Area Forecast Discussion
Issued by NWS Chicago, IL

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145
FXUS63 KLOT 230224
AFDLOT

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

.UPDATE...
924 PM CDT

No changes planned to forecast this evening. Seeing T/Td spreads
shrinking and would anticipate fog to begin to develop over the
next few hours. Webcams and satellite imagery show that fog as
cleared up over the lake, but could certainly re-develop tonight.
Confidence is still not high regarding how widespread and dense
fog will become tonight, so no plans for a dense fog advisory at
this point as confidence is certainly lower in coverage and
density of the fog tonight vs last night.

- Izzi

&&

.SHORT TERM...
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.

KJB

&&

.LONG TERM...
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 lakeside and a few/several miles inland. 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, do 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 be the 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 shore.

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. Next Tuesday may be a bit cooler with
more pronounced northeast flow from high pressure over the Great
Lakes, but still well above normal 80s inland.

Castro

&&

.AVIATION...
For the 00Z TAFs...

Primary aviation concern is with potential for fog/stratus
development tonight. There is a strong signal from the various
models suggesting dense fog and CIGS on the deck overnight into
very early Wednesday morning. However, unlike yesterday, boundary
layer has dried out a bit, dewpoint depressions today are around
10F vs 2F 24 hours ago. Still appears to be a decent set-up for
radiational fog development, which tends to favor non-urban areas.
Given observational trends are pointing toward conditions not as
low as this morning while model guidance suggests just as low,
have opted to make no changes to the previous forecast which is a
reasonable compromise at this point.

Any fog/stratus that develops tonight, would likely quickly
improve Wednesday morning with conditions likely to improve to
VFR by mid morning. Light winds tonight into Wednesday morning
should allow lake breeze to develop Wednesday afternoon, probably
fairly early again like today. Behind the lake breeze, could see a
period of winds around 10kt before easing a bit and probably
settling closer to 6-8kt.

- Izzi

&&

.MARINE...
330 PM CDT

There are no significant/impactful winds forecast through the
holiday weekend, with the strongest winds up to 25 kt over the
north half Friday into Friday evening. The main marine concern in
the near term is dense fog over the south half of the lake, with
the Dense Fog Advisory now in effect through Wednesday morning.
It`s again possible the advisory may need to be further extended
depending on trends on Wednesday.

Weak high pressure over the western Great Lakes will gradually
shift east to the eastern Great Lakes by Thursday. Weak flow will
be in place across Lake Michigan through this time frame. 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 south-southwest flow will develop, especially
over the north end of the lake, where again speeds could be up to
25 kt Friday PM. 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 Lakes. There are periodic low
end chances for thunderstorms. Will also need to watch for
possibility of dense fog at times into and through the weekend as
very and increasingly moist air mass moves over the still chilly
lake waters.

Over the IL/IN nearshore, winds will be driven by lake and land
breeze influences much of the time through the weekend, with
possible exception of Friday and Saturday.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 until 10 AM Wednesday.

&&

$$

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