Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220800

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
300 AM CDT Tue May 22 2018

258 AM CDT

Through tonight...

Main forecast concerns/challenges are with dense fog this morning
across much of northern IL and northwest IN.

Weak surface low and associated boundary continue to slowly
progress northeast across the CWA this morning, with much of the
CWA right along and north of this boundary. Very moist low levels
and light winds have allowed the dense fog to persist this
morning, and even expand south/southeast into other areas. Vis of
one quarter mile or less will continue, with vis near zero still
possible in some locations. Despite this low continuing northeast
of the area over the next several hours, moisture never really
scours out and with the gradient not increasing, won`t likely see
much of any improvement early this morning. May actually see
further south/southeast expansion of this dense fog into remaining
areas of the CWA on the backside of this low, south of the
Kankakee river valley. Will continue to monitor this possibility,
and add to current Advisory if needed. Current end time, mid
morning, of the Dense Fog Advisory looks to be on track at this

With weak ascent overhead, isolated showers will remain possible
over the next couple of hours. Do think any development will
remain rather isolated though, with much of the area remaining dry
early this morning. Low will depart this morning, however, weak
trough will remain situated on the backside of this low and this
could support some weak isolated showers later today mainly over
the far southern CWA. Have not included any mention in the
forecast, given how low chances appear at this time. This trough
quickly shifts out of the area today though, with high pressure
building across the region. Cloud cover will be a challenge later
today into tonight. Although latest guidance would suggest
clearing skies later this afternoon into tonight, not completely
confident with this solution. Did trend this way in the grids, but
could see clouds lingering tonight and fog once again developing.
Building high pressure would support a setup for any cloud cover
that does not scatter today, to likely remain and expand tonight.
Will need to monitor trends and latest guidance and adjust as



315 PM CDT

Tuesday night through Monday...

A summer-like pattern will return as we head toward and through
Memorial Day Weekend, including periodic lower end chances for
showers and thunderstorms. Conditions over at least the Friday-
Saturday period will likely be very warm and muggy.

Mid/upper ridging will build over the region during mid-week,
which should keep the local area mostly dry Wednesday-Thursday.
Can`t rule out elevated convection over the mid and upper
Mississippi Valley on Wednesday night sneaking into portions of
far northern IL (mainly northwest/north central) late Wednesday
night into Thursday morning. Surface high pressure over the Great
Lakes on Wednesday will allow for increasing onshore flow via lake
breeze enhancement, keeping highs near the lake in the mid to
upper 60s, while areas inland warm to the upper 70s to lower 80s.

The surface high will likely be close enough by on Thursday, along
with very strong land-lake differential to force lake breeze
development, and associated cooling during the afternoon. Highs
inland are favored to be well into the 80s, in the 82-87 range.

Most recent operational guidance casts some doubt on extent of
convection on Friday, despite modest height falls from troughing
from the Canadian Prairies to Northern Plains approaching the
western Lakes. Forecast soundings indicate a very warm profile
that could stay capped. Should this be the case, then forecast
highs in mid to upper 80s could be a bit conservative, with
possibility of making a run at 90 for at least portions of the
area. There is decent agreement in dew points over the corn belt
rising into the 60s, making for a more uncomfortable feel to the
warm temps. In addition, southwest low level flow should be strong
enough to keep lake breeze formation at bay barring major changes,
so shore areas appear likely to experience the warmth through the

Predictability remains on the lower side through the holiday
weekend regarding sensible weather. Stout mid-level ridging will
rebuild over the central CONUS, with local area on eastern
periphery of this ridge. Overall, this look is one that argues
against any noteworthy synoptic systems and organized strong-
severe convection due to weak mid and upper flow. Also big
picture wise, the above normal temperatures look to continue, with
question being whether a cold front passage occurs on Saturday
timeframe, which could assist with a better chance for
showers/storms beyond isolated to widely scattered pop-up type
convection. In addition, whether cold front passage occurs will
dictate wind direction and associated potential for lake cooling
returning, especially Sunday-Monday. Should cold fropa be delayed,
then Saturday-Monday temps and heat index values certainly could
be higher than in official forecast.



For the 06Z TAFs...

Fog (1/4-3SM) and low ceilings (002-004) have overspread much of
northern Illinois and northwest Indiana and will likely persist
into the mid morning hours Tuesday. Diffuse surface low centered
near JOT as of 04Z will gradually shift east overnight. Light
synoptic flow should back to the NNW over the next 1-2 hours as
the low moves across the region and then to the NW by Tuesday
morning. The flow should remain under 5 kt through the overnight
hours which may result in some variability at times. High pressure
will build into the Upper Midwest during the day Tuesday causing
winds to veer back to the north and eventually northeast as a lake
breeze forms during the afternoon. Fog should lift during the
morning, and expect a gradual improvement in the ceilings
throughout the daytime hours. As the surface high builds into
northern Illinois overnight, winds will become light and variable
and the threat for fog and low ceilings may return. Will have to
see how much sunshine/mixing there is today to get a better handle
on fog potential tonight. For now will hedge and introduce at
least MVFR conditions Wednesday night, but cannot rule out needing
IFR/LIFR at some point.



241 AM CDT

A weak low will move from northern Illinois to the eastern Great
Lakes today while a weak ridge will remain over the northern Great
Lakes region and consolidate over the western Great Lakes behind
the departing low later today. This high will then gradually shift
east to the eastern Great Lakes by Thursday. Weak flow will be in
place across Lake Michigan through this time frame. Fog will also
continue at times over the next 24-48 hours. A dense fog advisory
remains in effect for southern half of Lake Michigan through mid
morning. It`s possible it may need to be extended later into the
day. Late in the week, low pressure will form over the Canadian
Prairies and dig into the Upper Midwest, though is progged to
weaken during this time. Fresh southwest flow will develop,
especially over the north end of the lake. Winds are expected to
drop off again over the weekend as an expansive high builds over
Hudson Bay with ridging extending into portions of the Great



IL...Dense Fog Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ023 until 10 AM Tuesday.

IN...Dense Fog Advisory...INZ001-INZ002-INZ010-INZ011 until 10 AM

LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 until 9 AM Tuesday.




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