Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 282025

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
325 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.



320 PM CDT

Monday through Sunday...

The continued challenges of the past few days will continue Monday
and Tuesday, and possibly even Wednesday in the southern CWA. The
high precipitable water swath of 1.75+ inches will very gradually
spread back north on Monday as subtle return flow begins. Today
only subtle convergence in very moist air was able to blossom
storms as convective temps were reached by late morning. Think
that could be possible again Monday, and high-res models indicate
low-level streamlines with convergence draped from the Quad
Cities toward LaSalle-Peru to Gibson City. Challenging to say
where exactly that may be due to some influence of this afternoons well as just how active the boundary may be due to
such a light flow regime with general surface/upper high
pressure. But given the air mass it will not take much to spark
isolated/scattered storms. So have PoPs mainly in the western and
southern CWA. The conditional flooding threat will be there once
again due to the high rate efficiency and slow storm motion under
very weak 20 kt 3-6km flow.

A cold front will drop southward on Tuesday as influenced by a
northern stream jet developing back to the south. Upper lift in
advance of this jet will support better upper forcing than we had
today or will have Monday. The greater convergence looks to be
just north of the area in the afternoon, but again a modest to
possibly highly unstable air mass should be in place over the
region. So have chances for storms during the day and night as the
boundary shifts into the region. A localized severe wind threat
may exist as well as a continued flooding risk with any
training/slow moving storms. The chance for storms will continue
across the south on Wednesday ahead of the front, at least
depending on how quickly the front moves through.

Cooler than normal air will move in behind the front for
Wednesday with north-northeast winds gusting to 25 to possibly 30
mph along the lake front. This looks to probably bring favorable
conditions for rip currents on Wednesday possibly into at least
early Thursday.



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.



323 PM CDT

With moist conditions and light flow, fog will once again be a
threat over the lake tonight into Monday morning. Some of this may
be dense, probably more favored over the southern part of the
lake. Variable winds will be seen at times through Monday night. A
cool front will move southward down the lake late Tuesday and
Tuesday night. This will turn winds northerly and gusty. Some 30
kt gusts look very possible over the open water. Small craft
advisory conditions are probable in the Illinois and Indiana
nearshores during Wednesday through early Thursday.






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