Area Forecast Discussion
Issued by NWS Chicago, IL

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128
FXUS63 KLOT 200240
AFDLOT

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

.UPDATE...
940 PM CDT

Not planning any changes to going forecast, though fog is looking
to be less of a concern than previously feared.

Dewpoints depressions remain mostly in the 2-5F range across the
area, versus near 0 dewpoint depressions last night over IA where
dense fog was widespread. East to southeast winds of 4-7kt
continue this evening and with gradient not expected to get any
looser, would anticipate some wind to continue (perhaps increase a
bit western CWA) through the night. Would expect the result to be
mostly MVFR vsbys with denser fog probably confined to SE CWA
(where winds are lighter) and the typically more fog prone
sheltered/cold air drainage areas.

Also seeing some marine fog over the north shore areas already
this evening. It`s unclear how significant of a problem this will
become with many coastal webcams showing unrestricted vsby.

Otherwise, going forecast in good shape an no update planned to
text products at this time.

- Izzi

&&

.SHORT TERM...
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.

KJB

&&

.LONG TERM...
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).

MTF

&&

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

Have made little changes to the TAFs, with relatively quiet
conditions early this evening likely transitioning to a period of
low ceilings and visibility late tonight into early Wednesday
morning. In the near term, easterly wind will turn more southeast
while VFR conditions continue. Current low clouds across far NE IL
and SE WI should remain just to the north, and without any vis
restrictions. Am monitoring for the possibility of a sooner
arrival of the lower vis in fog, which could possibly come late
this evening or around midnight. Also monitoring possible lower
ceilings than currently forecast early Wednesday morning, with
ceilings below 500 ft possible. Lower confidence with these
scenarios, and so have continued current ceiling and vis forecast.
Any lower ceilings/vis Wednesday morning will scatter, with VFR
conditions and southerly winds expected through the remainder of
the period. Scattered showers/thunderstorms will likely develop to
the west of ORD towards the end of the period, and should remain
to the west this forecast period.

Rodriguez

&&

.MARINE...
315 PM CDT

Fairly quiet weather over the lake with an approaching cold front
midweek stalling to the west. Ahead of this front there will be a
southerly wind with 20-25 kt gusts probable over mid lake on
Wednesday. Otherwise, conditions look mainly rain-free over the
lake with little impacts from winds.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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