Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 192014

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
314 PM CDT Tue Sep 19 2017

244 PM CDT

Through Wednesday...

The main forecast concerns for the period are:
1.) The good threat for fog tonight into early Wednesday, some of
which may be dense.
2.) The very warm and humid conditions on Wednesday.

A moist airmass remains in place across the area this afternoon,
with many areas reporting surface dewpoints into the low to mid
60s. This is likely to set the stage for fog development across
much of northern Illinois and northwestern Indiana tonight as
skies clear and the surface winds abate. There was a large area of
dense fog this morning over western Illinois and Iowa within this
same airmass. With this in mind, I think that fog could become
dense again tonight over my area. In spite of this, I will not
issue any type of headline at this time. Instead, I plan to
allow the evening shift to get a better handle on the best areas
favored for dense fog development tonight. Any fog and associated
low clouds that develop tonight will likely linger into the early
to mid morning hours things improve with the passage of a surface
warm front.

Once this warm front passes over the area in the morning, expect a
much warmer summer-like airmass to shift northward into the area.
A lower level thermal ridge is expected to advect eastward over
the area in response to a potent storm system over south central
Canada. With 925 MB temperatures progged to increase up around 25C
Wednesday afternoon, high temperatures are likely to climb into
the upper 80s in most locations. Combine this with unseasonably
high dewpoints in the upper 60s to around 70, and heat indices
are likely to top out into the lower to possibly mid 90s.

Some overnight convection also looks to develop tonight across
portions of northern Iowa. While it appears this activity will
weaken, or even dissipate before it reaches Illinois, there is a
very small possibility for an isolated showers making it into
north central Illinois Wednesday morning. Otherwise, much of the
area will remain precipitation free until our next weather
disturbance approaches Wednesday night. See more on this in the
long term discussion.



309 PM CDT

Wednesday night through Tuesday...

Of note in the longer range forecast are thunderstorm chances
Wednesday night, mainly over north central Illinois, and
unseasonable heat for latter September that could be impacting to
some groups given the late season nature.

Strong low pressure over the Canadian prairie provinces will stall
Wednesday and its associated cold front will slowly inch toward
the area Wednesday evening, likely entering north central Illinois
overnight but on a weakening trend. The air mass in advance as
noted above will be unseasonably warm and humid and as such will
be a source of instability. The early evening frontal location is
anticipated to be draped across Wisconsin and the Illinois/Iowa
border into Missouri. With a southerly low-level jet forecast to
increase to around 30 kt in ambient high moisture (PWATs around
1.80 inches), isolated to scattered storms are probable along the
front. With the upper jet maximum hanging back across the Upper
Plains, deep layer shear is only forecast to be around 25 kt. So
there may be limited organization, especially with storms into the
overnight hours. The MUCAPE values though look to be in excess of
2000 J/kg within a plume along the immediate boundary, so that
along may support a few severe storms with a hail and wind threat
but these could be mainly a little west of the area toward
initiation, or inching just into north central Illinois.

The slightly greater threat may be heavy rain given the slowing
speed of the boundary and ambient moisture. Just from last night`s
rain we received a 3+" rainfall report in Iroquois County with
convection in a similar air mass, so will be something to watch.
Dry antecedent conditions should help though.

Looking ahead to the warmth, the front will weaken and the warm
air mass will prevail through early next week. The 850mb
temperatures are forecast on Thursday to be around 20C, which is
the upper 1-3 percentile for the time of year. There is potential
for low clouds, maybe even fog, along the remnant boundary
Thursday morning across mainly north central Illinois. That could
keep temperatures down from the otherwise upper 80s to low 90s
that the air mass would support. Continued high dew points in the
upper 60s to lower 70s would support mid 90s heat indices most
favored in southern locations and possibly into Chicago, but the
further north the higher uncertainty on Thursday.

Confidence for sunshine on Friday into the weekend is higher and
upper 80s to possible low 90s look to continue. For those areas
that do not receive much rain Wednesday night, the dry ground may
help with sensible warming into the low 90s. Dew points will
likely drop a tad without the moist pre-frontal pull from
Wednesday/Thursday, and presently forecast mid-upper 60s dew
points. Heat index values look to be noteworthy for this time of
year. The stretch of warmth this late in the season will be of
note, especially for any outdoor activities (e.g. high school
sports, etc).



For the 18Z TAFs...

The main weather concern during the period is the potential for
another period of fog and any associated very low CIGS late
tonight into Wednesday morning.

The low clouds and precipitation have shifted out of the area.
However, expect BKN low end VFR CIGS at times this afternoon as
diurnal Strato-CU develops over the area. Otherwise, expect light
easterly winds through the afternoon.

The winds should become very light across the area tonight, and
this combined with amble low-level moisture could set the stage
for the redevelopment of fog over the area overnight. Confidence
is not the highest with the fog extent for the main Chicago
terminals, as it is typically hard to get good fog to develop.
However, given the extent of the fog that occurred to our west
this morning, and the fact that this same airmass will be overhead
tonight, I think there is a decent possibility to have at least
some MVFR to IFR visibilities and possibly even some IFR to LIFR
CIGS at the Chicago terminals overnight. Even lower conditions,
with possible dense fog will be possible at KGYY and possibly even
KDPA. These low conditions may linger through mid-morning
Wednesday, but conditions are likely to improve thereafter
following a surface warm frontal passage. Expect southerly winds
to set up over the area by midday, with some gusts developing by
the afternoon.



222 AM CDT

High pressure over Quebec will continue northeast. East winds
become southeast tonight, and speeds increase to 15-
25 kt as the pressure gradient tightens over the lake Wednesday.
High pressure passes over Lake Superior Thursday, and winds will
become north briefly as the high passes by.  Southerly winds return
across the lake Friday and remain through early next week due to the
blocky pattern. A high pressure ridge remains stationary over the
eastern half of the U.S. while a low pressure trough extends from
the northern Mississippi Valley to the southern Plains through early
next week.






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