Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 191944
AFDLOT

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

.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...
222 AM CDT

Wednesday through Monday...

Transient upper ridge will traverse the region midweek with a more
blocky and amplified pattern developing late in the week and
continuing through early next week resulting in a warm pattern
throughout much of the extended time frame.

On Wednesday, closed upper low will be in place over the Pacific
Northwest with a lead shortwave ejecting east across the northern
Plains. Out ahead of the this feature, deep southerly flow will help
transport high theta-e air all the way north across the Great Plains
and into the Canadian Prairies. The thermal ridge axis with 850mb
temperatures of 19-20C will advect east across the local area
Wednesday contributing to very warm surface temperatures
accompanied by high dewpoints. Afternoon highs on Wednesday are
expected to be in the mid to upper 80s and a few spots may tag 90.
Meanwhile, dewpoints are expected to rise to around 70 resulting
in muggy conditions.

Wednesday night into Thursday looks to bring the best chance for any
precipitation over the area as the aforementioned upper level
disturbance lifts from the Upper Midwest into Canada while an weak
surface cold front pushes east across the Mississippi Valley. Models
are a bit mixed on how far east and south to bring precipitation
with this front, and for now would err on the drier end of the
spectrum given how far displaced we are from the upper wave and
forcing along the front may struggle to overcome warm mid level air
and capping. Moderately strong instability will be in place to
support thunderstorm development, though, if forcing ends up being
sufficient.

Meanwhile, the upper low over the Pacific Northwest is progged to
dig into the Intermountain West Wednesday through Friday with
further downstream amplification of the ridge expected to occur.
Friday through the weekend the pattern becomes very blocky with the
local area falling on the western periphery of a strong upper
ridge. While widespread convection is possible off to our west, it
looks like northern Illinois and northwest Indiana will remain
close enough to the ridge axis to stay primarily dry with
continued unseasonably warm temperatures.

Deubelbeiss

&&

.AVIATION...
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.

KJB

&&

.MARINE...
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.

JEE

&&

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

$$

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