Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 291807

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


1105 AM CDT

Weather concerns today will again focus on the threats for areas
of heavy rainfall.

Some of the latest AMDAR soundings out of KMDW indicate that the
atmosphere is becoming uncapped to near surface based convection
as the area warms into the 80s. In spite of this, however, there
is not much of a focus for convection. This makes it difficult to
pin point the exact areas for development. Some of the HiRes CAMs
do indicate the possibility of development over the western CWA by
late morning, with additional development possible along an
afternoon lake breeze boundary over portions of northeastern
Illinois and northwest Indiana. This is certainly plausible given
that it should not take much low level focus in this high dew
point air mass to get things going. Additionally, once some
storms do develop, outflow from them may also act as a focus for
additional areas of development. The storms will likely be very
slow moving (east up to ~10 KT) and efficient rainfall producers.
As a result, this could increase the threat for localized heavy
rainfall amounts, similar to yesterday.



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 18Z TAFs...

The main weather concerns/challenges will be the extent of
thunderstorms over or near the terminals this afternoon and again
on Tuesday afternoon.

A warm and humid air mass will remain in place over the area
through Tuesday. Recent AMDAR soundings out of MDW indicate that
thunderstorm development could begin initiation at any time. The
main question remains where will thunderstorms focus this
afternoon. At this time, it appears that low-level convergence
along a developing lake breeze boundary may help act as a focus
for SCT storms over or near some of the Chicago area terminals
after 19 Z. As a result, I have opted to add a VCTS mention from
19 to 22 Z this afternoon. However, overall confidence on coverage
remains a bit on the low side.

The next main threat for storms looks to arrive by Tuesday
afternoon as a cold front approaches the area. Given the range,
it is a bit difficult to pinpoint the best timing at ORD for
thunder. So, at this time I have just gone with a long (6hr)
PROB30 mention for such until a shorter time range can be

In addition to the threat for storms, some fog will be possible
later tonight, especially outside the Chicago area. The extent of
the fog will likely depend on the amount of cloud cover around
tonight, but given the high dew point air, some MVFR to IFR
visibilities are certainly possible late tonight.



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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