Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 222018

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

235 PM CDT

Through Wednesday...

Main forecast concern will be the threat of fog again tonight
across northern IL, some possibly dense.

Skies area expected to gradually clear this evening as surface
high pressure builds into northern IL. Overall low-level moisture
continues to be elevated across the area, with dew points in the
mid 50s to low 60s across the area. With light winds under the
high tonight, expect temperatures to radiate rather efficiently
through the 50s. This should set up ample conditions for fog
again tonight as temperature fall through the cross over
temperature. I would not be surprised for a some areas of dense
fog development as well. However, with low confidence on the
extent of the dense fog, no advisory is planed at this time, but
could be needed for some of the area later.

Any fog and low clouds should dissipate across the area Wednesday
morning, with more sun likely by the afternoon. Expect warmer
temperatures, with highs in the 70s to near 80 inland, but
continued cool near the lake.



315 PM CDT

Wednesday night through Tuesday...

The big story will be a mid summer-like warmth/heat likely through
Memorial Day Weekend and possibly beyond. On Wednesday night-
Thursday, mid-upper ridging will extend from Gulf northward across
our region. These height rises now appear likely to preclude any
spill-over convection from the Upper MS Valley from making into
the local area into Thursday morning. With the surface high still
overhead initially, will need to monitor how much dew points mix
out on Wednesday afternoon regarding potential for any patchy fog
development Wednesday night-early Thursday. Have left fog mention
out of grids for now.

Climo of progged 925 mb temps on Thursday favors generally mid
80s for highs inland. Surface high will slide slowly east but
remain close enough for light southeast gradient and weak flow
aloft supportive of lake breeze formation and associated PM
cooling. Dew points will remain in the 50s for much of the area,
so it should be a relatively comfortable day Thursday despite the
warm temps.

The mid-upper ridge will temporarily be knocked down on Friday by
troughing into the Canadian Prairies and Northern Plains. A
glancing blow of forcing any only weakly capped and moderately
unstable air mass could allow for isolated to perhaps widely
scattered PM convection. If any t-storms occur, would not
anticipate an appreciable strong to severe risk given weak mid and
upper level flow and resulting weak deep layer shear. Bigger
story on Friday will likely be temps further warming into the
upper 80s for much of the area and 90 a distinct possibility.
South- southwest low level flow should be sufficient to prevent
lake cooling from spreading inland, however, flow aloft appears it
might be weak enough for a weak lake breeze to hug the Illinois
shore during the afternoon and bring some cooling along immediate

Stout mid-upper ridging will rebuild over the central and western
CONUS over the holiday weekend and local area initially on eastern
periphery, with ridge then building eastward. A cold front
trailing from low pressure slowly moving east toward northern
Lakes now appears it may wash out, though confidence in details of
surface pattern is on lower side this far out. Mid-upper jet and
thus main belt of forcing will remain well north of the area, so
there likely won`t be much to initiate more than diurnal
isolated/widely scattered convection despite weak/minimal capping
and moderate instability.

925 mb temps will be into the mid 20s Celsius Saturday-Monday,
thus 90+ degree readings more widespread than in official
forecast are distinctly possible if not likely depending on how
the surface pattern evolves. Did adjust temps upward from previous
forecast to upper 80s to near/around 90 Sat-Sun and mid-upper 80s
Memorial Day. Lake cooling does appear to be a decent possibility
both Sunday and Monday, possibly greater inland extent of cooling
on Monday with signs pointing toward a potential backdoor cold
front. Regarding heat index values, dew points should be into the
60s, so humidity levels will feel a bit more uncomfortable along
with the July like warmth. Tuesday may be a bit cooler with more
pronounced northeast flow from high pressure over the Great Lakes,
but still well above normal inland.



For the 18Z TAFs...

Conditions continue to gradually improve across the area early
this afternoon. Overall, expect clouds to begin to scatter out
across the area over the next couple hours with a period of VFR
possible by mid to late this afternoon. However, forecast concerns
quickly focus on the threat of low conditions in fog again tonight
into early Wednesday morning.

A surface ridge of high pressure will build over the area for
tonight. While winds will initially be northeasterly this
afternoon, expect winds to abate quickly during the evening before
becoming light and variable overnight. With clear skies this
evening and ample low-level moisture remaining in place, expect
fog to develop across the area again sometime mid to late this
evening and continue overnight. There remains uncertainty in how
low conditions will get at the main Chicago terminals, but the
potential is there for another period of dense fog before
conditions quickly improve Wednesday morning. At the present I
have gone as low as 1/2 mile at DPA, but only down into the 1 to 2
mile range at the main Chicago terminals. We will have to watch
for a period of lower visibilities late tonight.

VFR conditions will return to the area by late Wednesday morning.
Otherwise, expect light easterly winds during the afternoon.



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



LM...Dense Fog Advisory...LMZ743-LMZ744-LMZ745-LMZ777-LMZ779 until 10
     AM Wednesday.




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