Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 281758

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1258 PM CDT SUN AUG 28 2016

1136 AM CDT

Main updates this morning were to adjust pop trends into this
afternoon, as well as raise temps a couple of degrees. Early
morning stratus and fog has since scattered out, while diurnal
development has replaced it. This will provide continued partly
cloudy to partly sunny skies this afternoon. Latest radar imagery
showing isolated thunderstorm development beginning to occur near
the Kankakee area. This is developing along lingering boundary
situated from near Pontiac northeast to Valparaiso. While flow
aloft continues to usher in subtle waves this afternoon and while
instability has increased, have focused pops along this boundary
with continued isolated to widely scattered thunderstorms to
persist in and around this boundary this afternoon. Due to the
coverage and duration of these storms, have kept pops at chance
today. Still not thinking any widespread severe weather will occur
today, but an isolated stronger storm will be possible with gusty
winds and heavy downpours/localized flooding the main threats.
This will be especially the case with the ILX morning sounding
showing near 2 inch PWATs still in place today and with RAP
analysis supporting this right over the surface boundary, as well
as a high potential for any storm to remain rooted in one location
for an extended period of time. Precip development will likely
diminish into early evening, with this trend continuing into



307 AM CDT

Through tonight...

Warm and humid conditions will once again be in place today across
northern Illinois and northwest Indiana. Strong upper level wave
is evident on water vapor imagery early this morning lifting
across Lake Superior with mid/upper level height rises spreading
over much of the upper midwest which will help suppress any more
widespread precip. Meanwhile, a weak cold front is analyzed from
near Kenosha southwest through Sterling, which has allowed some
very isolated convection to persist overnight. Given lack of upper
support expect coverage to remain very isolated at most through
the rest of the morning, with likely dry conditions until around
midday. As we head into the afternoon, dewpoints in the low to mid
70s and temperatures in the low to mid 80s will contribute to
moderate instability with forecast soundings indicating very
minimal convective inhibition. Some pop-up showers and
thunderstorms will be possible, especially south of I-80 in the
highest dewpoint air. Expect coverage to remain isolated to widely
scattered and severe weather is not expected due to the very weak
low/mid level flow. Any convective activity should begin to
diminish after sunset.



307 AM CDT

Monday through Saturday...

Warm and humid conditions will kick off the upcoming week, and
then a pattern change is expected to bring cooler temperatures
right on cue with the start of meteorological fall.

Weak upper ridging will be in place Monday with surface high
centered over the Great Lakes. Unstable conditions are still
expected to be in place along the periphery of the high where dew
points will be in the low 70s. Low level ridge axis is now
expected to poke into portions of far northeast Illinois which
will limit PoPs for much of the Chicago metro area, but PoPs will
increase some to the southwest as higher dew point air will be in
place along with slightly steeper mid level lapse rates. Despite
the instability, forecast soundings in the southwest do show a
weak cap in place and little in the way of forcing is expected.
Will limit PoPs to slight chance mainly southwest of a RPJ-IKK-RZL
line, favored from late morning into the afternoon. On Tuesday,
potent shortwave is progged to dive into northern Ontario and
across James Bay with attendant surface low pushing a cold front
into portions of the upper midwest. Showers and thunderstorms are
expected along the front; however, models do not drop the front
into northern Illinois until after 00Z Tuesday evening. With weak
mid-level flow and lack of any upper level support, thunderstorm
coverage is expected to diminish as we begin to stabilize after
sunset. Depending on the how far the front progresses Tuesday
night into early Wednesday, could see some thunderstorms redevelop
in the CWA Wednesday afternoon, with the best chances now
appearing to be south of I-80.

Behind the front Wendesday, winds turn northerly ahead of a strong
area of high pressure building down across the Canadian Prairies
which will eventually becoming centered across the northern Great
Lakes Region by Thursday. This high will result in dry weather
late Wednesday into Saturday. Cooler air will begin to trickle
into region Wednesday as the 850mb 0C isotherm drops into the
northern Great Lakes. Locally, the GFS and ECMWF bring 850mb
temperatures into the 8-12C range, with the ECMWF continuing to
run on the cooler side. Local climatology suggests that this would
support temperatures only in the low to mid 70s, and mixing down
925mb results in upper 60s to near 70 highs. While highs in the
upper 60s is supported by the raw MaxT output from the GFS and
ECMWF, various MOS guidance still support mid 70s so opted to not
make any significant changes just yet. Our forecast does appear to
be on the warm side of the possibility spectrum in the meantime.
Bottom line, cooler and drier conditions will be in place, at
least temporarily for the latter half of the week and to start
next weekend. Strong upper ridging over the mid section of the
country mid week is expected to inch east and eventually bring
more seasonable conditions along with additional chances for
showers and thunderstorms either late in the weekend or early next



For the 18Z TAFs...

Latest radar imagery showing thunderstorm development occurring
just south of the terminals at this time. These storms are
developing along a lingering boundary, and expect continued
development over the next several hours near this boundary. Don`t
anticipate these storms to reach any of the terminals, just
staying to the south before diminishing completely this evening.
Then expect tonight to be mainly dry, but with scattered
thunderstorm development possible away from the terminals again on
Monday. VFR conditions will likely prevail, but it is possible for
fog and low ceilings to once again develop late tonight, similar
to what occurred earlier this morning. Have lower confidence of
this possibility at this time, but will need to continue to
monitor this.



307 AM CDT

High pressure will return across Lake Michigan today as weak low
pressure exits east across Ontario. Low pressure across the
Canadian prairies will spread east to James Bay on Tuesday, then
east to the Atlantic seaboard late Wednesday night. This low will
send a cold front across the northern half of Lake Michigan
Tuesday. The front will get shoved a bit farther south as high
pressure strengthens behind the departing low and build waves
across the south half of the lake Wednesday. The high will
overspread the western Great Lakes from mid to late week.






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