Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 291934
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
234 PM CDT MON AUG 29 2016
234 PM CDT
Very slow moving thunderstorms continue to develop over portions
of the area this afternoon. As a this writing, the most pronounced
heavy rain producer is the cluster of storms over Lee county.
These storms have been dropping up to 2" per hour over a small
portion of the county south of the Dixon area. While the
individual storm cells are moving slowly eastward, the cluster of
storms is propagating slowly north-northwestward into the Dixon
area as new development occurs along the outflow. If this heavy
rain continues into the Dixon area a flash flood warning may be
needed in the near future. Otherwise, expect SCT thunderstorms to
continue to develop and impact portions of the area for the next
few hours before dissipating by early this evening. The slow
eastward movement of these storms could result in some isolated
areas of flash flooding due to very heavy rainfall.
Some isolated storms are possible over the area tonight, but most
areas will likely remain dry. The high dew point air mass and the
expectation of light winds tonight could allow for some areas of
dense fog to develop. As a result, I have added patchy fog to the
forecast for tonight.
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
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