Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220743

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

231 PM CDT

Through Tuesday...

The main forecast concerns first focus on the threat for a few
additional showers and thunderstorms developing across mainly
southern sections of the area late this afternoon through early
evening. However, the threat of low clouds and possibly low
visibilities in fog will be the primary concern across much of
northern IL tonight.

An MCV moved across northeastern IL earlier and this produced a
period of thunderstorms with very heavy rain and flooding over
portions of the Chicago metro. This activity has since shifted
northeastward across southern Lake Michigan, with only some
lighter rains over the area in its wake. While cloud cover has
been persistent across much of the area through the day, there
has been an area of decreased cloudiness to our southwest over
portions of northeast MO and west central IL. This is in the
vicinity of the weak surface low and the associated warm front,
which is expected to track northeastward over northeastern IL and
northwestern IN tonight.

Temperatures in the vicinity of the warm front, which is
currently shifting northward into my far southern counties, are
generally in the middle 70s with dew points in the mid to upper
60s. This is resulting in rather modest surface based instability
of 500-1000 J/KG given rather weak mid-level lapse rates. Overall,
it appears that some shower and thunderstorm development is
likely to occur yet this afternoon, mainly within the vicinity of
the surface approaching tough. It appears the threat for severe
storms will be fairly low, due to the weaker mid-level lapse
rates, but an isolated strong storm could produce some locally
gusty winds late this afternoon. The primary threat for these
stronger storms should remain well south of I-80 across my
southern counties.

A few showers and storms may linger for a bit this evening over
the area, but the main concern then turns to fog and very low
ceilings as the surface tough moves overhead. Given the ample
low-level moisture with cool conditions near the lake, I cant
rule out a period of dense fog moving in off the lake tonight.
This possibility will need to be monitored this evening.

Any low ceilings and visibilities should improve during the
morning on Tuesday as the winds turn northerly. Expect dry
conditions with decreasing clouds in the afternoon. High
temperatures should generally be in the 70s inland, but in the
low to mid 60s near the lake.



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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