into the midwest with chances
for showers and thunderstorms returning early next week.



For the 12Z TAFs...

Patchy low clouds and fog will quickly ease after daybreak similar to
yesterday, with visibility holding in the 3-5SM range in the fog.

Concern will shift to the slow moving showers and thunderstorms expected
again today. Lingering subtle flare up convection south of the
terminals have since eased but there is some redevelopment
southwest of the area which current guidance does not have handled
the eastward extent well at the moment. Nonetheles, additional
shortwave energy across Iowa will glide northeast-east along the
back side of the upper ridge axis and right through Northeast
Illinois today. Expect that this will trigger additional
thunderstorms today as the airmass destabilizes. Current forecast
would suggest the potentially more organized activity would be in
Northwest and into North Central Illinois with impacts only for
RFD, but with the moist airmass and subtle forcing is such that
isolated storms are possible closer to the Chicago terminals this
afternoon as well. Have kept the thunder out of ORD/MDW/DPA for
now but will need to monitor the north and eastward progression
today especially with any outflow boundaries or subtle onshore
wind convergence to act as triggers closer to ORD/MDW.

Light winds and moist conditions will again support some fog tonight,
again likely limited by clouds as upper level energy continues to
rotate along the northern fringes of the ridge axis. We could even
see some showers and storms return overnight.



259 AM CDT

A few days of quiet conditions across Lake Michigan will occur
today and most of Tuesday as high pressure slowly departs east.
Low pressure will continue east from northern Saskatchewan today
to James Bay Tuesday, and then east through Quebec through mid
week. This low will send a cold front to northern Lake Michigan
Tuesday morning. As the low continues east, high pressure will
strengthen across north central Canada mid week, with its
associated ridge axis extending south across the western Great
Lakes. The initial cold front will reach southern Lake Michigan by
daybreak Wednesday, and as the high strengthens a secondary surge
of cooler air later Wednesday into Thursday will maintain a
seasonably strong north-northeast wind across the lake. With a 2
day period of NNE winds Weds/Th with winds reaching 25 to possibly
30 kt at times, expect waves to build across southern Lake
Michigan. Small craft advisory conditions can be expected
Wednesday and into Thursday. The high will then shift east Friday
and Saturday.






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