Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191747

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1247 PM CDT Tue Sep 19 2017

1105 AM CDT

The latest GOES-16 Water vapor imagery channels indicate that the
main mid-level disturbance driving the showers this morning over
northeastern Illinois is now beginning to shift eastward over
southern Lake Michigan. As a result, the shower threat should be
coming to an end for much of the area by midday as the better
focus sets up east of the area. We still cant rule out a few
isolated showers this afternoon, especially over northwestern
Indiana, but most areas are likely to remain dry this afternoon.

In spite of the low chances for additional showers this afternoon,
it does appear that lower level cloud cover is likely to remain
with us for much of the afternoon, as any clearing areas are
likely to experience a quick uptick in diurnal Strato-CU
development. Because of this, high temperatures are likely to be
held down into the 70s for much of the area. So, highs this
afternoon have been lower a bit to reflect this thinking.



222 AM CDT

Through tonight...

Early this morning, a shortwave trough is lifting across the
Midwest while a weak surface low is located over west central
Illinois. Showers and a few thunderstorms will continue this
morning out ahead of these features. HRRR has had a decent handle
on conditions overnight and have leaned heavily on the latest
runs for timing PoPs through the remainder of the morning. Expect
most of the area to be dry by mid to late this morning with
perhaps a few showers hanging on over northwest Indiana by early
afternoon, if any at all. Mid/upper level height rises overspread
the region behind the departing trough helping to erode the cloud
cover through the afternoon. Expect afternoon highs in the mid
70s east to around 80 west. Temperatures along the lake front will
be held in the low 70s with onshore flow through the day.



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

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.



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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